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Having a strange problem with the timer0 interrupt. I'm trying to generate a square wave to send to data lines on a pair of servos (from parallax). I tried to calculate the frequency using similar methods I applied on the Motorolla 68hc11/12, but the frequency didn't quite work out. I then used the trial-and-error method and was able to produce a 1kHz square wave. So far so good. It was quite interesting that the frequency gain was not linear or exponential as I tried different values for TMR0H and TMR0L. My motors are configured in such a way that I need one to recieve a 285Hz and the other a 350Hz square wave so my platform will go in a straight line. My intent was to create the two waves from the 1kHz interrupt. I added a few lines of code into my interrupt routine (see below) that I expected to do just that. Instead, I ended up with two waves, one was 12.6Hz the other was 7.58Hz. I wondered if my few instructions were using a lot of CPU time, so I changed my main loop to be: main: btg LATC,5 goto main The frequency on this pin is 333kHz, so this theory seems unlikely. I guess my question is: Am I doing anything fundamentally wrong with my timer interrupt? (Code posted below) The fastest square wave I was able to get from the timer interrupt was 1.887kHz from an 8MHz crystal. To me, it seems that I should be able to get a much faster wave. Sorry this is so long- and thanks in advance, Wayne ----Code TMR0_ISR btg LATC,7 ; Creates a square wave on RC7 btg LATC,6 ; Creates a square wave on RC6 ; dcfsnz TimC1 ; call FIREM1 ; dcfsnz TimC2 ; call FIREM2 movff TIMER1H_C,TMR0H ; Resetting place to count from movff TIMER1L_C,TMR0L ; for timer overflow (do i ; have to do this?) bcf INTCON,T0IF ; Clear Overflow Bit retfie ; Return From Interrupt ;FIREM1 btg LATC,6 ; movff TimR1,TimC1 ; return ; ;FIREM2 btg LATC,7 ; movff TimR2,TimC2 ; return ---- End Code NOTE: The commented code is the code that I added to split the 1kHz square wave into two square waves. TimR1, TimC1 are initially set to 285. TimR2, TimC2 are initially set to 350. ---- If needed, timer init code bcf INTCON,T0IF ; clear Timer0 interrupt flag bsf INTCON,TMR0IE ; enable Timer0 interrupts clrf T0CON ; Clear Timer Register ; This sets the timer: ; TMR0ON: OFF ; T08BIT: 16BIT Operating Mode ; T0CS: Set to Timer Mode ; T0SE: Set to do low-to-high edge ; PSA: Use Prescaler ; P0PS2-P0PS0: Set Prescaler 1:2 clrf TMR0H ; Clear Timer0 High bits clrf TMR0L ; Clear Timer0 Low bits bsf T0CON,TMR0ON ; Turn on the timer bsf INTCON,GIE ; Turn on interrupts ---- End timer init code |
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Timer0 interrupt on 18f452
Started by ●January 16, 2005
Reply by ●January 22, 20052005-01-22
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Somehow this message took a few days to get posted. Anyhow, I solved most of the problem I was having.. I was attempting to use timer1 as an 8 bit timer. :( I switched to using timer2 instead. Now, i'm still curious to know how to calculate the output frequency. The formula i was using is: DesiredFrequency = (1/(((256-PreLoadedVariable)*Pr escaler)/(OscFreq/4)) Wayne --- In , Wayne Fuchs <grok@x> wrote: > > > Having a strange problem with the timer0 interrupt. I'm trying to > generate a square wave to send to data lines on a pair of servos (from > parallax)......... > Sorry this is so long- and thanks in advance, > > Wayne |
Reply by ●January 23, 20052005-01-23
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Take a closer look at the datasheet...
There are a special procedure for reloading the timer registers, doing it the wrong way could cause a rollover error while reloading. Wayne wrote:
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