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MARK it ! MARK millwood0 !

here is the same code, running on a 8Mhz AVR (clock divider to be 1:8, so each timer tick is 1us).

=========code============
#include <ioavr.h>
#include <intrinsics.h>
#include "gpio.h"

//hardware configuration
#define PWM_PORT        PORTB
#define PWM_DDR                 DDRB
#define PWM_OUT                 (1<<0)        //pwm output on p2.0
//end hardware configuration

#define PWM_ON(bits)        IO_CLR(PWM_PORT, bits)        //turn on bits
#define PWM_OFF(bits)        IO_SET(PWM_PORT, bits)        //turn off bits
#define PWM_STEPPING        150                 //pwm stepping, in  us - should leave enough time to fully execute the isr, 0x00 - 0xff
#define PWM_PERIOD                100                //pwm period, 0x00 - 0xff
unsigned char PWM_DC = 4;        //desired pwm duty cycle, from 0x00 - PWM_PERIOD

///////////////////???0????PWM////////////////////
#pragma vector = TIMER0_OVF_vect
__interrupt void timer0(void) {  
        static unsigned char pwm_index=0;   //t?????????????????  
       
        TCNT0+= -PWM_STEPPING;        //update tmr0 timer
        pwm_index++;    //?15????1
       
        if (pwm_index==PWM_PERIOD) {   //1.5???????  
                pwm_index=0;                //reset pwm_index
                PWM_ON(PWM_OUT);        //turn on pwm_out               
        }
        if (pwm_index==PWM_DC)  //???????????????  
                PWM_OFF(PWM_OUT);        //turn off pwm
}  

void mcu_init(void) {
        PWM_ON(PWM_OUT);         //drive pwm_out off
        IO_OUT(PWM_DDR, PWM_OUT);        //pwm_out as output
}

void tmr0_init(void) {
        TCCR0A = 0x00;                                                //normal port operation
        TCCR0B = 0x01;                                                //no scalling
        TCNT0 = -PWM_STEPPING;                                //initiate the tmr0 counter
        TIMSK0 = 0x01;                                                //enable tmr0 interrupt
        __enable_interrupt();        //?????
}

//////////////////?????//////////////////////

void main(void) {  

        volatile unsigned short i;

        mcu_init();         //reset the mcu
        tmr0_init();         //set up tmr0
             
        while(1) {      
                //for (i=0; i<0x02ff; i++)
                        //continue;                                //waste some time
                //PWM_DC++;                                        //update pwm duty cycle
                //if (PWM_DC==PWM_PERIOD) PWM_DC=0;
                //(PWM_DC == PWM_PERIOD)? PWM_DC=0: PWM_DC++;
        }         
}  
========================end code============

as you can see, the code is remarkably similar to the 8051 code, with the exception of some unique AVR stuff (like the avr tmr0 doesn't have the automatic reload function).

since our tmr offset / stepping is 150 (ticks) per interrupt, and our pwm period is 100 interrupts, we expect the pwm period to be 150 * 100 = 15ms.


that's confirmed via simulation as well.



(原文件名:USB1286 software PWM.PNG)

that's the beauty of C - portability.

you can port the same code to a PIC, or any other mcus and it would remain remarkably the same.

谢谢学习中