[time-nuts] Raspberry Pi Sidereal clock display

Nick Sayer nsayer at kfu.com
Fri May 12 11:05:16 EDT 2017

A while ago, I took my GPS clock board (https://hackaday.io/project/18501-gps-clock) and sort of rearranged it to instead be a Raspberry Pi Zero clock display. I turned it into a product on Tindie for folks who can more easily get NTP over WiFi or Ethernet than GPS. Recently, someone asked me about a Sidereal GPS clock, but that’s problematic because the PPS interrupts are GPS time, which isn’t synchronized at all with sidereal time. Also, the traditional method of converting to sidereal time involves lots of floating point math, and little bitty microcontrollers aren’t at home doing that.

Instead, I wrote an alternative driver program for my Pi Zero clock board, so now it can be a sidereal clock display. If you run the daemon with no arguments, you get Greenwich Mean Sidereal Time, but if you specify your longitude, you get local mean sidereal time.

In principle, I believe you could design an alternate clock board with two independent displays with different SPI chip select lines and display local civil time and local MST simultaneously. I haven’t contemplated going down that road (yet).

The code is in the Pi Zero clock repo at https://github.com/nsayer/SPI_Clock/, and the clock hardware project is at https://hackaday.io/project/20156-raspberry-pi-zero-w-desk-clock

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