Making tachometers and speedometers

[tonyfoale]

Hello Tony - MANY thanks for your ongoing supply of fascinating and informative posts.

So - the question is:- do you think the approach you have used measuring the duration between ignition pulses via an interrupt is as good as it gets?

As good as it gets? That seems a strange question. It drinks pulses synced to a rotary speed and displays an analog scaled representation of the measured frequency. What more do you want? Are you questioning the accuracy. Well it is digital all the way through to the display but then it relies on the accuracy with which the scale was drawn, the alignment of the pointer and our eye's ability to read it. The interrupt will have some latency and there will be the usual +/- count error, but those are smaller than any display/reading errors. There are basically two methods to measure frequency in this context.
1. Measure the time between pulses, which you question.
2. Count how many pulses you get in a set time. I used this method around 50 years ago because with the discrete components available then it was the easiest way. However, it tends to give a jerky reading. You can smooth it out by using a very short capture period but then accuracy suffers. A long period gives a jerky meter movement and if acceleration takes place during that period, the reading will be the average over that period and not the value now, so accuracy suffers.