how the knock sensor works? you can explain this in two ways: easier and not easy.
essentially, it's a very small and narrow banded microphone. when it is vibrated by the engine knocking(or what can be suspected as knock), the current limited 5V signal that is fed into it starts moving toward the 5V or 0V range (in a no knock condition, line voltage will be near 2.5 volts) due to the sensor resistance going from roughly 3900 ohms to more or less than 3900, causing the voltage fluctuation.
since the signal basically looks like an AC waveform using a scope, it's one way to think of it.
after the sensor itself is the knock filter, who's job is to further refine the signal coming out of the sensor so that only real knock gets detected as knock. when that happens, a signal is sent to the MCU (68HC11 in your case) and the processor will respond VERY quickly(within a few cycles, and at 2.1 million cycles per second, it is essentially instant) and start pulling timing the next time the timing calculation is done (80 times per second is a normal rate).
basically, the filter takes the roughly AC looking signal and when one of the peaks suddenly moves outside of the range that it's been seeing, that's considered knock and it will react. otherwise, a very slow change doesn't really trigger it.
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