Re: OT: MC12080 prescaler chip weirdness

Tom Lee

That is a very fussy chip. The most common error is to drive the control signals with TTL level signals. That won't work reliably. They have to be very close to the rails. I don't remember the specs any longer, but IIRC you have to be within a diode drop of the positive supply, and the signals have to be clean. And, of course, the input signal has to be clean as well.



Prof. Thomas H. Lee
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On 3/20/2021 10:46, Michael Dunn wrote:
A Fluke counter I’ve been fixing up and adding homebuilt options to is missing the prescaler option. The vast majority of current chips seem to be binary, so, short of also dividing the reference clock, I decided to use a decade divider, an NOS MC12080 ÷10/20/40/80 chip.

Funny thing is, instead of dividing by 10, it’s dividing by 2! The chip internals consist of a fixed ÷5, followed by a programmable ÷2/4/8/16 stage. It’s like the ÷5 isn’t there, or is bypassed.

After some searching, I found a few other people bemoaning this exact same behaviour. So…at least I’m not crazy. Anyone here experienced something similar?

Gotta wonder what’s going on. Is there some sort of flaw in the chip design that leads to this failure mode? Was it a custom version? Hmm…


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