This repository contains a trivial program to set the frequency based on the
demo code at https://www.qrp-labs.com/synth/si5351ademo.html

It has been tested on a Raspberry Pi 4, but will probably work on any Pi or
similar device.

Some setup is required to enable the I2C peripheral. See
https://learn.adafruit.com/adafruits-raspberry-pi-lesson-4-gpio-setup/configuring-i2c

The `test` program that is built will set the output frequency to 4 times the
first command-line argument. The 4x multiplier is intended for driving a
receiver with a quadrature clock divider, like the QRP Labs receiver:
https://www.qrp-labs.com/receiver.html