Crossbar-Switch PCBs on the way

A project log for ONAN 1111

A Retrobrew TTL CPU - Classic yet Baroque

nicolas-reimenNicolas REIMEN 09/20/2022 at 08:170 Comments

The 50 PCBs for the crossbar switch are on the way! (thanks to PCBWay) Wait a minute?! Did you say fifty?!

Yes, fifty.

The crossbar switch, the heart of ONAN 1111's design, will allow data to be exchanged between (nearly all) internal components of the processor on each clock cycle. It consists of ten 20-bit wide 8-to-1 multiplexers. Each multiplexer is split into five 4-bit wide slices. 10 x 5 equals 50. QED.

Each board will contain four 74x151 clones (К155КП7), for a total of 200. Each chip is a 1-bit wide 8-to-1 multiplexer. The boards will also contain 8 LEDs and a 74x138 clone (К555ИД7) to indicate which data-path is selected, when the processor is running in single-step mode or at a low clock speed. This alone will contribute 400 blinkenlights to the overall design. What is a retrobrew project without the blinkenlights ? ...

Below is a Kicad-generated 3D picture of one of the boards:

This one is the multiplexer that will feed a loading value to the microprogram counter (uPC) whenever necessary (jumps, interrupts, ...) The dip-switches will be used to enter the address of the interrupt-handling routine.