SimulIDE_1.1.0-R250918_Lin64 atmega8 no PWM on PB3 (OC2)

#1 · October 21, 2025, 8:37 pm

Quote from unbreakmyheart on October 21, 2025, 8:37 pm

Good day, so bear metal MCU (AtMega8-16PU) outputs PWM on a PB3 pin just fine but i cant get simulator to do this.



#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>

static inline void timer2_init_pwm_pc_prescale8_noninv(void) {
    /* 1) Make Timer2 synchronous and wait for any async updates to finish */
    ASSR &= ~(1 << AS2);  // ensure CPU-clocked Timer2
    /* If AS2 was set before, writes to TCCR2/OCR2/TCNT2 need busy-wait: */
    while (ASSR & ((1 << TCR2UB) | (1 << OCR2UB) | (1 << TCN2UB))) { /* wait */
    }

    /* 2) Disable SPI so PB3 (MOSI/OC2) is free */
    SPCR = 0;
    SPSR = 0;

    /* 3) PB3 as output, start low */
    DDRB |= (1 << PB3);
    PORTB &= ~(1 << PB3);

    /* 4) Program Timer2: phase-correct 8-bit (WGM20=1), prescaler=8 (CS21=1) */
    TCCR2 = 0;
    TCNT2 = 0;
    OCR2 = 128;                                      // 50% duty to start
    while (ASSR & ((1 << OCR2UB) | (1 << TCN2UB))) { /* (paranoia) wait */
    }

    TCCR2 = (1 << WGM20) | (1 << CS21);  // phase-correct, clk/8
    while (ASSR & (1 << TCR2UB)) {       /* wait if needed */
    }

    /* 5) Non-inverting on C (COM21=1, COM20=0) */
    TCCR2 &= ~(1 << COM20);
    TCCR2 |= (1 << COM21);
}

int main(void) {
    timer2_init_pwm_pc_prescale8_noninv();

    /* Steady PWM forever: ~8 MHz / (8 * 510) ≈ 1.96 kHz */
    /* (If you want ~7.8 kHz, use prescaler=1: CS20=1, WGM20=1) */
    while (1) {
        // do nothing — PWM runs continuously
        // (Optional: blink another pin to show the MCU is alive)
        _delay_ms(1000);
    }
}


main.hex:



:1000000012C019C018C017C016C015C014C013C044
:1000100012C011C010C00FC00EC00DC00CC00BC06C
:100020000AC009C008C011241FBECFE5D4E0DEBF5E
:10003000CDBF02D029C0E4CF82B5877F82BD82B513
:1000400087708111FCCF1DB81EB8BB9AC39815BC30
:1000500014BC80E883BD82B586708111FCCF82E438
:1000600085BD02B400FCFDCF85B58F7E85BD85B50D
:10007000806285BD2FEF89E698E121508040904055
:0C008000E1F700C00000F6CFF894FFCFBD
:00000001FF

Please help you are my last hope!)

Good day, so bear metal MCU (AtMega8-16PU) outputs PWM on a PB3 pin just fine but i cant get simulator to do this.



#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>

static inline void timer2_init_pwm_pc_prescale8_noninv(void) {
    /* 1) Make Timer2 synchronous and wait for any async updates to finish */
    ASSR &= ~(1 << AS2);  // ensure CPU-clocked Timer2
    /* If AS2 was set before, writes to TCCR2/OCR2/TCNT2 need busy-wait: */
    while (ASSR & ((1 << TCR2UB) | (1 << OCR2UB) | (1 << TCN2UB))) { /* wait */
    }

    /* 2) Disable SPI so PB3 (MOSI/OC2) is free */
    SPCR = 0;
    SPSR = 0;

    /* 3) PB3 as output, start low */
    DDRB |= (1 << PB3);
    PORTB &= ~(1 << PB3);

    /* 4) Program Timer2: phase-correct 8-bit (WGM20=1), prescaler=8 (CS21=1) */
    TCCR2 = 0;
    TCNT2 = 0;
    OCR2 = 128;                                      // 50% duty to start
    while (ASSR & ((1 << OCR2UB) | (1 << TCN2UB))) { /* (paranoia) wait */
    }

    TCCR2 = (1 << WGM20) | (1 << CS21);  // phase-correct, clk/8
    while (ASSR & (1 << TCR2UB)) {       /* wait if needed */
    }

    /* 5) Non-inverting on C (COM21=1, COM20=0) */
    TCCR2 &= ~(1 << COM20);
    TCCR2 |= (1 << COM21);
}

int main(void) {
    timer2_init_pwm_pc_prescale8_noninv();

    /* Steady PWM forever: ~8 MHz / (8 * 510) ≈ 1.96 kHz */
    /* (If you want ~7.8 kHz, use prescaler=1: CS20=1, WGM20=1) */
    while (1) {
        // do nothing — PWM runs continuously
        // (Optional: blink another pin to show the MCU is alive)
        _delay_ms(1000);
    }
}


main.hex:



:1000000012C019C018C017C016C015C014C013C044
:1000100012C011C010C00FC00EC00DC00CC00BC06C
:100020000AC009C008C011241FBECFE5D4E0DEBF5E
:10003000CDBF02D029C0E4CF82B5877F82BD82B513
:1000400087708111FCCF1DB81EB8BB9AC39815BC30
:1000500014BC80E883BD82B586708111FCCF82E438
:1000600085BD02B400FCFDCF85B58F7E85BD85B50D
:10007000806285BD2FEF89E698E121508040904055
:0C008000E1F700C00000F6CFF894FFCFBD
:00000001FF

Please help you are my last hope!)

#2 · October 22, 2025, 9:25 am

Hi. Thanks for reporting.

I found the problem in Simulide.
As a workaround you can try this:

At the end of void timer2_init_pwm_pc_prescale8_noninv(void)

Change this:

    TCCR2 = (1 << WGM20) | (1 << CS21);  // phase-correct, clk/8
    while (ASSR & (1 << TCR2UB)) {       /* wait if needed */
    }

    /* 5) Non-inverting on C (COM21=1, COM20=0) */
    TCCR2 &= ~(1 << COM20);
    TCCR2 |= (1 << COM21);

By this:

    TCCR2 = (1 << WGM20) | (1 << CS21) | (1 << COM21);  // phase-correct, clk/8
    while (ASSR & (1 << TCR2UB)) {       /* wait if needed */
    }

unbreakmyheart has reacted to this post.

#3 · October 22, 2025, 9:59 am

HOORAY! It worked! Thank you.

I cant appreciate enough how much you project saving me time to develop a working board.

except completely unusable Proteus i don't know any other software that allows fully develop a project almost exclusively 'in silica', it saved me tons of hours in past and still do. Thank you!

KerimF has reacted to this post.