Controlling GPIO pins over SPI from a Gigatron

Gigatron TTL microcomputer

This Gigatron has a prototype I/O expander with 4 serial peripheral interface (SPI) ports. Here one port is hooked to the MCP23S17 chip on the little breadboard. This chip has 16 GPIO pins, I connected 8 to the LEDs. Next I toggled in (using the WozMon!) a little vCPU program that makes them count.

If we can control LEDs this way, we can also read MicroSD cards. That will be next...