18F4431 - Complementary mode PWM

[AndrewC]

Hi Bruce, yes that worked (but I would expect nothing less from you )

But why ?

- I've got a period of $07E8 which is 2024 in decimal
- I ramp duty to 2024 which is seems is not 100% of a period (led watching was misleading, I guess, because one channel was ramping 0-25% (?) and the other 100-75% which isn't really much of a change in output intensity)
- so why does a duty cycle of 8000 get me 99% ?

Andrew

[Bruce]

0 TO 2024 wasn't giving you enough range.

    ' At 20MHz, to figure a PWM frequency of 19.455kHz
    '
    ' TPWM = time period of PWM frequency
    ' PTPER = 12-bit period register PTPERL and PTPERH
    ' PTMRPS = PWM time base prescaler
    ' 
    '           (PTPER+1)*PTMRPS         257
    ' TPWM = ----------------  =  ------------ = 0.0000514 
    '                  Fosc/4                5000000
    '
    ' Frequency = 1/TPWM = 1/0.0000514 = 19.455kHz
    '
    ' PWM resolution (bits resolution for duty cycle)
    '
    '                   log(PTPER+1)*4)          3.01
    ' Resolution = ------------------ = ----------- = 10 bits
    '                      .301                       .301

You have PTPER at 2024. PTMRPS at 16. Plug these into the above, and you get 0.00648.
1/0.00648 = 154.32 so you have a PWM frequency arouund 154hZ.

Your resolution is about 13-bits. %0001111111111111 = 8191.

So 8000 instead of 2024 is just giving you full range duty cycle.

[AndrewC]

Got it now, for some reason I had it in my head that the calculated resolution was just a max you could go to - not the actual max you had to go to for 100% duty cycle.

Thanks for the guidance - where's the book ? !!!

Andrew