Re: [avr-gcc-list] Fine Tuning code timings

[David Kelly]

On Dec 28, 2007, at 11:00 AM, Jan Menzel wrote:

> Hi Alex!
> 	I'd suggest to take larrys approach and generate the start bit from  
> within the ISR. For precise timings you might add the bit time to  
> the current OCR value (insight the ISR). Using this schema you can  
> have an arbitrary (but constant) phase difference between the  
> interrupt and the time of changing the tx-pin. To start a  
> transmission I'd just set the OCR to TIMER + x where x shall be set  
> such that the ISR is called as fast as you like the transmission to  
> start.


That works so long as your interrupt doesn't have to wait on another  
before it runs.

I recently had a problem where I used one interrupt to service an A/D  
converter and another for management of the A/D. The A/D responded  
much faster than I expected and these two interrupts overlapped. Most  
of the time that was fine but when a 3rd interrupt was running the A/D  
interrupts queued and were processed in priority order which was  
reverse of arrival times. Thankfully it was repeatable and occurred at  
least once every 10 seconds.

If your timing accuracy doesn't mind pseudorandom latencies then  
manually driving output bits from interrupt is fine. If its picky then  
connect the OCR timer to its output bit and let the hardware do it.

--
David Kelly N4HHE, address@hidden
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Whom computers would destroy, they must first drive mad.