please dont rip this site

SX Microcontroller IO Routine

Pulse Width Modulation

From: Scott Dattalo

;  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.
; pwm0-pwm7 - pwm counters [ed: initialized with 0..255, but not greater
;	then PWM_PERIOD (0..255). 0 corresponds to 256 cycles period]
; rising_edge - rising edge counter [ed.: initialized with PWM_PERIOD]
; pwm_state - current state of the pwm outputs.
; 23 instructions
;Execution time
; 23 cycles


	CLR	W			;Build the bit mask for turning
                                ;off the PWM outputs. Assume that
                                ;all of the outputs will be turned
	DECSZ	pwm0			;If the first counter has not reached 0
	OR	W, #%00000001	;then we don't want to turn it off.
	DECSZ	pwm1			;Same for the second one
	OR	W, #%00000010	;
	DECSZ	pwm2			;and so on...
	OR	W, #%00000100	;
	DECSZ	pwm3			;
	OR	W, #%00001000	;
	DECSZ	pwm4			;
	OR	W, #%00010000	;
	DECSZ	pwm5			;
	OR	W, #%00100000	;
	DECSZ	pwm6			;
	OR	W, #%01000000	;
	DECSZ	pwm7			;
	OR	W, #%10000000	;
	AND	W, pwm_state		; Clear all of those pwm outputs
                                ;that have reached zero.
	XOR	W, #%11111111	;Toggle the current state.
	DECSZ	rising_edge		;If the rising edge counter has not
	XOR	W, #%11111111	;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

;[ed. by Nikolai Golovchenko
;Update pwm counters with new values when rising_edge reaches zero.
;If update is not necessary and PWM period is 256 cycles, then this portion
;of code may be skipped because rising_edge counter has 256 cycles period
;and all initial values of pwm counters are restored in the end of each period.
;Additional 22 instructions.
	TEST	rising_edge
	SB	Z			;update if zero
	MOV	W, PWM_PERIOD	;init period
	MOV	rising_edge, W
	MOV	W, pwm0_init		;init pwm0
	MOV	pwm0, W
	MOV	W, pwm1_init		;init pwm1
	MOV	pwm1, W
	MOV	W, pwm7_init		;init pwm7
	MOV	pwm7, W
;end of ed.]

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) {
    rising_edge_counter = PWM_PERIOD;

  if(--falling_edge_counter == 0) {
    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



See also:

file: /Techref/scenix/lib/io/osi1/pwm/PWM_sx.HTM, 5KB, , updated: 2004/6/10 14:40, local time: 2017/10/18 22:44,

 ©2017 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE! / 

<A HREF=""> PIC Microcontroller IO Routine Pulse Width Modulation from Scott Dattalo</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.

Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?

  PICList 2017 contributors:
o List host: MIT, Site host, Top posters @20171018 RussellMc, Van Horn, David, James Cameron, Sean Breheny, IVP, alan.b.pearce, Neil, David C Brown, Bob Blick, Denny Esterline,
* Page Editors: James Newton, David Cary, and YOU!
* Roman Black of Black Robotics donates from sales of Linistep stepper controller kits.
* Ashley Roll of Digital Nemesis donates from sales of RCL-1 RS232 to TTL converters.
* Monthly Subscribers: Gregg Rew. on-going support is MOST appreciated!
* Contributors: Richard Seriani, Sr.

Welcome to!