Guys,

        I've been working on using Hardware PWM to control the speed of a 12v dc
motor connected through a Mosfet.  I'm using 2 buttons to speed up and
speed down.
        It works really well with a PWM frequency above something like 4kHz.  If I
try to go below that frequency, it acts goofy.  For example, below is my
code for PWM at 3kHz.
        The way I understand it, I should get 0% - 100% for a CCPR1L from 0 -
333.  For these lower frequencies, if I slowly increase the DUTY cycle up
to about CCPR1L =75 it ramps up to 100%.  They it stays at 100% until
~CCPR1L =260 at which it instantly drops to 0%.  From CCPR1L =260 to 333 it
again ramps up to 100%.  It does the exact opposite when I ramp down in speed.
        Does anyone have a clue why this is happening?  If I reset the code for
anything from like 4kHz and up to work exactly as it's suppose to.  I
really think it's a software issue, but I'm not sure.  I really want to run
at lower frequency (don't care about audible noise) to minimize switching
losses.  Help?!?!?!

Thanks,

Gary


________________________________________
duty            var     word
btnwk           var     byte
clr             con     1               'LCD clear screen instruction
I               con     254             'Instruction prefix value

'******** Initialize ***************************************

TRISA = %000000         'Set porta   = output
TRISB = %11111100               'set RB2:RB7 = input
TRISC = %00000000               'Set portc   = output

btnwk=0
PORTC.2=0                       'try to keep it from jumping when 12v connected

OPTION_REG = %10000111  'set timer source as internal instuction cycle clock
                                'assign prescaler to tmr0 (not WDT)
                                'set prescaler to 1:64
INTCON = %10100000              'enable the tmr0 interrupt (intcon)
                                'Enable Global Interrupt Enable bit
                                'Disable RB0 interrupt
ADCON1 = %00000110      'Make PortA into digital I/O instead of analog I/O

PR2=332                         'Set PR2 to 3 kHz

TRISC.2=0                       'Set CCP1 bit as output
T2CON=%00000100                 'Timer on prescale is 1
CCP1CON=%00001100       'Set to PWM Mode
duty=0
CCPR1L = duty           'Set Pulse width - CCPR1L must be less than PR2.
                                'Lower number(pulse width)
                                'PR2 value is 100% duty cycle.
                                'PR2/2 = 50% duty cycle.

'******** Main Program *************************************

DEFINE BUTTON_PAUSE 50          'change button pause to 50 milliseconds

main:
        if duty > 60000 then            'did this b/c if <0, it rolls over to 65676
                duty = 0
        endif

        if duty > 333  and duty < 1000 then     'for 3khz operation
                duty = 333
        endif

        CCPR1L = duty   'Set Pulse width - CCPR1L must be less than PR2.
                                'Lower number(pulse width)
                                'PR2 value is 100% duty cycle.
                                'PR2/2 = 50% duty cycle.

        serout 1,n9600,[I, clr]         'Clear LCD screen
        serout 1,n9600,[" ", #duty]     'Display seconds variable on LCD

        button PORTB.3, 1, 255, 255, btnwk, 1, speedup
        button PORTB.2, 1, 255, 255, btnwk, 1, speeddown

goto main

speedup:

        duty=duty+15            'for 3khz operation
        goto main

speeddown:

        duty=duty-40            'for 3khz operation
        goto main


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