Today I started to experiment with PWM in a 16F877. Got both PWM modules working in minutes.

While period can be finely ajusted with 10-bits precision, the duty cycle seems more complicate due to the two LSB in CCP1CON<5:4>.

I have no specific application in mind but when tried to increase/decrease the duty cycle value I've realized that trying to change it down to the LSB would demand too much code other than INCF / DECF.

For an eventual application requiring a fast adjustment, is it any way to do it or we have to accept adjusting only the value in CCPR1L?

Agustín Tomás

 

You can't modify the PWM duty-cycle to its 10-bit precision using only CCPR1L with the 16F87x series. However, with many other PICs, you can specify the justifiction of the 10-bit value, as in CCP1CON<5:4> can represent the LSBs or the MSBs.

However, when you're running this thing at 20MHz, a few extra lines of code won't make that much of a difference, as it takes a while for the CCP module to settle anyways.

Lets say the MSBs are in DCH, and the two LSBs are in DCL<5:4>
you can change the PWM with

movfw DCH
movwf CCPR1L
movfw CCP1CON
andlw b'11001111'
addwf DCL, W
movfw CCP1CON

However, you are correct in saying that INCF and DECF are slower, but the alternative is quite short

 

Originally posted by nerd256@Apr 12 2005, 08:25 PM
However, when you're running this thing at 20MHz, a few extra lines of code won't make that much of a difference, as it takes a while for the CCP module to settle anyways.

Yes, that's the point! The time frame I am working in counts a lot!
Thanks.

Agustín Tomás