I desoldered a Holtek HT9290A from a old alarm system and tried loading a number into it's registers with various methods to get it to generate DTMF. The HT9290A and B use a 2 wire bus for communication in 5 bit chunks. Thought it would be relatively quick and easy hack.
Initially, I used BusPirate 4 to generate the signals and the HT9290A test circuit from the datasheet on a breadboard. Some success, but inconsistent results.
I switched to an Arduino Nano to have more control over timing and to try to read the data after writing. This led to putting the test circuit on a prototype PCB. Hooking it up to a scope and eventually using extra pins on the Arduino Nano to trigger the scope and give some more debugging information about timing of events.
Inspired by a tweet regarding Ken Shirriff's juror duty.