FAQ(3)

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💖 FAQ(3) — avr-libc 💖

okay bestie, pull up a chair, grab a boba 🧋, because this is the big one. this is the avr-libc FAQ — Frequently Asked Questions — and it is long, so we're doing it in 7 parts, and this is part 1. no sign-off, we're not done, keep scrolling, stay hydrated 🫶

✨ NAME ✨

FAQ — Frequently Asked Questions. that's it. that's the name. iconic of it honestly 💅

🎀 FAQ Index 🎀

the whole vibe map, in order, no cap:

My program doesn't recognize a variable updated within an interrupt routine
I get 'undefined reference to...' for functions like 'sin()'
How to permanently bind a variable to a register?
How to modify MCUCR or WDTCR early?
What is all this _BV() stuff about?
Can I use C++ on the AVR?
Shouldn't I initialize all my variables?
Why do some 16-bit timer registers sometimes get trashed?
How do I use a #define'd constant in an asm statement?
Why does the PC randomly jump around when single-stepping through my program in avr-gdb?
How do I trace an assembler file in avr-gdb?
How do I pass an IO port as a parameter to a function?
What registers are used by the C compiler?
How do I put an array of strings completely in ROM?
How to use external RAM?
Which -O flag to use?
How do I relocate code to a fixed address?
My UART is generating nonsense! My ATmega128 keeps crashing! Port F is completely broken!
Why do all my 'foo...bar' strings eat up the SRAM?
Why does the compiler compile an 8-bit operation that uses bitwise operators into a 16-bit operation in assembly?
How to detect RAM memory and variable overlap problems?
Is it really impossible to program the ATtinyXX in C?
What is this 'clock skew detected' message?
Why are (many) interrupt flags cleared by writing a logical 1?
Why have 'programmed' fuses the bit value 0?
Which AVR-specific assembler operators are available?
Why are interrupts re-enabled in the middle of writing the stack pointer?
Why are there five different linker scripts?
How to add a raw binary image to linker output?
How do I perform a software reset of the AVR?
I am using floating point math. Why is the compiled code so big? Why does my code not work?
What pitfalls exist when writing reentrant code?
Why are some addresses of the EEPROM corrupted (usually address zero)?
Why is my baud rate wrong?
On a device with more than 128 KiB of flash, how to make function pointers work?
Why is assigning ports in a 'chain' a bad idea?

🌸 My program doesn't recognize a variable updated within an interrupt routine

bestie. the optimizer is not gaslighting you, but it is lowkey ignoring you 😭 when you've got the optimizer on, and you write a loop like this one:

uint8_t flag;
...
ISR(SOME_vect) {
  flag = 1;
}
...

    while (flag == 0) {
        ...
    }

the compiler will typically access flag only once, and optimize further accesses completely away, since its code path analysis shows that nothing inside the loop could change the value of flag anyway. like, it did the math, and the math said "this variable does not move, bestie." 💀

so to tell the compiler that this variable could be changed outside the scope of its code path analysis (e. g. from within an interrupt routine), the variable needs to be declared like:

volatile uint8_t flag;

volatile is the "actually, watch this one 👀" keyword. periodt.

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🌈 I get 'undefined reference to...' for functions like 'sin()'

so you called sin() and the linker is screaming at you. that's because to access the mathematical functions that are declared in <math.h>, the linker needs to be told to also link the mathematical library, libm.a. the math is there, the linker just hasn't been introduced to it yet 🙈

typically, system libraries like libm.a are given to the final C compiler command line that performs the linking step by adding a flag -lm at the end. (that is, the initial lib and the filename suffix from the library are written immediately after a -l flag. so for a libfoo.a library, -lfoo needs to be provided.) this will make the linker search the library in a path known to the system. ✨ the pattern is right there, it's giving very "the name is the flag."

an alternative would be to specify the full path to the libm.a file at the same place on the command line, i. e. after all the object files (*.o). however, since this requires knowledge of where the build system will exactly find those library files, this is deprecated for system libraries. don't do it, it's giving 2009.

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🦋 How to permanently bind a variable to a register?

yesss you can literally hand a variable its own register, this can be done with:

register unsigned char counter asm("r3");

typically, it should be safe to use r2 through r7 that way. 👑

registers r8 through r15 can be used for argument passing by the compiler in case many or long arguments are being passed to callees. if this is not the case throughout the entire application, these registers could be used for register variables as well.

⚠️ extreme care should be taken that the entire application is compiled with a consistent set of register-allocated variables, including possibly used library functions. like, entire application. every file. every library. no exceptions, fr fr. 🙏

See C Names Used in Assembler Code for more details.

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💅 How to modify MCUCR or WDTCR early?

the method of early initialization (MCUCR, WDTCR or anything else) is different (and more flexible) in the current version. basically, write a small assembler file which looks like this:

;; begin xram.S

#include <avr/io.h>

        .section .init1,"ax",@progbits

        ldi r16,_BV(SRE) | _BV(SRW)
        out _SFR_IO_ADDR(MCUCR),r16

;; end xram.S

assemble it, link the resulting xram.o with other files in your program, and this piece of code will be inserted in initialization code, which is run right after reset. see the linker script for comments about the new .initN sections (which one to use, etc.). slay. 💖

the advantage of this method is that you can insert any initialization code you want (just remember that this is very early startup -- no stack and no __zero_reg__ yet 😳), and no program memory space is wasted if this feature is not used. so cute, so efficient.

there should be no need to modify linker scripts anymore, except for some very special cases. it is best to leave __stack at its default value (end of internal SRAM -- faster, and required on some devices like ATmega161 because of errata), and add -Wl,-Tdata,0x801100 to start the data section above the stack.

for more information on using sections, see Memory Sections. there is also an example for Using Sections in C Code. note that in C code, any such function would preferably be placed into section .init3 as the code in .init2 ensures the internal register __zero_reg__ is already cleared. 🧚

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⭐ What is all this _BV() stuff about?

when performing low-level output work, which is a very central point in microcontroller programming, it is quite common that a particular bit needs to be set or cleared in some IO register. while the device documentation provides mnemonic names for the various bits in the IO registers, and the AVR device-specific IO definitions reflect these names in definitions for numerical constants, a way is needed to convert a bit number (usually within a byte register) into a byte value that can be assigned directly to the register. however, sometimes the direct bit numbers are needed as well (e. g. in an SBI() instruction), so the definitions cannot usefully be made as byte values in the first place. classic bit-vs-byte identity crisis 😭

so in order to access a particular bit number as a byte value, use the _BV() macro. of course, the implementation of this macro is just the usual bit shift (which is done by the compiler anyway, thus doesn't impose any run-time penalty), so the following applies:

_BV(3) => 1 << 3 => 0x08

however, using the macro often makes the program better readable. (yes bestie, readability is self-care 💅)

example: clock timer 2 with full IO clock (CS2x = 0b001), toggle OC2 output on compare match (COM2x = 0b01), and clear timer on compare match (CTC2 = 1). make OC2 (PD7) an output.

