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I'm writing Arduino UNO (=ATMega328P-PU) programs in assembly to save memory, so I use avra.exe (same as atmel studio's avrasm32) to compile and avrdude to upload, and simple programs like blinking run fine. But now i tried to half-bright a LED with pwm. I checked up my code for errors but I didn't find any, and the LED just full brighs. I checked wiring too. I'm getting struck :c. Here's my pwm.asm code:

.nolist
.include "m328pdef.inc"
.list

.cseg
.org 0x00
     rjmp start
.org 0x34
 start: sbi ddrb, 5 ;pin 13
        sbi portb, 5 ;pin 13 on, just to compare with the PWMed led
        sbi ddrd, 5 ;pin 5 pwm
        ldi r16, 0b00100011 ;fast pwm mode, non inverted pwm at oc0b = pin 5 (right?)
        out tccr0a, r16 ;I'm using Timer0
        ldi r16, 0b00000001 ;no prescaler
        out tccr0b, r16
        ldi r16, 128 ; duty cycle = 50%
        out ocr0b, r16

  loop: rjmp loop
  • Try writing the code in C and see if you get any other output using avr-objdump -S <filename>.elf. From my limited knowledge of assembly, the code looks file. What happens if you connect the led the other way around (VCC--resistor--led--pwm-pin vs. pwm-pin--resistor--led--GND)? Out eyes don't perceive different light levels in a linear scale. – Gerben Jul 7 '16 at 15:59
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I tried your program, after converting it to GNU-as syntax. It works as expected, as seen on the scope. Your problem may be simply that the difference between 50% ON and 100% ON is not very obvious to the eye because of its logarithmic perception.

| improve this answer | |
  • Got it! i tried to set the duty cycle to 16 (6.25% duty cycle) and I noticed a difference. I didn't know about its logarithmic perception. Thanks! – Frazzo Jul 7 '16 at 19:33

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