From: Scott Dattalo
;------------------------------------------------------------ ;pwm_multiple ; ; The purpose of this routine is to generate 8 pulse width ;modulated waveforms. The algorithm consists of 9 counters ;that change the state of the pwm bits whenever they roll over. ;One of the counters, rising_edge, will drive the pwm bits ;high when it rolls over. The other 8 counters pwm0-pwm7 will ;drive their corresponding bits low when they roll over. ; ; ;RAM: ; pwm0-pwm7 - pwm counters ; rising_edge - rising edge counter ; pwm_state - current state of the pwm outputs. ;ROM ; 23 instructions ;Execution time ; 23 cycles pwm_multiple CLR W ;Build the bit mask for turning ;off the PWM outputs. Assume that ;all of the outputs will be turned ;off. ; DECSZ pwm0 ;If the first counter has not reached 0 OR W, #00000001b ;then we don't want to turn it off. ; DECSZ pwm1 ;Same for the second one OR W, #00000010b ; ; DECSZ pwm2 ;and so on... OR W, #00000100b ; ; DECSZ pwm3 ; OR W, #00001000b ; ; DECSZ pwm4 ; OR W, #00010000b ; ; DECSZ pwm5 ; OR W, #00100000b ; ; DECSZ pwm6 ; OR W, #01000000b ; ; DECSZ pwm7 ; OR W, #10000000b ; ; ; AND W, pwm_state ; Clear all of those pwm outputs ;that have reached zero. ; XOR W, #11111111b ;Toggle the current state. INCSZ rising_edge ;If the rising edge counter has not XOR W, #11111111b ;rolled over then toggle them again. ;Double toggle == no effect. However, ;if the rising edge counter does roll ;over then a single toggle will turn ;the pwm bits on, unless of course the ;pwm counter has just rolled over too. ; MOV pwm_state, W ;Save the state MOV PWM_PORT, W ;update the outputs Now this routine has been optimized for speed. It would be difficult (but not impossible) to modify it to: 1) change the duty cycle on the fly and 2) have different frequencies for each pwm. One technique is to maintain two counters for each pwm. Call them say, rising_edge_counter and falling_edge_counter. Make the counters the same size and do something like this: if(--rising_edge_counter == 0) { turn_PWM_on(); rising_edge_counter = PWM_PERIOD; } if(--falling_edge_counter == 0) { turn_PWM_off(); falling_edge_counter = PWM_PERIOD; } In other words, every time the rising edge counter rolls over, you make the PWM output high and when the falling edge counter rolls over you make it low. Now, to get duty cycle, you simply adjust the phase between the counters: set_duty_cycle(int new_duty_cyle) { falling_edge_counter = rising_edge_counter + new_duty_cyle; if(falling_edge_counter > PWM_PERIOD) falling_edge_counter -= PWM_PERIOD; } Now this routine assumes that the new_duty_cycle is less than the PWM_PERIOD. Scott
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