TCCR2 = _BV(COM20)|_BV(CTC2)|_BV(CS20);
DDRD = _BV(PD7);

look at that. immaculate. ✨

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👑 Can I use C++ on the AVR?

basically yes, C++ is supported (assuming your compiler has been configured and compiled to support it, of course). source files ending in .cc, .cpp or .C will automatically cause the compiler frontend to invoke the C++ compiler. alternatively, the C++ compiler could be explicitly called by the name avr-c++. 🔥

however, there's currently no support for libstdc++, the standard support library needed for a complete C++ implementation. this imposes a number of restrictions on the C++ programs that can be compiled. among them are:

obviously, none of the C++ related standard functions, classes, and template classes are available. 💀
the operators new and delete are not implemented, attempting to use them will cause the linker to complain about undefined external references. (this could perhaps be fixed.)
some of the supplied include files are not C++ safe, i. e. they need to be wrapped into

extern "C" { . . . }

(this could certainly be fixed, too.)

exceptions are not supported. since exceptions are enabled by default in the C++ frontend, they explicitly need to be turned off using -fno-exceptions in the compiler options. failing this, the linker will complain about an undefined external reference to __gxx_personality_sj0. 🙈

constructors and destructors are supported though, including global ones. so that's cute. 🎀

when programming C++ in space- and runtime-sensitive environments like microcontrollers, extra care should be taken to avoid unwanted side effects of the C++ calling conventions like implied copy constructors that could be called upon function invocation etc. these things could easily add up into a considerable amount of time and program memory wasted. thus, casual inspection of the generated assembler code (using the -S compiler option) seems to be warranted. like, look at what you made. 👀

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okay that's part 1, bestie — more parts are coming, don't go anywhere 🫶✨

✨ Shouldn't I initialize all my variables? ✨

okay bestie, real talk: global and static variables are guaranteed to be initialized to 0 by the C standard. 💖 avr-gcc does this by placing the appropriate code into section .init4 (see The .initN Sections). And yeah, with respect to the standard this sentence is somewhat simplified (because the standard allows for machines where the actual bit pattern used differs from all bits being 0), but for the AVR target, in general: all integer-type variables are set to 0, all pointers to a NULL pointer, and all floating-point variables to 0.0. periodt. 💅

As long as these variables are not initialized (i. e. they don't have an equal sign and an initialization expression to the right within the definition of the variable), they go into the .bss section of the file. 🎀 This section simply records the size of the variable, but otherwise doesn't consume space, neither within the object file nor within flash memory. (Of course, being a variable, it will consume space in the target's SRAM.) no cap.

In contrast, global and static variables that have an initializer go into the .data section of the file. This will cause them to consume space in the object file (in order to record the initializing value), and in the flash ROM of the target device. The latter is needed since the flash ROM is the only way that the compiler can tell the target device the value this variable is going to be initialized to. 😭

Now if some programmer "wants to make doubly sure" their variables really get a 0 at program startup, and adds an initializer just containing 0 on the right-hand side, they waste space. 💀 While this waste of space applies to virtually any platform C is implemented on, it's usually not noticeable on larger machines like PCs, while the waste of flash ROM storage can be very painful on a small microcontroller like the AVR. it's giving wasteful fr fr.

So in general, variables should only be explicitly initialized if the initial value is non-zero. ✨👑

Note: Recent versions of GCC are now smart enough to detect this situation, and revert variables that are explicitly initialized to 0 to the .bss section. 🧚 Still, other compilers might not do that optimization, and as the C standard guarantees the initialization, it is safe to rely on it. 🫶

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🕰️ Why do some 16-bit timer registers sometimes get trashed? 🕰️

okay so some of the timer-related 16-bit IO registers use a temporary register (called TEMP in the Atmel datasheet) to guarantee an atomic access to the register despite the fact that two separate 8-bit IO transfers are required to actually move the data. 🌸 Typically, this includes access to the current timer/counter value register (TCNTn), the input capture register (ICRn), and write access to the output compare registers (OCRnM). Refer to the actual datasheet for each device's set of registers that involves the TEMP register. iykyk 📖

When accessing one of the registers that use TEMP from the main application, and possibly any other one from within an interrupt routine, care must be taken that no access from within an interrupt context could clobber the TEMP register data of an in-progress transaction that has just started elsewhere. ⚠️

To protect interrupt routines against other interrupt routines, it's usually best to use the ISR() macro when declaring the interrupt function, and to ensure that interrupts are still disabled when accessing those 16-bit timer registers. 🛡️

Within the main program, access to those registers could be encapsulated in calls to the cli() and sei() macros. If the status of the global interrupt flag before accessing one of those registers is uncertain, something like the following example code can be used. 👇

uint16_t
read_timer1(void)
{
    uint8_t sreg;
    uint16_t val;

    sreg = SREG;
    cli();
    val = TCNT1;
    SREG = sreg;

    return val;
}

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🌈 How do I use a `#define`'d constant in an asm statement? 🌈

So you tried this:

asm volatile("sbi 0x18,0x07;");

Which works. ✅ When you do the same thing but replace the address of the port by its macro name, like this:

asm volatile("sbi PORTB,0x07;");

you get a compilation error: 'Error: constant value required'. 😭💀

PORTB is a precompiler definition included in the processor specific file included in avr/io.h. As you may know, the precompiler will not touch strings and PORTB, instead of 0x18, gets passed to the assembler. One way to avoid this problem is:

asm volatile("sbi %0, 0x07" : "I" (_SFR_IO_ADDR(PORTB)):);

Note: For C programs, rather use the standard C bit operators instead, so the above would be expressed as PORTB |= (1 << 7). 👑 The optimizer will take care to transform this into a single SBI instruction, assuming the operands allow for this. ✨

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🦋 Why does the PC randomly jump around when single-stepping through my program in avr-gdb? 🦋

When compiling a program with both optimization (-O) and debug information (-g) which is fortunately possible in avr-gcc, the code watched in the debugger is optimized code. While it is not guaranteed, very often this code runs with the exact same optimizations as it would run without the -g switch. 🙈

This can have unwanted side effects. Since the compiler is free to reorder code execution as long as the semantics do not change, code is often rearranged in order to make it possible to use a single branch instruction for conditional operations. Branch instructions can only cover a short range for the target PC (-63 through +64 words from the current PC). If a branch instruction cannot be used directly, the compiler needs to work around it by combining a skip instruction together with a relative jump (rjmp) instruction, which will need one additional word of ROM. 💅

Another side effect of optimization is that variable usage is restricted to the area of code where it is actually used. So if a variable was placed in a register at the beginning of some function, this same register can be re-used later on if the compiler notices that the first variable is no longer used inside that function, even though the variable is still in lexical scope. When trying to examine the variable in avr-gdb, the displayed result will then look garbled. 😵‍💫

So in order to avoid these side effects, optimization can be turned off while debugging. However, some of these optimizations might also have the side effect of uncovering bugs that would otherwise not be obvious, so it must be noted that turning off optimization can easily change the bug pattern. In most cases, you are better off leaving optimizations enabled while debugging. 🫶

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🧚 How do I trace an assembler file in avr-gdb? 🧚

When using the -g compiler option, avr-gcc only generates line number and other debug information for C (and C++) files that pass the compiler. Functions that don't have line number information will be completely skipped by a single step command in gdb. This includes functions linked from a standard library, but by default also functions defined in an assembler source file, since the -g compiler switch does not apply to the assembler. 😭

