C++ code to parse the Japanese JJY radio signal using a Seiko SM9501A decoder IC
The Japanese government maintains two AM transmitters that combine to cover the entirety of the country. These transmitters send a time signal that can be used by clocks and other devices to automatically set their time.
I discovered this by accident when I found a radio in a store that claimed to set itself and cost USD $8.
I tore apart the clock to find the RF radio and decoder chip so I could sniff the signals
Wrote the software found here to reverse the signals and how to interface with the decoder chip.
Then built a self-setting Nixie Tube clock
The C++ code in jjy.cpp was written for the Arduino environment and a very cheap Atmel microcontroller. It runs in about 1K each of RAM/FLASH.
The process of reverse engineering the protocol etc. is documented here Pulling Time from the Sky