I don't have a problem of using one for inherently computational intensive tasks, such as Ethernet or Wi-Fi, but for a mere keyboard, seriously?Ĩ042. But to me, having a microcontroller with processing power comparable to (or greater than) the Z80 CPU defeats the spirit of a retrocomputing project. It's the most common, easiest, flexible solution used in various retrocomputing projects. Historically, it seems a SPI controller is never used in this way. But I think it's just another clever trick to exploit the functionality on a modern microcontroller than a standard solution. Obviously, putting the widely-used SPI interface in slave mode can allow the serial transmission to be clocked by the keyboard. However, PS/2 is clocked by the keyboard and its frequency lies at anywhere between 10.0–16.7 kHz, again, this method is NOT in compliance of the protocol at all, and could only work by chance, so it's more of a clever trick than a proper solution. It sounds convenient since the USART is general-purpose, may perform buffering, parity check, and interrupt generation. Although PS/2 is not designed as a USART-compatible protocol, there are reports of success by simply running the USART at 12,000 bauds and connecting the DATA line to a USART interface, and handling the keymap decoding in software. So a parity bit is needed, also, we need to follow the timing, have some meaningful interrupts control - all of these requires extra parts, which makes it difficult to design and program (especially when routing it on a single-layer board). I don't think parity is only for farmers, especially when bidirectional transmission is also needed for controlling Caps Locks, etc. But PS/2 is 8-bit data, 1-bit parity, not 7-bit data! Both the Z80 and the shift register can only handle 8-bit of data, so the parity bit is ignored (or requires additional parts), not elegant. A simple shift register like 74HC595N should be able to translate the serial data from the keyboard to parallel, and it can be clocked by the keyboard, so it has no problem to follow the PS/2 protocol.

According to my research, this problem has been solved by the following approaches. I'd like to build a machine with straightforward I/O, that it, the design requirement: one should be able to use the computer by simply plugging a PS/2 keyboard, rather than hooking it to a RS232, USB-TTL stick or some exotic CRT terminal.īut how to incorporate a PS/2 keyboard input to my computer is unclear to me.

I planned to start from something simple, like a ROM, some SRAM, a video chip and a Z80-PIO.

I'm attempting to build a Z80 homebrew computer to teach myself the basis of electronic design.

Transistor keyboard or controller