So in order to debug an assembler input file (possibly one that has to be passed through the C preprocessor), it's the assembler that needs to be told to include line-number information into the output file. (Other debug information like data types and variable allocation cannot be generated, since unlike a compiler, the assembler basically doesn't know about this.) This is done using the (GNU) assembler option --gstabs. ✨

Example:

$ avr-as -mmcu=atmega128 --gstabs -o foo.o foo.s

When the assembler is not called directly but through the C compiler frontend (either implicitly by passing a source file ending in .S, or explicitly using -x assembler-with-cpp), the compiler frontend needs to be told to pass the --gstabs option down to the assembler. This is done using -Wa,--gstabs. Please take care to only pass this option when compiling an assembler input file. ⚠️ Otherwise, the assembler code that results from the C compilation stage will also get line number information, which confuses the debugger. 💀

Note: You can also use -Wa,-gstabs since the compiler will add the extra - for you. 🎀

Example:

$ EXTRA_OPTS="-Wall -mmcu=atmega128 -x assembler-with-cpp"
$ avr-gcc -Wa,--gstabs ${EXTRA_OPTS} -c -o foo.o foo.S

Also note that the debugger might get confused when entering a piece of code that has a non-local label before, since it then takes this label as the name of a new function that appears to have been entered. 🌀 Thus, the best practice to avoid this confusion is to only use non-local labels when declaring a new function, and restrict anything else to local labels. Local labels consist just of a number only. References to these labels consist of the number, followed by the letter b for a backward reference, or f for a forward reference. These local labels may be re-used within the source file, references will pick the closest label with the same number and given direction. 👑

Example:

myfunc: push    r16
    push    r17
    push    r18
    push    YL
    push    YH
    ...
    eor r16, r16    ; start loop
    ldi YL, lo8(sometable)
    ldi YH, hi8(sometable)
    rjmp    2f      ; jump to loop test at end
1:  ld  r17, Y+     ; loop continues here
    ...
    breq    1f      ; return from myfunc prematurely
    ...
    inc r16
2:  cmp r16, r18
    brlo    1b      ; jump back to top of loop

1:  pop YH
    pop YL
    pop r18
    pop r17
    pop r16
    ret

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🌸 How do I pass an IO port as a parameter to a function? 🌸

okay bestie, let's talk about passing an IO port into a function — because this is one of those things that looks innocent and then absolutely does not work the way you think. 😭 Consider this example code:

#include <inttypes.h>
#include <avr/io.h>

void
set_bits_func_wrong (volatile uint8_t port, uint8_t mask)
{
    port |= mask;
}

void
set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
{
    *port |= mask;
}

#define set_bits_macro(port,mask) ((port) |= (mask))

int main (void)
{
    set_bits_func_wrong (PORTB, 0xaa);
    set_bits_func_correct (&PORTB, 0x55);
    set_bits_macro (PORTB, 0xf0);

    return (0);
}

so the first function, set_bits_func_wrong, is giving major red-flag energy. 🚩 It will generate object code which is not even close to what is intended. The major problem arises when the function is called. When the compiler sees this call, it will actually pass the value of the PORTB register (using an IN instruction), instead of passing the address of PORTB (e.g. memory mapped io addr of 0x38, io port 0x18 for the mega128). This is seen clearly when looking at the disassembly of the call:

    set_bits_func_wrong (PORTB, 0xaa);
 10a:   6a ea           ldi     r22, 0xAA       ; 170
 10c:   88 b3           in      r24, 0x18       ; 24
 10e:   0e 94 65 00     call    0xca

so, the function, once called, only sees the value of the port register and knows nothing about which port it came from. At this point, whatever object code is generated for the function by the compiler is irrelevant. The interested reader can examine the full disassembly to see that the function's body is completely fubar. 💀

the second function, set_bits_func_correct, is the glow-up. ✨ It shows how to pass (by reference) the memory mapped address of the io port to the function so that you can read and write to it in the function. Here's the object code generated for the function call:

    set_bits_func_correct (&PORTB, 0x55);
 112:   65 e5           ldi     r22, 0x55       ; 85
 114:   88 e3           ldi     r24, 0x38       ; 56
 116:   90 e0           ldi     r25, 0x00       ; 0
 118:   0e 94 7c 00     call    0xf8

you can clearly see that 0x0038 is correctly passed for the address of the io port. Looking at the disassembled object code for the body of the function, we can see that the function is indeed performing the operation we intended:

void
set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
{
  f8:   fc 01           movw    r30, r24
    *port |= mask;
  fa:   80 81           ld      r24, Z
  fc:   86 2b           or      r24, r22
  fe:   80 83           st      Z, r24
}
 100:   08 95           ret

notice that we are accessing the io port via the LD and ST instructions. 👑

The port parameter must be volatile to avoid a compiler warning.

Note: Because of the nature of the IN and OUT assembly instructions, they can not be used inside the function when passing the port in this way. Readers interested in the details should consult the Instruction Set datasheet.

finally we come to the macro version of the operation. In this contrived example, the macro is the most efficient method with respect to both execution speed and code size:

    set_bits_macro (PORTB, 0xf0);
 11c:   88 b3           in      r24, 0x18       ; 24
 11e:   80 6f           ori     r24, 0xF0       ; 240
 120:   88 bb           out     0x18, r24       ; 24

of course, in a real application, you might be doing a lot more in your function which uses a passed by reference io port address and thus the use of a function over a macro could save you some code space, but still at a cost of execution speed.

care should be taken when such an indirect port access is going to one of the 16-bit IO registers where the order of write access is critical (like some timer registers). All versions of avr-gcc up to 3.3 will generate instructions that use the wrong access order in this situation (since with normal memory operands where the order doesn't matter, this sometimes yields shorter code). ⚠️

See http://mail.nongnu.org/archive/html/avr-libc-dev/2003-01/msg00044.html for a possible workaround.

avr-gcc versions after 3.3 have been fixed in a way where this optimization will be disabled if the respective pointer variable is declared to be volatile, so the correct behaviour for 16-bit IO ports can be forced that way.

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🎀 What registers are used by the C compiler? 🎀

Data types: char is 8 bits, int is 16 bits, long is 32 bits, long long is 64 bits, float and double are 32 bits (this is the only supported floating point format), pointers are 16 bits (function pointers are word addresses, to allow addressing up to 128K program memory space). There is a -mint8 option (see Options for the C compiler avr-gcc) to make int 8 bits, but that is not supported by avr-libc and violates C standards (int must be at least 16 bits). It may be removed in a future release.
Call-used registers (r18-r27, r30-r31): May be allocated by gcc for local data. You may use them freely in assembler subroutines. Calling C subroutines can clobber any of them - the caller is responsible for saving and restoring.

Note: For the AVR_TINY architecture (ATtiny10 and relatives), r18 and r19 are call-saved.

Call-saved registers (r2-r17, r28-r29): May be allocated by gcc for local data. Calling C subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. r29:r28 (Y pointer) is used as a frame pointer (points to local data on stack) if necessary. The requirement for the callee to save/preserve the contents of these registers even applies in situations where the compiler assigns them for argument passing.

Note: For the AVR_TINY architecture (ATtiny10 and relatives), r18 and r19 are call-saved, and r0 through r16 do not exist.

