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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Tight loop timing |
 Posted: Sun Jul 24, 2005 10:52 am |
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PCWH v. 3.228, 18F452 & 18F4610 running at 4 MHz.
I'm trying to flash an IR LED. Specifically, I'm trying to recreate the on/off etc codes to run A/V equipment. This involves "bursting" the IR LED at about 38 kHz, 50% duty cycle, then turning it off, then "bursting" again, etc.
Anyway, I can't use the PWM module(s), so I was bit-banging it myself.
I know that at 4 MHz, the system clock is only 1 us. To create the 38 kHz IR carrier, I coded a loop as follows:
| Code: | for (i = 0; i < 328; i++) { // lead in burst of 8.5 ms
output_high(PIN_C0);
delay_us(12);
output_low(PIN_C0);
delay_us(12);
} |
One instruction to set the pin high, another 12 in the delay, one more to set the pin low, then wait another 12. Repeat. A total of 26 cycles, 26 us, which corresponds to a frequency of 38.5 kHz, which is what I need. Only problem is that if you actually measure this waveform with a scope, it's not even close to a 50% duty cycle, and not 38.5 kHz either.
Turns out the high time is 13 us, exactly as I thought, but the low time is about 19 us. Enough to throw it off so that a standard IR decoder can't detect it.
I had to tweak the second delay to just 9 us. With that, the output waveform is a perfect 50% duty cycle, with a 26 us period.
In addition, the lead in pulse must be about 8.5 ms. 328 times through the loop x 26 us = 8.53 ms. Only it isn't. In order to get that lead in pulse to be 8.5 ms, I have to change the loop to run 260 times, not 328. This is with the low time delay changed to 9 us.
Obviously, the loop overhead is not being taken into account, by me or by CCS. My question is this: doesn't CCS take these things into account? I've done loops with tight timing restrictions on earlier versions of the compiler without problems. Is this a bug?
And I know, I should have used the PWM module. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Sun Jul 24, 2005 12:02 pm |
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| Quote: | Obviously, the loop overhead is not being taken into account, by me or by CCS. My question is this: doesn't CCS take these things into account? I've done loops with tight timing restrictions on earlier versions of the compiler without problems. Is this a bug?
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Now why do you think they should take into account the instructions for the loop? They execute the delay routine. The loop overhead is after the delay and the error is in your technique. |
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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Posted: Sun Jul 24, 2005 12:14 pm |
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| Mark wrote: | | Now why do you think they should take into account the instructions for the loop? They execute the delay routine. The loop overhead is after the delay and the error is in your technique. |
I think I know what was causing me so much grief.
I prototyped this system about 6 months ago and was able to control anything I wanted using this code. The difference was that when I prototyped it, I'm pretty sure I was running the PIC at 20 MHz, not 4. The overhead in executing the loops is then minute compared with the large delays (12 us).
Let this be a lesson to everyone: keep good notes!!!  |
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