I somehow interpreted what was written as a 7:1 rather than 70:1 range of input and output frequency.

Nevertheless, it's unclear how the output frequency is applied. Is there an existing display that takes 10x the input from the engine tachometer and displays it?

With an 805x at, say, 25 MHz, that frequency taken from the frequency at which a small inexpensive SiLabs 805x type operates, using its resources including an ADC, by means of which a slope at zero-crossing of the input waveform can be derived, among other methods, an output frequency can be generated using any of a number of methods, not the least of which is the on-chip counter array.

By emulating in firmware, any of a number of hardware schemes, an all-firmware scheme can be applied without any changing of passive components.

Now, it's just a guess, but I think he can manage that.

RE