Fixed registers (r0, r1): Never allocated by gcc for local data, but often used for fixed purposes:
- r0 - temporary register, can be clobbered by any C code (except interrupt handlers which save it), may be used to remember something for a while within one piece of assembler code
- r1 - assumed to be always zero in any C code, may be used to remember something for a while within one piece of assembler code, but must then be cleared after use (clr r1). This includes any use of the [f]mul[s[u]] instructions, which return their result in r1:r0. Interrupt handlers save and clear r1 on entry, and restore r1 on exit (in case it was non-zero).

Note: For the AVR_TINY architecture (ATtiny10 and relatives), r16 is used as temporary register, and r17 as zero.

Function call conventions: Arguments - allocated left to right, r25 to r8. All arguments are aligned to start in even-numbered registers (odd-sized arguments, including char, have one free register above them). This allows making better use of the movw instruction on the enhanced core.

If too many, those that don't fit are passed on the stack.

Return values: 8-bit in r24 (not r25!), 16-bit in r25:r24, up to 32 bits in r22-r25, up to 64 bits in r18-r25. 8-bit return values are zero/sign-extended to 16 bits by the called function (unsigned char is more efficient than signed char - just clr r25). Arguments to functions with variable argument lists (printf etc.) are all passed on stack, and char is extended to int.

Warning: There was no such alignment before 2000-07-01, including the old patches for gcc-2.95.2. Check your old assembler subroutines, and adjust them accordingly.

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How do I put an array of strings completely in ROM? 🎀

okay bestie, sometimes you need an array of strings that is literally never going to be modified. and in that case?? you do NOT want to be burning your precious RAM on constant strings. so the most obvious thing to do is… also the wrong thing. it's this:

#include <avr/pgmspace.h>

PGM_P array[2] PROGMEM = {
    "Foo",
    "Bar"
};

int main (void)
{
    char buf[32];
    strcpy_P (buf, array[1]);
    return 0;
}

and the result is not what you want, bestie. what you actually end up with is the array sitting in ROM while the individual strings are chilling in RAM (in the .data section). it's giving half a job. 😭

to work around this you need to do something like this instead:

#include <avr/pgmspace.h>

const char foo[] PROGMEM = "Foo";
const char bar[] PROGMEM = "Bar";

PGM_P array[2] PROGMEM = {
    foo,
    bar
};

int main (void)
{
    char buf[32];
    PGM_P p;
    int i;

    memcpy_P(&p, &array[i], sizeof(PGM_P));
    strcpy_P(buf, p);
    return 0;
}

and if we look at the disassembly of the resulting object file we can see that array is in flash like this:

00000026 <array>:
 26:   2e 00           .word   0x002e  ; ????
 28:   2a 00           .word   0x002a  ; ????

0000002a <bar>:
 2a:   42 61 72 00                                         Bar.

0000002e <foo>:
 2e:   46 6f 6f 00                                         Foo.

so like, for the record:

foo is at addr 0x002e.
bar is at addr 0x002a.
array is at addr 0x0026.

then in main we see this:

  memcpy_P(&p, &array[i], sizeof(PGM_P));
70:   66 0f           add     r22, r22
72:   77 1f           adc     r23, r23
74:   6a 5d           subi    r22, 0xDA       ; 218
76:   7f 4f           sbci    r23, 0xFF       ; 255
78:   42 e0           ldi     r20, 0x02       ; 2
7a:   50 e0           ldi     r21, 0x00       ; 0
7c:   ce 01           movw    r24, r28
7e:   81 96           adiw    r24, 0x21       ; 33
80:   08 d0           rcall   .+16            ; 0x92

so this code reads the pointer to the desired string from the ROM table array into a register pair. ✨

the value of i (in r22:r23) gets doubled to accommodate for the word offset required to access array[], and then the address of array (0x26) is added, by subtracting the negated address (0xffda). the address of variable p is computed by adding its offset within the stack frame (33) to the Y pointer register, and memcpy_P is called.

  strcpy_P(buf, p);
82:   69 a1           ldd     r22, Y+33       ; 0x21
84:   7a a1           ldd     r23, Y+34       ; 0x22
86:   ce 01           movw    r24, r28
88:   01 96           adiw    r24, 0x01       ; 1
8a:   0c d0           rcall   .+24            ; 0xa4

and this is what finally copies the ROM string into the local buffer buf. 💅

variable p (located at Y+33) gets read, and passed together with the address of buf (Y+1) to strcpy_P. this will copy the string from ROM to buf.

heads up bestie: when you're using a compile-time constant index, omitting the first step (reading the pointer from ROM via memcpy_P) usually remains unnoticed, since the compiler would then optimize the code for accessing array at compile-time. sneaky. 👀

back to the FAQ Index 🫶

How to use external RAM? 🌈

okay so, plot twist: there is no universal answer to this one. it 100% depends on what the external RAM is actually going to be used for. main character energy but make it situational.

basically, the bit SRE (SRAM enable) in the MCUCR register needs to be set in order to enable the external memory interface. depending on the device you're using, and the application details, further registers affecting the external memory operation like XMCRA and XMCRB, and/or further bits in MCUCR might be configured. refer to the datasheet for details. 📖

if the external RAM is going to be used to store the variables from the C program (i. e., the .data and/or .bss segment) in that memory area, it is essential to set up the external memory interface early during the device initialization so the initialization of these variables will take place. refer to How to modify MCUCR or WDTCR early? for a description of how to do this using a few lines of assembler code, or to the chapter about memory sections for an example written in C. 🦋

the explanation of malloc() contains a whole discussion about the use of internal RAM vs. external RAM, in particular with respect to the various possible locations of the heap (the area reserved for malloc()). it also explains the linker command-line options that are required to move the memory regions away from their respective standard locations in internal RAM. so if that's your question, that's your answer, iykyk.

finally, if the application simply wants to use the additional RAM for private data storage kept outside the domain of the C compiler (e. g. through a char * variable initialized directly to a particular address), it would be sufficient to defer the initialization of the external RAM interface to the beginning of main(), so no tweaking of the .init3 section is necessary. the same applies if only the heap is going to be located there, since the application start-up code does not affect the heap. 🎀

it is not recommended to locate the stack in external RAM. in general, accessing external RAM is slower than internal RAM, and errata of some AVR devices even prevent this configuration from working properly at all. so like… don't. 🚫

back to the FAQ Index 🫶

Which `-O` flag to use? 👑

there's a common misconception that larger numbers behind the -O option might automatically cause 'better' optimization. first of all bestie, there's no universal definition for 'better' — optimization is often a speed vs. code size trade off. see the detailed discussion for which option affects which part of the code generation. ✨

a test case was run on an ATmega128 to judge the effect of compiling the library itself using different optimization levels. the following results are iconic, honestly:

-O3 — size of .text: 6898, time for test #1: 903 µs, time for test #2: 19.7 ms
-O2 — size of .text: 6666, time for test #1: 972 µs, time for test #2: 20.1 ms
-Os — size of .text: 6618, time for test #1: 955 µs, time for test #2: 20.1 ms
-Os -mcall-prologues — size of .text: 6474, time for test #1: 972 µs, time for test #2: 20.1 ms

the test case consisted of around 2 KB of strings to sort. test #1 used qsort() using the standard library strcmp(), and test #2 used a function that sorted the strings by their size (so it had two calls to strlen() per invocation). 🌸

when comparing the resulting code size, it should be noted that a floating point version of fvprintf() was linked into the binary (in order to print out the time elapsed) which is entirely not affected by the different optimization levels, and added about 2.5 KB to the code. so account for that.

