Post by thread:Timing using RTCC/TMR0

Hi Folks, I thought that I would pass this along just in case anyone else finds it useful.... One of my favorite ways of producing timing loops is to make use of the TMR0 free running timer. Basically I sample the value of TMR0 at the start of the period event and then simply subtract this value from the current value of TMR0 to check on the actual time. Obviously this requires that the period to be measured falls within the range of 256 ticks of TMR0. I use the PIC a lot to do radio control stuff both decoding standard RC signals and generating pulses for servos etc. To do this you need to be able to both generate and time pulses in the range 1-2ms. My usual setup is to arrange for the TMR0 period to be just over 2ms with a 4Mhz pic. (Actually the setup I use gives a 1ms as a count of 125 in TMR0). To do this I load the prescaler with the value 2 which gives a divide by 8). With this setup the obvious way to generate a 1ms period would be movf TMR0, W ; Get current tick count movwf start ; save it loop movwf start, W ; Get tick count now subwf TMR0, W ; Subtract start time addlw -.125 ; Subtract 1ms count btfss STATUS, Z ; have we finished goto loop ; no so carry on waiting However if you actually use this type of code to drive a servo you will find that the servo 'twitches'. The reason for this is that at the start of the loop the pre-scaler register may have any value in it and so you will have an uncertainty of up to 1/125ms in the actual period. The 'fix' for this simply clear the pre-scaler at the same time as saving the start point by saving the TMR0 value back to the timer... movf TMR0, W ; Get current tick count movwf TMR0 ; clear pre-scaler counter movwf start ; save start time Anyway what a ramble this turned out to be! Hope somebody out there finds this of some use. It took me a long time to learn this particular lesson! BTW I know you can probably do this sort of timing stuff without having to use TMR0. But I hate all of that instruction counting stuff! also I find having a standard timer very useful, I use it to schedule various tasks, time pulse lengths etc. It also makes it very easy to change timer resolution and/or clock frequency! Andy