I'm no expert on IR links, but the ones I've used had much lower than 50%
duty cycles.
It seems to me, that if you used a shorter on time, it'd make the looping
and testing easier,
not to mention letting you drive the LED harder.

Either way, it'll be a tight fit.
-Denny



----- Original Message ----- 
From: "Jinx" <joecolquitt@clear.net.nz>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, May 03, 2005 11:11 PM
Subject: Re: [PIC] 38KHz from 12f675 TMR0 interrupt?


> > 13.15usec per half cycle.  I don't think you have enough time to
> > save and restore context and reload timer0 at 4MHz, let alone
> > keep track of other timing.
>
> You could be right - as a straight 38kHz generator it would work
> but what's going to make it a tight squeeze at 4MHz is the testing
> and branching
>
> Maybe put a 32kHz xtal on Timer1 and use that to measure the
> 5 & 20 second intervals (T1 OSC available only if you use the
> internal osc), keeping TMR0 @ 4MHz just for the 38kHz
>
> Or even WDT + a timer ?
>
> OK OK, run it at 10MHz ;-)
>
> BTW Tom, I'd recommend ZTX transistors for driving IR LEDs.
> They switch faster and better and more current than other common
> types (eg TO92 like BCxxx or 2Nxxxx). Their higher current rating
> means a low value series R can be used eg 1 or 2 ohms for good
> range, but watch the wattage dissipated by the IR LED
>
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