(the difference between 955 µs and 972 µs was just a single timer-tick, so take this with a grain of salt.) 🧂

so generally, it seems -Os -mcall-prologues is the most universal 'best' optimization level. only applications that need to get the last few percent of speed benefit from using -O3. periodt. 💅

back to the FAQ Index 🫶

How do I relocate code to a fixed address? 📍

first, the code should be put into a new named section. this is done with a section attribute:

__attribute__ ((section (".bootloader")))

in this example, .bootloader is the name of the new section. this attribute needs to be placed after the prototype of any function to force the function into the new section. 💖

void boot(void) __attribute__ ((section (".bootloader")));

to relocate the section to a fixed address, the linker flag --section-start is used. this option can be passed to the linker using the -Wl compiler option:

-Wl,--section-start=.bootloader=0x1E000

the name after section-start is the name of the section to be relocated. the number after the section name is the beginning address of the named section. easy as that, bestie. ✨

back to the FAQ Index 🫶

My UART is generating nonsense! My ATmega128 keeps crashing! Port F is completely broken! 💀

okay so certain odd problems arise out of the situation that the AVR devices as shipped by Atmel often come with a default fuse bit configuration that doesn't match the user's expectations. so before you spiral, here is a list of things to care for:

all devices that have an internal RC oscillator ship with the fuse enabled that causes the device to run off this oscillator, instead of an external crystal. this often remains unnoticed until the first attempt is made to use something critical in timing, like UART communication. 🙈
the ATmega128 ships with the fuse enabled that turns this device into ATmega103 compatibility mode. this means that some ports are not fully usable, and in particular that the internal SRAM is located at lower addresses. since by default, the stack is located at the top of internal SRAM, a program compiled for an ATmega128 running on such a device will immediately crash upon the first function call (or rather, upon the first function return). 😭
devices with a JTAG interface have the JTAGEN fuse programmed by default. this will make the respective port pins that are used for the JTAG interface unavailable for regular IO. 🎀

back to the FAQ Index 🫶

💾 Why do all my `'foo...bar'` strings eat up the SRAM?

okay bestie so by default? every single string gets treated exactly like all your other initialized variables. Which means it lives in RAM (even though the compiler might yell at you when it catches you trying to write to those RAM locations 😭), and it also eats up the same amount of flash ROM, because the startup code needs to copy it in there so it can be initialized to the actual string. The compiler can merge multiple identical strings into one single copy — but obvi only within one compilation unit (i.e., a single C source file). No cross-file shenanigans. ✨

The upside of all this? Any string literal is a valid argument to any C function that expects a const char * argument. Convenient! 🌸

But of course, this is gonna burn through a lot of SRAM. In Program Space String Utilities, there's a method described for how you can move constant data like this out to flash ROM. BUT — and this is the catch, bestie — a constant string living in flash ROM is no longer a valid argument to pass to a function expecting a const char *-type string, because the AVR processor needs the special instruction LPM to access these strings. So you need separate functions that take this into account. A bunch of the standard C library functions have equivalents available where one of the string arguments can be located in flash ROM. And your own private functions in your applications need to handle this too. 😤

For example, this here is how you'd implement simple debugging messages that get sent through a UART:

#include <inttypes.h>
#include <avr/io.h>
#include <avr/pgmspace.h>

int
uart_putchar(char c)
{
  if (c == '0)    ');
    uart_putchar('
  loop_until_bit_is_set(USR, UDRE);
  UDR = c;
  return 0; /* so it could be used for fdevopen(), too */
}

void
debug_P(const char *addr)
{
  char c;

  while ((c = pgm_read_byte(addr++)))
    uart_putchar(c);
}

int
main(void)
{
  ioinit(); /* initialize UART, ... */
  debug_P(PSTR("foo was here0));
  return 0;
}

Note: 🎀 By convention, the suffix _P on the function name is used as an indication that this function is gonna accept a 'program-space string'. Also peep the use of the PSTR() macro. iykyk 💅

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🤔 Why does the compiler compile an 8-bit operation that uses bitwise operators into a 16-bit operation in assembly?

fr fr, bitwise operations in Standard C will automatically promote their operands to an int, which is (by default) 16 bits in avr-gcc. So yeah. It's giving ✨type promotion✨.

To work around this, use typecasts on the operands — including literals — to declare that the values are meant to be 8-bit operands. 🫶

This is especially important when you're clearing a bit:

var &= ~mask;  /* wrong way! */

The bitwise 'not' operator (~) will also promote the value in mask to an int. So to keep it an 8-bit value, typecast before the 'not' operator:

var &= (unsigned char)~mask;

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🔍 How to detect RAM memory and variable overlap problems?

You can simply run avr-nm on your output (ELF) file. Run it with the -n option and it'll sort the symbols numerically (by default, they're sorted alphabetically, which is lowkey useless here 💀).

Look for the symbol _end — that's the first address in RAM that is not allocated by a variable. (avr-gcc internally adds 0x800000 to all data/bss variable addresses, so please just ignore this offset, bestie.) Then the run-time initialization code initializes the stack pointer (by default) to point to the last available address in (internal) SRAM. So the region between _end and the end of SRAM is what's available for the stack. 👑

(If your application uses malloc(), which e.g. also can happen inside printf(), the heap for dynamic memory is also located there. See Memory Areas and Using malloc().)

The amount of stack required for your application cannot be determined that easily. Like, for example, if you recursively call a function and forget to break that recursion, the amount of stack required is infinite. :-) 😭 iconic. You can look at the generated assembler code (avr-gcc ... -S) — there's a comment in each generated assembler file that tells you the frame size for each generated function. That's the amount of stack required for this function, and you have to add up that for all functions where you know the calls could be nested. ✨

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🧚 Is it really impossible to program the ATtinyXX in C?

So like, some small AVRs are not directly supported by the C compiler since they do not have a RAM-based stack (and some do not even have RAM at all 💀), but it is possible anyway to use the general-purpose registers as a RAM replacement, since they are mapped into the data memory region. slay.

Bruce D. Lightner wrote an excellent description of how to do this, and offers it together with a toolkit on his web page:

http://lightner.net/avr/ATtinyAvrGcc.html

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🕐 What is this 'clock skew detected' message?

It's a known problem of the MS-DOS FAT file system, bestie. Since the FAT file system has only a granularity of 2 seconds for maintaining a file's timestamp, and it seems that some MS-DOS derivative (Win9x) perhaps rounds up the current time to the next second when calculating the timestamp of an updated file in case the current time cannot be represented in FAT's terms, this causes a situation where make sees a 'file coming from the future'. 🔮

Since all make decisions are based on file timestamps and their dependencies, make warns about this situation.

Solution: don't use inferior file systems / operating systems. 🙈 Neither Unix file systems nor HPFS (aka NTFS) experience that problem.

