MC6850Tester: Using a Raspberry Pi to test a MC6850 ACIA

One challenge of retro-computing is obtaining chips that are no longer manufactured. Some sellers offer damaged or even falsified chips. In this project, I used a Raspberry Pi Pico to build a tester for the MC6850, a popular chip for adding serial communication to old microprocessors.

You can find a description of the project at hackester.io. Here, I will only highlight some RP2040 specifics:

• The software was written using the Pico SDK, for better control of the pins. Using the SDK functions, the eight data pins can be turned around, read, and written with a single call.
• The PWM peripheral is used to generate the MC6850 clock. The RP2040 is clocked at 200 MHz, and the PWM's fractional divider provides high-precision output.
• The MC6050 operates at 5 V. The Pico accepts 5 V signals if it is already powered. To ensure this condition and allow safe changes to the MC6850, the 5 V power is controlled by the Pico.

In the future, I may implement the reading from and writing to the MC6850 with the PIO, to get more precise timings.

The full code and schematic can be found at https://github.com/dquadros/MC6850Tester