Quote Originally Posted by Nicmus View Post
If I understand your application right you are sending a 0 – 80 mS pulse from your main processor to your 683.
It's not so much the pulse width as it is the pulse freq, and it's not so much a pulse as it is a 50% duty square wave. I'm using the E-PWM output from the '4685 to drive the OSC on the 12F675 (EC_OSC mode). The '675 only runs thru a loop turning an output on and off with a bit of a pause in between steps, like a really slow blinky LED program, only 23 words of code or something like that. The '675 ends up being a 'divide-by-586' chip.
I do lose a little bit of 'accuracy' at the very top end (rather large steps), and the tach jumps a bit at the very low end, below about 100 rpm as indicated on the tach (which can display 0-8000 rpm) which is actually .1MPG . I don't really need accuracy on the tach as I've got a color LCD displaying all of my raw numbers. The tach idea was just for a quick look solution. The other problem is the friction error caused by the movement in the tach. If I slowly come up to 4000 rpm from under, it reads 3800. If I slowly come down to 4000 rpm from above, it reads 4200 rpm. Common problem I'm sure. It's an old tach I used to use in my old '76 Blazer.

I took it out for a test drive last night. Works like a champ. For grins, I'm going to transfer the program over to a 10F200 and see how much crap I can cram into that.

It's kinda neat...that's all I wanted
Maybe I'll start building boxes that convert tach's into MPG meters.