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Setting up a 16F87x for PWM Output

Tony Nixon [sales at picnpoke.com] of ICmicro's  says:

Assume PIC runs at 4MHz 1000Hz wanted at 50% duty cycle

These PWM enable steps are taken from the 16F87X data sheet

  1. Set the PWM period (1000Hz = 0.001S) by writing to the PR2 register.
    PWM Period = [PR2 + 1]*4*Tosc*TMR2 PreScale value
    

    Use 4 for TMR2 prescale. You can try other values but make sure it works with the duty cycle calcs as well.

    Tosc = 1/4000000 = 2.5*10-7
    

    Rearranging the equation gives...

    PR2 = (Period/(4 * Tosc * TMR2 Prescale)) - 1
    

    Therefore...

    PR2 = (0.001/(4 * 2.5 * 10-7 * 4)) - 1 = 249
    
       bsf STATUS,RP0
       movlw d'249'    ; when TMR2 = 249 = end of Period
       movwf PR2
       bcf STATUS,RP0
    
    
  2. Set the PWM Duty Cycle by writing to the CCPR1L register and CCP1CON<5:4> bits.

    CCPR1L contains the upper 8 bits of the 10 bit Duty Cycle value CCP1CON<5:4> contain the lower 2 bits

    PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>)*Tosc*TMR2 Prescale Value
    
    CCPR1L:CCP1Con<5:4> = PWM Duty Cycle / (Tosc * TMR2 Prescale)
    
    PWM Duty Cycle = 50% of Period
    
    PWM Duty Cycle = 50% of 0.001 = 0.0005
    
    CCPR1L:CCP1Con<5:4> = 0.0005 / (2.5 * 10-7 * 4)
    
    CCPR1L:CCP1Con<5:4> = 500
    
    500 in 10 bit binary = 0111110100
    
    CCPR1L = 01111101    CCP1Con<5:4> = 00
    

    Write the 10 bit value

       movlw b'01111101'  ; set bits 9 - 2
       movwf CCPR1L
       bcf CCP1CON,CCP1X  ; set bit 1
       bcf CCP1CON,CCP1Y  ; set bit 0
    
    
  3. Make the CCP1 pin an output by clearing the TRISC<2> bit
       bsf STATUS,RP0
       movlw b'11111011'
       andwf TRISC
       bcf STATUS,RP0
    
  4. Set The TMR2 prescale value and enable TMR2 by writing to T2CON.

    We decided previously to set the prescale value to 4.

    TMR2 prescale are bits 1 and 0 in T2CON
    TMR2 enable is bit 2

       movlw b'00000101'   ; TMR2 = on, prescale = 1:4
       movwf T2CON
    
  5. Configure the CCP1 module for PWM
       movf CCP1CON,W
       andlw b'00110000'  ; mask all but previously set Duty Cycle bits
       iorlw b'00001111'  ; and enable PWM mode
       movwf CCP1CON
    

    A 1000 Hz square wave with a 50% duty cycle should now be coming out of RC2.

Liew Danny Jiefu says:

how do you use the pwm after setting it up, for example like on the leds? could u write a sample program?

Comments:

See also:


file: /Techref/microchip/16F877/pwm.htm, 5KB, , updated: 2012/10/1 11:36, local time: 2017/10/23 06:33,
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