This code is for adding 50Hz/60Hz selection to the Sega Master System.
The selection is via the existing reset button. A bi-colour LED shows the current selection.
It came from a PCB project that Frank_fjs on Aussie Arcade needed help with.
I rewrote the code to simplify it and to alter the operation a little:
Hex file Download and Source code (C, compiles with XC8) Download
A Microchip PIC16F630 micro-controller is used, but the code could be altered to use a smaller PIC.