Best way to count pulses? Why is pulse counter blocking?

hugo · August 2, 2020, 2:33am

I recently came up with some interesting pulse interval code - using the UART timing mode to essentially time-stamp falling edge events.

The example code is for RPM counting, but it should be pretty understandable. Each time there’s a falling edge, the UART sees a start bit and we get a microsecond timestamp since the last event. The byte is malformed (there’s no stop bit so technically there’s a framing error) but I just ignore that. As imps tend to have plenty of UARTs this is a nice way to build a totally background RPM counter.

gist.github.com

https://gist.github.com/hfiennes/0e7f0c14f44578122b830b8a7650fe61

device.nut

// RPM counter with UART in timing mode
// hfiennes@twilio.com 20200611
//
// This uses the falling edge of the input signal to trigger a UART receive.
// There will be a framing error generated, but we ignore this (and the data) and just
// use the timing information.
//
// The baudrate must be fast enough that the input signal is LOW for at least 1 bit time
// and the high time is at least 9 bit times - ie max RPM that can be detected is
// 60 * (baudrate/10).

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Obviously, to just count pulses, you simply need to count received bytes - as long as the minimum low period is >1 bit time, and the minimum high >9 bit times (configure the UART appropriately) youl won’t even need to use timing mode. This should work fine for fairly high pulse rates - what’s the range you’re looking at?