pwm help needed

Mike said:

Some notes that will help us to help you;

(1) Use the processor include file instead of equating register names.
[COLOR="darkorange"]i had read that if i just in the project wizard of mplab say its a pic16f87 that ill b using it will be enough. [/COLOR]





org 0x0000

RESET clrf STATUS ; |B0
clrf PORTA ; clear Port A data latches |B0
clrf PORTB ; clear Port B data latches |B0
why is this done to clear the data latches. ?
movlw h'07' ; |B0
movwf CMCON ; turn comparator off |B0
bsf STATUS,RP0 ; select bank 1 |B1
clrf ANSEL ; digital I/O, no ADC pins |B1
clrf TRISA ; port A all outputs |B1
clrf TRISB ; port B all outputs |B1
bcf STATUS,RP0 ; select bank 0 |B0
;
; setup PWM (20-MHz clock)
;
movlw b'00000010' ; |B0
movwf T2CON ; TMR2 prescale:16 (3.2-usecs) |B0
movlw d'010' ; |B0
movwf CCPR1L ; 10% duty cycle?
1)what if i want to change the duty cycle according to the input .say for eg my input keeps varying from 7 to 10 volts and i want a stable 6volts. how do i code that using assembleer ?

(2)and another question just by writing d'010' does it mean it is 10 percent,hence if i write d'060' it becomes 60 percent duty cycle ?.. arnt there calculations involved ..? |B0
bsf STATUS,RP0 ; select bank 1 |B1
movlw d'100'-1 ; 100 3.2-usec 'ticks' |B1
movwf PR2 ; Period=320-usecs, Freq=3125-Hz |B1
bcf STATUS,RP0 ; select bank 0 |B0
movlw b'00001100' ; |B0
movwf CCP1CON ; put CCP module in PWM mode |B0
bsf T2CON,TMR2ON ; turn on TMR2 |B0
;
; now test to see if the LED on RB3 is glowing at 10% brightness
;
LOOP goto LOOP ;loop indefinately |B0
;
;******************************************************************
end
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Mike said:

For the case when using a value for PR2 of d'100'-1, a CCPR1L value of 10 will provide a duty cycle of 10% and a CCPR1L value of 60 will provide a duty cycle of 60%.

For the case when using a value for PR2 of d'200'-1, a CCPR1L value of 20 will provide a duty cycle of 10% and a CCPR1L value of 120 will provide a duty cycle of 60%.

A point of confusion with PIC PWM is that PR2 uses 'ticks' of Tcyc (reciprocal oscillator freq/4) while CCPR1L:CCP1X:CCP1Y use 'ticks' of Tosc (reciprocal oscillator freq). If you ignore the CCP1X:CCP1Y bits you've effectively multiplied Tosc by 4 and CCPR1L represents 'ticks' in the same time base that PR2 is using. For example; if PR2 = d'256'-1 then a 50% duty cycle is achieved when CCPR1L is set to d'128' (ignoring CCP1X:CCP1Y).

As for modifying CCPR1L:CCP1X:CCP1Y "on the fly" to change PWM duty cycle, you'll have to figure that out on your own. I can't imagine how you'll do it for this application without an ADC module but I suspect there are several creative minds on the forum who can help.

It seems you may be a beginner, and if so, I'd recommend you search for the Elmer 160 tutorial series and study.

Good luck with your project. Kind regards, Mike

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