Workaround: after saving the file, wait a second before starting make. Or simply ignore the warning. If you are paranoid, execute a make clean all to make sure everything gets rebuilt.

In networked environments where the files are accessed from a file server, this message can also happen if the file server's clock differs too much from the network client's clock. In this case, the solution is to use a proper time keeping protocol on both systems, like NTP. As a workaround, synchronize the client's clock frequently with the server's clock. 🌈

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⚡ Why are (many) interrupt flags cleared by writing a logical 1?

Usually, each interrupt has its own interrupt flag bit in some control register, indicating the specified interrupt condition has been met by representing a logical 1 in the respective bit position. When you're working with interrupt handlers, this interrupt flag bit usually gets cleared automatically in the course of processing the interrupt — sometimes by just calling the handler at all, sometimes (e.g. for the U[S]ART) by reading a particular hardware register that will normally happen anyway when processing the interrupt.

From the hardware's point of view, an interrupt is asserted as long as the respective bit is set, while global interrupts are enabled. So it is essential to have the bit cleared before interrupts get re-enabled again (which usually happens when returning from an interrupt handler). 👑

Only few subsystems require an explicit action to clear the interrupt request when using interrupt handlers. (The notable exception is the TWI interface, where clearing the interrupt indicates to proceed with the TWI bus hardware handshake, so it's never done automatically.)

However — and here's the plot twist 🎀 — if no normal interrupt handlers are to be used, or in order to make extra sure any pending interrupt gets cleared before re-activating global interrupts (e.g. an external edge-triggered one), it can be necessary to explicitly clear the respective hardware interrupt bit by software. This is usually done by writing a logical 1 into this bit position. Which seems illogical at first, right? The bit position already carries a logical 1 when you read it, so why does writing a logical 1 to it clear the interrupt bit?? 🤔

The solution is simple: writing a logical 1 to it requires only a single OUT instruction, and it is clear that only this single interrupt request bit will be cleared. There is no need to perform a read-modify-write cycle (like, an SBI instruction), since all bits in these control registers are interrupt bits, and writing a logical 0 to the remaining bits (as it is done by the simple OUT instruction) will not alter them, so there is no risk of any race condition that might accidentally clear another interrupt request bit. 🔥

So instead of writing

TIFR |= _BV(TOV0); /* wrong! */

simply use

TIFR = _BV(TOV0);

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🔥 Why have 'programmed' fuses the bit value 0?

Basically, fuses are just a bit in a special EEPROM area. For technical reasons, erased E[E]PROM cells have all bits set to the value 1, so unprogrammed fuses also have a logical 1. Conversely, programmed fuse cells read out as bit value 0. Wild, right? 💅

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🛠️ Which AVR-specific assembler operators are available?

See Pseudo-ops and operators.

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🧵 Why are interrupts re-enabled in the middle of writing the stack pointer?

When setting up space for local variables on the stack, the compiler generates code like this:

/* prologue: frame size=20 */
    push r28
    push r29
    in r28,__SP_L__
    in r29,__SP_H__
    sbiw r28,20
    in __tmp_reg__,__SREG__
    cli
    out __SP_H__,r29
    out __SREG__,__tmp_reg__
    out __SP_L__,r28
/* prologue end (size=10) */

It reads the current stack pointer value, decrements it by the required amount of bytes, then disables interrupts, writes back the high part of the stack pointer, writes back the saved SREG (which will eventually re-enable interrupts if they have been enabled before), and finally writes the low part of the stack pointer.

At the first glance, there's a race between restoring SREG and writing SPL. 😱 BUT — after enabling interrupts (either explicitly by setting the I flag, or by restoring it as part of the entire SREG), the AVR hardware executes (at least) the next instruction still with interrupts disabled, so the write to SPL is guaranteed to be executed with interrupts disabled still. So the emitted sequence ensures interrupts will be disabled only for the minimum time required to guarantee the integrity of this operation. queen behavior honestly 👑

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📜 Why are there five different linker scripts?

From a comment in the source code:

Which one of the five linker script files is actually used depends on command line options given to ld.

A .x script file is the default script
A .xr script is for linking without relocation (-r flag)
A .xu script is like .xr but do create constructors (-Ur flag)
A .xn script is for linking with -n flag (mix text and data on same page).
A .xbn script is for linking with -N flag (mix text and data on same page).

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🫶 How to add a raw binary image to linker output? 🫶

okay so bad news first: the GNU linker avr-ld literally cannot handle binary data directly. it's not a skill issue, it just can't. but there's a companion tool called avr-objcopy, and if that name feels familiar it's because you've already met it on the output side — it's the thing that extracts the contents of the linked ELF file into an Intel Hex load file. 💅

turns out avr-objcopy can also create a relocatable object file from arbitrary binary input, like so:

avr-objcopy -I binary -O elf32-avr foo.bin foo.o

this makes a file named foo.o with the contents of foo.bin inside it. the contents default to section .data, and two symbols get created, named _binary_foo_bin_start and _binary_foo_bin_end. you can refer to those symbols inside a C source to access the data. ✨

now if the goal is to have those data go to flash ROM (basically the same vibe as having used the PROGMEM attribute in C source code), the sections have to be renamed while copying, and it's also useful to set the section flags:

avr-objcopy --rename-section .data=.progmem.data,contents,alloc,load,readonly,data -I binary -O elf32-avr foo.bin foo.o

ngl, all of this can be conveniently wired into a Makefile, so whenever foo.bin changes, it triggers the recreation of foo.o, and a subsequent relink of the final ELF file. automation slay 🤖

below are two Makefile fragments that provide rules to convert a .txt file to an object file, and to convert a .bin file to an object file:

$(OBJDIR)/%.o : %.txt
    @echo Converting $<
    @cp $(<) $(*).tmp
    @echo -n 0 | tr 0 ' 00' >> $(*).tmp
    @$(OBJCOPY) -I binary -O elf32-avr     --rename-section .data=.progmem.data,contents,alloc,load,readonly,data     --redefine-sym _binary_$*_tmp_start=$*     --redefine-sym _binary_$*_tmp_end=$*_end     --redefine-sym _binary_$*_tmp_size=$*_size_sym     $(*).tmp $(@)
    @echo "extern const char" $(*)"[] PROGMEM;" > $(*).h
    @echo "extern const char" $(*)_end"[] PROGMEM;" >> $(*).h
    @echo "extern const char" $(*)_size_sym"[];" >> $(*).h
    @echo "#define $(*)_size ((int)$(*)_size_sym)" >> $(*).h
    @rm $(*).tmp

$(OBJDIR)/%.o : %.bin
    @echo Converting $<
    @$(OBJCOPY) -I binary -O elf32-avr     --rename-section .data=.progmem.data,contents,alloc,load,readonly,data     --redefine-sym _binary_$*_bin_start=$*     --redefine-sym _binary_$*_bin_end=$*_end     --redefine-sym _binary_$*_bin_size=$*_size_sym     $(<) $(@)
    @echo "extern const char" $(*)"[] PROGMEM;" > $(*).h
    @echo "extern const char" $(*)_end"[] PROGMEM;" >> $(*).h
    @echo "extern const char" $(*)_size_sym"[];" >> $(*).h
    @echo "#define $(*)_size ((int)$(*)_size_sym)" >> $(*).h

