oh but a TPIC8101 does exist.

http://focus.ti.com/docs/prod/folder.../tpic8101.html

Cool stuff on the windowing.

Matt, the output of my circuit isn't "0-5V". It's a bandpassed, amplified version of the knock signal. You'll need a peak detector or AM circuit after it.

Also, the MS's knock input is kind of backwards. +5 indicates no knock, 0v means that knock is occurring.

Also II, this input goes directly into a pin on the microprocessor, so the threshold is pretty damned nondetermenistic. The specs for the '908 only guarantee that the threshold will be somewhere between 0.2 * VDD and 0.7 * VDD, which is pretty typical. And, of course, you need to hold the sample for long enough the the uP has a chance to actually read it. A quick transient is likely to get missed by the processor as it only polls the inputs on a "best effort" basis. The closest thing I've found to an official spec says you need to hold the low state for at least 400ms, which is an eternity for a high-frequency audio waveform. Granted, that spec was written for the MS1, but even the MS3 isn't polling those ports continuously.




Now that is damned cool.

I may have to build myself another MS and sacrifice the engine in my '90 on the altar of figuring out how to tune that little bugger.

Anybody know were I can buy some 70 octane gas?

A rectifier + peak detector + peak&hold circuit to process the output of the bandpass circuit is pretty simple.

5 samples free on that tpic.

It occurs to me that we are all complete idiots, myself included.

The time-window of interest to us can be expressed as either n degrees of crankshaft revolution or n number of milliseconds after the ignition event, right? (I'd need to do a bit of reading to recall which.)

How hard would it be to rig up a little external device to take, as inputs, a one-pulse-per-ignition-event signal and something resembling an RPM signal (which could be as simple as a freq-to-voltage converter, if we're working in the analog domain) and use it to drive a monostable multivibrator to gate the sensor signal?

Easy. Just use an analog CMOS switch e.g. CD4013B for gating.

You'll need a pulse train that goes 'hi' during said ignition window e.g. from 10* to 90* ATDC to gate the CMOS switch.

Since most knock occurs after the ignition event, just use the actual ignition event to gate the window. And since burn time is 3 ms max IIRC, use a 3 ms monostable.