assembly - How to make PWM pulse with 20% duty cycle in AVR? -


i want generate pwm wave in pwm mode of timer0 in atmega8 figure below:

enter image description here

it has 20% duty cycle can't implemented pwm mode alone. have tried using fast pwm mode in reversed mode , tried check tcnt0 until gets 64h can clear oc0 pin when reaches it.

i wondering if method works correctly when clearing oc0 manually?

and here code:

.def = r16             ;general purpose accumulator  .org $0000  on_reset:     sbi ddrb,3           ;set portb3(oc0) output     ldi a,0b01011011    ;set fast pwm mode     out tccr0,a    ;set prescaler/divider /32         ldi a,32             ;different value     out ocr0,a          ;for compare   main_loop: ploop: in   a,tcnt0        ;compare tcnt0        andi a,0x64h   ;compare tcnt0 64 make 0        brneq ploop        cbi  pinb,3      rjmp main_loop;a check timer loop

pwm cycles in avr start timer overflow (except phase correct , phase/frequency correct pwm) ... in other words have control on duty cycle not on start. in example - whatever reasons - want control start of pwm cycle (at 32h timer overflow) duty cycle (ca. 20% ... 32h of ffh).

so may want consider

  • to use free running timer generating interrupt every 1/ffh
  • use single 8 bit register increment during each call interrupt (it overflow after ff ... that's ok)
  • if register reads 32h or 64h, invert output pin (or set / reset below)
  • initialize program setting output pin 0 before sei

i hijacked 1 of pwm led things on at90usbkey2 - using at90usb1287 processor - , modified acc. above (sorry code bit lengthy :-o ... see below)

edit:

strictly speaking makes sense if have point of synchronisation ... if @ 20% waveform in isolation cannot determine if starts @ timeslot 0x00 or 0x32 ... oscilloscope sync @ rising (or falling) edge of pulse. need provide reference start of pwm frame output of pulse on different pin @ overflow of pwm_sublevel. using pulse other pin sync source oscilloscope start see move of phase start changing pwm_on.

/*  * asmfile1.asm  *  *  created: 19.05.2015 22:01:49  *   author: miked  */  .nolist .include <usb1287def.inc> .list  .def tmp1 = r16 .def tmp2 = r17 .def pwm_sublevel = r18 .def pwm_on = r19 .def pwm_off = r20  .cseg .org 0x0000     jmp v_reset ;            1 $0000 reset               external pin, power-on reset, brown-out reset, watchdog reset, , jtag avr reset     jmp v_noint ;            2 $0002 int0                external interrupt request 0     jmp v_noint ;            3 $0004 int1                external interrupt request 1     jmp v_noint ;            4 $0006 int2                external interrupt request 2     jmp v_noint ;            5 $0008 int3                external interrupt request 3     jmp v_noint ;            6 $000a int4                external interrupt request 4     jmp v_noint ;            7 $000c int5                external interrupt request 5     jmp v_noint ;            8 $000e int6                external interrupt request 6     jmp v_noint ;            9 $0010 int7                external interrupt request 7     jmp v_noint ;           10 $0012 pcint0              pin change interrupt request 0     jmp v_noint ;           11 $0014 usb general         usb general interrupt request     jmp v_noint ;           12 $0016 usb endpoint/pipe   usb endpoint/pipe interrupt request     jmp v_noint ;           13 $0018 wdt                 watchdog time-out interrupt     jmp v_noint ;           14 $001a timer2 compa        timer/counter2 compare match     jmp v_noint ;           15 $001c timer2 compb        timer/counter2 compare match b      jmp v_t2ovf ;           16 $001e timer2 ovf          timer/counter2 overflow      jmp v_noint ;           17 $0020 timer1 capt         timer/counter1 capture event     jmp v_noint ;           18 $0022 timer1 compa        timer/counter1 compare match     jmp v_noint ;           19 $0024 timer1 compb        timer/counter1 compare match b     jmp v_noint ;           20 $0026 timer1 compc        timer/counter1 compare match c     jmp v_noint ;           21 $0028 timer1 ovf          timer/counter1 overflow     jmp v_noint ;           22 $002a timer0 compa        timer/counter0 compare match     jmp v_noint ;           23 $002c timer0 compb        timer/counter0 compare match b     jmp v_noint ;           24 $002e timer0 ovf          timer/counter0 overflow     jmp v_noint ;           25 $0030 spi, stc            spi serial transfer complete     jmp v_noint ;           26 $0032 usart1 rx           usart1 rx complete     jmp v_noint ;           27 $0034 usart1 udre         usart1 data register empty     jmp v_noint ;           28 $0036 usart1tx            usart1 tx complete     jmp v_noint ;           29 $0038 analog comp         analog comparator     jmp v_noint ;           30 $003a adc                 adc conversion complete     jmp v_noint ;           31 $003c ee ready            eeprom ready     jmp v_noint ;           32 $003e timer3 capt         timer/counter3 capture event     jmp v_noint ;           33 $0040 timer3 compa        timer/counter3 compare match     jmp v_noint ;           34 $0042 timer3 compb        timer/counter3 compare match b     jmp v_noint ;           35 $0044 timer3 compc        timer/counter3 compare match c     jmp v_noint ;           36 $0046 timer3 ovf          timer/counter3 overflow     jmp v_noint ;           37 $0048 twi                 2-wire serial interface     jmp v_noint ;           38 $004a spm ready           store program memory ready  v_reset: ; prepare stack ... special write procedure     ldi tmp1, low(ramend)     ldi tmp2, high(ramend)     out spl, tmp1     out sph, tmp2  ; increase clkio 1 8mhz ... special write procedure     ldi tmp1, 0b1000_0000     ldi tmp2, 0b0000_0000     sts clkpr, tmp1     sts clkpr, tmp2  ; initialize variables     clr pwm_sublevel      ldi pwm_on, 0x32     ldi pwm_off, 0x64  ; prepare led ports ; d2-rd on portd4  ; d2-gn on portd5  ; d5-rd on portd7  ; d5-gn on portd6      ldi tmp1, 0b1111_0000     out ddrd, tmp1  ; timer2 (8bit) without prescaler in normal mode ; generates interrupt every 256 cpu clock cycles (32 us) pwm sublevel ; use pwm timer2 has highest priority amongst timers      clr tmp1     sts tccr2a, tmp1                ; normal mode, port pins disabled      ldi tmp1, (1 << cs20)     sts tccr2b, tmp1                ; internal clock, no prescaler      ldi tmp1, (1 << toie2)      sts timsk2, tmp1                ; overflow interrupt enable      sei                             ; set general interupt enable flag  main:     rjmp main  v_t2ovf: ; fires every 32      inc pwm_sublevel                  ; overflows every 8.192 ms, f=122.07... hz     cp  pwm_sublevel, pwm_on     breq go_hi     cp  pwm_sublevel, pwm_off     breq go_lo     reti go_hi:     sbi portd, portd4     reti go_lo:     cbi portd, portd4     reti  v_noint: ; fire error led ... if here wrong     sbi pind, pind7                 ; invert output writing 1 input bit in output mode     reti

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