delay makes wrong values ?

[Andreas]

Hello Friends

After starting this project I had troubles to get the delay_ms(10) function to work I reduced my project to the minimum and now I got a very interesting behaviour.

The delay_ms(10) works perfect as long as the timer1 Interrupt is not active, but as soon I start the timer1 interrupt the delay_ms(10) makes a value of about 18ms !!!

Can somebody out there explain this ?

What I am doing wrong ??

Here is my code:

[Gabriel]

Its not "interesting" behaviour...

interrupts take time to handle...


if you need to generate a frequency that is not affected by interrupts you need to use the PWM hardware module...

G.
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CCS PCM 5.078 & CCS PCH 5.093

[ckielstra]

When the interrupt is triggered the interrupt software handler routine (ISR) is called. The ISR has the task to figure out which hardware event triggered the interrupt, save all chip registers that might get changed by your interrupt routine. On interrupt exit the saved registers have to be restored.
Depending on chip model there will be more/less registers to store. On an average PIC18 it takes about 60 instruction times to enter the ISR and another 60 to exit again.

I'm even surprised your delay_ms is only slowed down by 55%. With your Timer1 interrupt rate I'd expected it to be (almost) completely blocked.

[gpsmikey]

As the others have implied - the delay_ms() function is implemented by CCS by creating a delay with a calculated number of machine cycles (NOP or whatever) to get that delay. It will function correctly as long as nothing else adds instructions inside the delay loop. In this case, every time the timer interrupt goes off, if you are in the delay loop, the ISR adds probably a minimum of 120 cycles to the loop. If you are using interrupts, the only way to get accurate time is to have a timer that goes off every 10 ms or whatever works for you which you then use as your timebase (increment a timer counter inside the ISR).

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[Ttelmah]

The other key thing to mention, is just how often you are interrupting.
8bit counter, running off the instruction clock. The interrupt will be called every 256 instructions....

[Andreas]

Hello Friends


Thanks very much to all who answerd, I was in the impression that the ccs compiler is compensating for interrupts.

Of course now is clear for me what happens.


Thanks and best regards
Andreas