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🦋 How do I perform a software reset of the AVR? 🦋

so the canonical way to do a software reset of non-XMega AVR's is to use the watchdog timer. you enable the watchdog timer to the shortest timeout setting, then go into an infinite, do-nothing loop. the watchdog will then reset the processor. periodt. 🐕

XMega parts are built different 💅 — they have a specific bit RST_SWRST_bm in the RST.CTRL register that generates a hardware reset. RST_SWRST_bm is protected by the XMega Configuration Change Protection system.

now why is using the watchdog timer or RST_SWRST_bm preferable over jumping to the reset vector? because when the watchdog or RST_SWRST_bm resets the AVR, the registers get reset to their known, default settings. jumping to the reset vector, on the other hand, leaves the registers in their previous state, which is generally not a good idea. 🙈

CAUTION! Older AVRs will have the watchdog timer disabled on a reset. For these older AVRs, doing a soft reset by enabling the watchdog is easy, as the watchdog will then be disabled after the reset. On newer AVRs, once the watchdog is enabled, then it stays enabled, even after a reset! For these newer AVRs a function needs to be added to the .init3 section (i.e. during the startup code, before main()) to disable the watchdog early enough so it does not continually reset the AVR. 😭

here's some example code that creates a macro that can be called to perform a soft reset:

#include <avr/wdt.h>

...

#define soft_reset()        do                          {                               wdt_enable(WDTO_15MS);      for(;;)                     {                           }                       } while(0)

for newer AVRs (such as the ATmega1281) also add this function to your code to then disable the watchdog after a reset (e.g., after a soft reset):

#include <avr/wdt.h>

...

// Function Pototype
void wdt_init(void) __attribute__((naked)) __attribute__((section(".init3")));

...

// Function Implementation
void wdt_init(void)
{
    MCUSR = 0;
    wdt_disable();

    return;
}

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💀 I am using floating point math. Why is the compiled code so big? Why does my code not work? 💀

bestie. you are not linking in the math library from AVR-LibC. that's it, that's the whole tea. 🍵

GCC has a library that is used for floating point operations, but it is not optimized for the AVR, and so it generates big code, or it could be incorrect. and the wild part? this can happen even when you are not using any floating point math functions from the Standard C library — you're just doing floating point math operations and boom, cursed code. 💀

when you link in the math library from AVR-LibC, those routines get replaced by hand-optimized AVR assembly and it produces much smaller code. icon behavior honestly. 👑

see I get 'undefined reference to...' for functions like 'sin()' for more details on how to link in the math library.

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🧚 What pitfalls exist when writing reentrant code? 🧚

okay so first, the definition: reentrant code means the ability for a piece of code to be called simultaneously from two or more threads. attention to re-enterability is needed when using a multi-tasking operating system, or when using interrupts — since an interrupt is really a temporary thread. 👀

the code generated natively by gcc is reentrant. good for her. but only some of the libraries in avr-libc are explicitly reentrant, and some are known not to be. in general, any library call that reads and writes global variables (including I/O registers) is not reentrant. why? because more than one thread could read or write the same storage at the same time, unaware that other threads are doing the same, and create inconsistent and/or erroneous results. 🫠

a library call that is known not to be reentrant will work if it is used only within one thread and no other thread makes use of a library call that shares common storage with it. iykyk. ✨

below is a table of library calls with known issues.

rand(), random() — Uses global variables to keep state information.
- Workaround/Alternative: Use special reentrant versions: rand_r(), random_r().
strtod(), strtol(), strtoul() — Uses the global variable errno to return success/failure.
- Workaround/Alternative: Ignore errno, or protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. Or use sccanf() or sccanf_P() if possible.
malloc(), realloc(), calloc(), free() — Uses the stack pointer and global variables to allocate and free memory.
- Workaround/Alternative: Protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. If using an OS, use the OS provided memory allocator since the OS is likely modifying the stack pointer anyway.
fdevopen(), fclose() — Uses calloc() and free().
- Workaround/Alternative: Protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. Or use fdev_setup_stream() or FDEV_SETUP_STREAM().
- Note: fclose() will only call free() if the stream has been opened with fdevopen().
eeprom_*(), boot_*() — Accesses I/O registers.
- Workaround/Alternative: Protect calls with cli()/sei(), ATOMIC_BLOCK(), or use OS locking.
pgm_*_far() — Accesses I/O register RAMPZ.
- Workaround/Alternative: Starting with GCC 4.3, RAMPZ is automatically saved for ISRs, so nothing further is needed if only using interrupts.
- Some OSes may automatically preserve RAMPZ during context switching. Check the OS documentation before assuming it does.
- Otherwise, protect calls with cli()/sei(), ATOMIC_BLOCK(), or use explicit OS locking.
printf(), printf_P(), vprintf(), vprintf_P(), puts(), puts_P() — Alters flags and character count in global FILE stdout.
- Workaround/Alternative: Use only in one thread. Or if returned character count is unimportant, do not use the *_P versions.
- Note: Formatting to a string output, e.g. sprintf(), sprintf_P(), snprintf(), snprintf_P(), vsprintf(), vsprintf_P(), vsnprintf(), vsnprintf_P(), is thread safe. The formatted string could then be followed by an fwrite() which simply calls the lower layer to send the string.
fprintf(), fprintf_P(), vfprintf(), vfprintf_P(), fputs(), fputs_P() — Alters flags and character count in the FILE argument. Problems can occur if a global FILE is used from multiple threads.
- Workaround/Alternative: Assign each thread its own FILE for output. Or if returned character count is unimportant, do not use the *_P versions.
assert() — Contains an embedded fprintf(). See above for fprintf(). See above for fprintf().
clearerr() — Alters flags in the FILE argument.
- Workaround/Alternative: Assign each thread its own FILE for output.
getchar(), gets() — Alters flags, character count, and unget buffer in global FILE stdin.
- Workaround/Alternative: Use only in one thread. ***
fgetc(), ungetc(), fgets(), scanf(), scanf_P(), fscanf(), fscanf_P(), vscanf(), vfscanf(), vfscanf_P(), fread() — Alters flags, character count, and unget buffer in the FILE argument.
- Workaround/Alternative: Assign each thread its own FILE for input. ***
- Note: Scanning from a string, e.g. sscanf() and sscanf_P(), are thread safe.

Note:

It's not clear one would ever want to do character input simultaneously from more than one thread anyway, but these entries are included for completeness.

An effort will be made to keep this table up to date if any new issues are discovered or introduced.

