topic Re: MCF51JM Input Capture values in ColdFire/68K Microcontrollers and Processors

MCF51JM Input Capture values

Bloodhound — Fri, 05 Jun 2009 14:54:34 GMT

Hi All,

I have a MCF51JM running off an 8MHz XTAL.

I want to use one of the timers to monitor pulse data coming in and measure the input capture toggle times.

What is the best method for calculating out how I need to set up the Timer Registers (Prescalers etc) to be able to measure bit times between 16uS and 300uS?.

I assume the best method is once I get an Int from the Timer I will read the TPM-Counter Registers (TPMxCNTH:TPMxCNTL) to get the value and compare it to 'x' to determine the time.

I'm just not sure how to calculate out for a given clock frequency and prescaler value what 100 or 400 (as examples) in the (TPMxCNTH:TPMxCNTL) registers corresponds to as a uS time.

Thanks,
Ross

Re: MCF51JM Input Capture values

Bloodhound — Mon, 08 Jun 2009 09:19:15 GMT

Okay, I answered my own question above by experimenting, however, the problem I have now is the Timer count values are just incrementing on each interrupt, not resetting back to zero after each int is processed.

I'm not sure what I am doing wrong.

For each IC Int I read the values from (TPMxCnVH:TPMxCnVL) which are approximatly what I expect when read (value increases by approx 280 per read), which is about 64uS trigger time.

I can't seem to get (TPMxCnVH:TPMxCnVL) to revert back to zero after they are read.

The CF doc states "The latching mechanism may be manually reset by writing to the TPMxCnSC register", I am doing this by clearing bit 7 of the (TPMxCnSC) which is CHnF, what else am I missing?

Thanks,

Ross