PWM with TPM Timer at low frequency

[Thunder]

Hi,  I need a PWM function to use with a frequency of as low as a minute or two.  For example, a freq of 1 minute would provide a 30 secs on/30 secs off with a 50% duty cycle.  I'm using a 32.768kHz external crystal attached to the XTAL pins, and have a bus frequency of 4MHz.  Unfortunately, dividing the clock by 128 and doing a full count in the TPM counter 0xFFFF does not provide a sufficiently long time period. 

There is a statement in my user manual that an internal "XCLK" could be used.  After reading what I could find in this forum, I still cannot figure out if there's a way to provide a lower clock freq to the TPM without changing the bus clock frequency.  If using the "XCLK" is an option, any suggestions about what registers to alter to provide a lower clock freq for the TPM ?  I am using the HCS08LG32 (C coding).   Thanks.

[Thunder]

Thanks Rocco, your suggestions work well.

I also went back to look into changing the clock input to the TPM.  It seems that XCLK, the internal clock, is the signal pre-filter to the DCO.  In my case, it is correlated to the 32.768kHz crystal frequency which is being used external to the processor.  With RDIV=0 and setting this clock for TPM input (plus divide by 128), I am able to get a very low frequency.  I'm still having a few problems setting the modulus (the program memory doesn't seem to want to accept any changes to this register for some reason), but that's another problem.

I should be able to use either method for generating the PWM.  Thanks again.

[rocco]

Hi Thunder,

With a frequency that low, you don't need a PWM module at all. You could more easily bit-bang it. Here is how I have done it:

Expand the timer to 24 or 32 bits. Simply allocate the byte or two in ram, and increment it in the timer-overflow interrupt-service routine.

Check the expanded timer at a predetermined interval to see if it is time to toggle the output-bit. You can simply use the timer-overflow if you don't need much precision, or if you do need precision, you can use the timer's compare function.

I've used this technique for a PWM with a 20 second cycle-time and 1/8 second resolution.

mark

Message Edited by rocco on 2010-02-01 06:05 PM