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😭 Why are some addresses of the EEPROM corrupted (usually address zero)?

okay bestie this one is a classic and it hurts every time. there are two big reasons your EEPROM goes feral: either you're writing to the EEPROM beyond the datasheet endurance specification, or you're resetting the AVR while an EEPROM write is in progress. that's it, that's the whole mystery. 💀

EEPROM writes can take up to tens of milliseconds to finish. so that the CPU isn't just sitting there tied up that whole time, an internal state-machine handles the EEPROM write requests for you. the way it works: the EEPROM state-machine expects to have all of the EEPROM registers set up first, and then an EEPROM write request kicks off the process. once that state-machine has started, changing EEPROM related registers during an EEPROM write is guaranteed to corrupt the EEPROM write process. periodt. the datasheet always shows you the proper way to tell when a write is in progress, so your program knows not to touch those registers. and heads up: the EEPROM state-machine will always complete the write in progress unless power is removed from the device.

and look, as with all EEPROM technology: if power fails during an EEPROM write, the state of the byte being written is undefined. not vibes. undefined.

now here's the messy part 🫠 in older generation AVRs the EEPROM Address Register (EEAR) is initialized to zero on reset — whether that reset came from Brown Out Detect, Watchdog or the Reset Pin. so if an EEPROM write has just started at the time of the reset, the write will be completed… but now at address zero instead of the requested address. and if the reset happens later in the write process, then both the requested address and address zero may end up corrupted. iconic in the worst way.

want to know which AVRs can do this address-zero corruption while a write is in process during a reset? look at the 'initial value' section for the EEPROM Address Register. if EEAR shows the initial value as 0x00 or 0x0000, then address zero — and possibly the one being written — will be corrupted. newer parts show the initial value as 'undefined', and those will not corrupt address zero during a reset (unless it was address zero that was being written, obviously).

last thing, and it's important, so listen up 🎀 EEPROMs have limited write endurance. the datasheet specifies the number of EEPROM writes that are guaranteed to function across the full temperature specification of the AVR, for a given byte. so a read should always be performed before a write, to check whether the value in the EEPROM actually even needs to be written — that way you don't cause unnecessary EEPROM wear. bestie self-care for your memory.

the failure mechanism for an overwritten byte is generally one of 'stuck' bits — i. e. a bit will stay at a one or zero state regardless of the byte written. also, a write followed by a read may return the correct data, but the data will change with the passage of time, due the EEPROM's inability to hold a charge from the excessive write wear. it's giving tired. 😮‍💨

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🐌 Why is my baud rate wrong?

so some AVR datasheets hand you this formula for calculating baud rates:

(F_CPU/(UART_BAUD_RATE*16L)-1)

and unfortunately that formula does not work with all combinations of clock speeds and baud rates, because of integer truncation during the division operator. it's just math being math. 🤷‍♀️

here's the tea: when doing integer division it is usually better to round to the nearest integer, rather than to the lowest. to do this, add 0.5 (i. e. half the value of the denominator) to the numerator before the division, which gives you:

((F_CPU + UART_BAUD_RATE * 8L) / (UART_BAUD_RATE * 16L) - 1)

and slay, this is also the way it is implemented in <util/setbaud.h>: Helper macros for baud rate calculations. 🦋

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✨ On a device with more than 128 KiB of flash, how to make function pointers work?

okay so function pointers beyond the 'magical' 128 KiB barrier(s) on larger devices are supposed to be resolved through so-called trampolines by the linker, so the actual pointers used in the code can remain 16 bits wide. cute, right? 🎀

but for this to actually work, the option -mrelax must be given on the compiler command-line that is used to link the final ELF file. and ngl, older compilers did not implement this option for the AVR — use -Wl,--relax instead. iykyk. 💅

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🚫 Why is assigning ports in a 'chain' a bad idea?

bestie. this one is a trap and it looks SO innocent. suppose a number of IO port registers should get the value 0xff assigned. conveniently, someone implements it like this:

DDRB = DDRD = 0xff;

and according to the rules of the C language, this causes 0xff to be assigned to DDRD, then DDRD is read back, and that value is assigned to DDRB. the compiler stands no chance to optimize the readback away, as an IO port register is declared 'volatile'. so chaining that kind of IO port assignment would better be avoided — use explicit assignments instead:

DDRB = 0xff;
DDRD = 0xff;

and then there's the even worse version of this, e. g. on an ATmega1281:

DDRA = DDRB = DDRC = DDRD = DDRE = DDRF = DDRG = 0xff;

the same happens as outlined above. however 🚨 when reading back register DDRG, this register only implements 6 out of the 8 bits, so the two topmost (unimplemented) bits read back as 0! consequently, all remaining DDRx registers get assigned the value 0x3f, which does not match the intention of the developer in any way. not even a little. 💀

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and that's a wrap on the whole FAQ, bestie!! 🫶 this page was Version 2.1.0 and it was dated Sat Jan 29 2022 — so if you're reading this in the future, hi, the vibes are archived, go check for a newer version. thanks for sticking with me through all seven parts, you absolutely ate. now go write some embedded C that doesn't corrupt address zero. 💖✨🎀🦋🌸

📦 where this page comes from

fedora 43: avr-libc 2.2.1-3.fc43 · BSD-3-Clause
fedora 44: avr-libc 2.2.1-5.fc44 · BSD-3-Clause
opensuse-leap 16.0: avr-libc 2.1.0-bp160.1.33 · BSD-3-Clause
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms

FAQ(3)

💖 FAQ(3) — avr-libc 💖

✨ NAME ✨

🎀 FAQ Index 🎀

🌸 My program doesn't recognize a variable updated within an interrupt routine

🌈 I get 'undefined reference to...' for functions like 'sin()'

🦋 How to permanently bind a variable to a register?

💅 How to modify MCUCR or WDTCR early?

⭐ What is all this _BV() stuff about?

👑 Can I use C++ on the AVR?

✨ Shouldn't I initialize all my variables? ✨

🕰️ Why do some 16-bit timer registers sometimes get trashed? 🕰️

🌈 How do I use a #define'd constant in an asm statement? 🌈

🦋 Why does the PC randomly jump around when single-stepping through my program in avr-gdb? 🦋

🧚 How do I trace an assembler file in avr-gdb? 🧚

🌸 How do I pass an IO port as a parameter to a function? 🌸

🎀 What registers are used by the C compiler? 🎀

How do I put an array of strings completely in ROM? 🎀

How to use external RAM? 🌈

Which -O flag to use? 👑

How do I relocate code to a fixed address? 📍

My UART is generating nonsense! My ATmega128 keeps crashing! Port F is completely broken! 💀

💾 Why do all my 'foo...bar' strings eat up the SRAM?

🤔 Why does the compiler compile an 8-bit operation that uses bitwise operators into a 16-bit operation in assembly?

🔍 How to detect RAM memory and variable overlap problems?

🧚 Is it really impossible to program the ATtinyXX in C?

🕐 What is this 'clock skew detected' message?

⚡ Why are (many) interrupt flags cleared by writing a logical 1?

🔥 Why have 'programmed' fuses the bit value 0?

🛠️ Which AVR-specific assembler operators are available?

🧵 Why are interrupts re-enabled in the middle of writing the stack pointer?

📜 Why are there five different linker scripts?

🫶 How to add a raw binary image to linker output? 🫶

🦋 How do I perform a software reset of the AVR? 🦋

💀 I am using floating point math. Why is the compiled code so big? Why does my code not work? 💀

🧚 What pitfalls exist when writing reentrant code? 🧚

😭 Why are some addresses of the EEPROM corrupted (usually address zero)?

🐌 Why is my baud rate wrong?

✨ On a device with more than 128 KiB of flash, how to make function pointers work?

🚫 Why is assigning ports in a 'chain' a bad idea?

🌈 How do I use a `#define`'d constant in an asm statement? 🌈

Which `-O` flag to use? 👑

💾 Why do all my `'foo...bar'` strings eat up the SRAM?