Hello,

I am having some major problems with the duty cycle of a PWM setup on
the 18F452 device.

The problems is when I set the following conditions for the PWM:
PR2 = 0X7C
TMR2[0,1]= 1 ;for 4 bit prescale on timer 2
TOSC = 40MHz system clock

With these above settings, that should yield me a 20KHz PWM period,
which it does. But the problems is that according to the data sheet,
this should allow me a 12-bit resolution, but do to the limits of the
PIC this can only be 10-bit.  Which also is file.  However, the problem
occurs when I set the duty cycle.  If CCPR1L = 0x80 then I should have a
50% duty cycle, this is without taking into consideration CCPR1CON<5:4>
are set, however I am getting a 100% duty cycle.  In fact it appears
that the value of CCPR1L is 2 times the value of the output duty cycle.
I.E if CCPR1L = 128 then the output is 100% if the CCPR1L is anything
more that 128, then the output is high perminately.

What is the problem.  I want to be able to have the full 10-bit range,
but how?  Below is code that I am using.

PulseLaser:
            GLOBAL           PulseLaser
            ;We begin by moving the high byte in the CCPR1L register.
            ;Then, we stripp all bits in CMDDATA1(Low 2 bits) but 4 and
5
            ;We proceed to clear the previous value in the CCP1CON for
the two LSB.
            ;Finally, we move the two lowest bits in CMDDATA1 into
working and then
            ;IOR that value with the CCP1CON and place the result back
into the CCP1CON
            ;register.
            ;CMDDATA2 = High 8bits of duty cycle
            ;CMDDATA1 = low 2 bits of duty cycle
            ;TRICKEL_PRESCALE=0X04      FOR 16 TMR2 PRESCALE VALUE
            ;WORK_PRESCALE = 0X01       FOR 4BIT TMR2 PRESCALE VALUE
            ;TRICKEL_FREQ_PERIOD = 0X7C FOR PR2 yielding a 5KHz PWM
frequency
            ;WORK_FREQ_PERIOD = 0X7C FOR PR2 yielding a 20KHz PWM
frequency
            ;The two above frequencies at determined by the combination
of the PR2 and TMR2 prescale settings.

MOVFF             CMDDATA2,CCPR1L
            MOVLW            B'00110000'
            ANDWF            CMDDATA1,F,A
            MOVLW            B'11001111'
            ANDWF            CCP1CON,F,A
            MOVF               CMDDATA1,W,A
            IORWF             CCP1CON,F,A
            ;Here we check the value to determine if both values are
zero.
            ;If they are, then pulsing is to be terminated.
            MOVLW            0X00
            CPFSEQ           CMDDATA1,A
            BRA                 StartPulseTimer
            CPFSEQ           CMDDATA2,A
            BRA                 StartPulseTimer
            ;If we are here, then we are terminating the pulsing of the
laser.
            MOVFF TRICKEL_FREQ_PERIOD,PR2
            MOVLW            B'00000011'
            ANDWF            TRICKEL_PRESCALE,F,BANKED
            IORLW  B'11111100'
            ANDWF            T2CON,F,A
            MOVF   TRICKEL_PRESCALE,W,BANKED
            IORWF T2CON,F,A
;Should now be setup.
            BCF                  T2CON,TMR2ON,A                    ;Turn
off timer.
            RETURN
StartPulseTimer
            ;Move frequency into registers.
            MOVFF             WORK_FREQ_PERIOD,PR2
            MOVLW            B'00000011'
            ANDWF            WORK_PRESCALE,W,BANKED
            IORLW              B'11111100'
            ANDWF            T2CON,F,A
            BSF                  T2CON,TMR2ON,A
            RETURN
END

Regards,

James

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