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I want to make a piezo buffer beeping with assembly code, but i haven't managed to yet. I have an Arduino UNO board (ATMega328p) and using avra and avrdude to build and load. The buzzer just stay silent. Here's my code:

.nolist
.include "m328pdef.inc"
.list

.equ _a = 141 ;(16000000 / 256) / 440(frequency of A) - 1

.cseg
  ;wgm02..0 = 7 (fast pwm, top = ocr0a)
  ;cs02..0 = 4 (N=256)
  ;com0b1..0 = 2 (clear on compare match, set at bottom)
  ldi r16, 0b00100011
  sts tccr0a, r16 ;need out!
  ldi r16, 0b00001100
  sts tccr0b, r16 ;need out!
  ;ocr0a = _a to obtain 440Hz
  ldi r16, 141
  out ocr0a, r16
  ;ocr0b = ocr0a/2 to obtain a duty cycle of 50%
  ldi r16, 71
  out ocr0b, r16
  ;enable output at D5 aka oc0b aka Arduino pin 5
  sbi ddrd, 5
  loop:
  rjmp loop

Wiring is good and the buzzer too, since it works with a C sketch

EDIT: I found my answer by myself (with the help of answerers). I was accessing IO ports (tccr0a and tccr0b) with sts, while i had to use out. (tccr0a and tccr0b are IO ports defined in "m328pdef.inc" as IO addresses, not as memory addresses)

  • A common way to gain more insight in these sort of cases is to: * compile with optimizations turned off: -O0 * tell gcc to preserve intermediate artifacts (the assembly file generated: -S or --save-temps) – Igor Stoppa Sep 6 '16 at 20:52
  • I don't use gcc. I use avra – Frazzo Sep 6 '16 at 21:04
  • You said that the buzzer works with a C sketch. How did you compile it? – Igor Stoppa Sep 6 '16 at 21:10
  • For compiling C sketches i use Arduino IDE, that uses gcc i think, but for asm compiling i use avra, the AVRStudio assembler – Frazzo Sep 6 '16 at 21:12
  • 1
    Right. My advice was to observe the intermediate file produced by gcc (you can tell it to preserve the assembly sources) and use them as reference for debugging your own asm program. – Igor Stoppa Sep 6 '16 at 21:15
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I translated you program to GNU as syntax, and it works as expected: I get a 440 Hz square wave on digital pin 5. Here is the translated program:

#include <avr/io.h>

; I'm too lazy to always type the full name of these macros
#define io(reg) _SFR_IO_ADDR(reg)
#define mem(reg) _SFR_MEM_ADDR(reg)

_a = 141 ;(16000000 / 256) / 440(frequency of A) - 1

  ;wgm02..0 = 7 (fast pwm, top = ocr0a)
  ;cs02..0 = 4 (N=256)
  ;com0b1..0 = 2 (clear on compare match, set at bottom)
  ldi r16, 0b00100011
  sts mem(TCCR0A), r16
  ldi r16, 0b00001100
  sts mem(TCCR0B), r16
  ;ocr0a = _a to obtain 440Hz
  ldi r16, 141
  out io(OCR0A), r16
  ;ocr0b = ocr0a/2 to obtain a duty cycle of 50%
  ldi r16, 71
  out io(OCR0B), r16
  ;enable output at D5 aka oc0b aka Arduino pin 5
  sbi io(DDRD), 5
  loop:
  rjmp loop

As shown in my comment to your question, I suspect the problem is a misunderstanding of the meaning of things like tccr0a and ocr0a. In the translated version above, I admit I do not know what TCCR0A and co. mean. And I need not to: I can treat these as opaque identifiers, as the avr-libc recommends to always use them with the macros _SFR_IO_ADDR() and _SFR_MEM_ADDR(), which provide the I/O address and the data address of the register respectively.

I do not know how to use avra, so I cannot tell you how the get the I/O or data address of a register. But there certainly must be a way. You should always use the data address with sts and the I/O address with out and sti.

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