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84 lines
3.3 KiB
Markdown
84 lines
3.3 KiB
Markdown
# Interfacing a PS/2 keyboard
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Serial connection through ACIA is nice, but you are probably plugging a modern
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computer on the other side of that ACIA, right? Let's go a step further away
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from those machines and drive a PS/2 keyboard directly!
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## Goal
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Have a PS/2 keyboard drive the stdio input of the Collapse OS shell instead of
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the ACIA.
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## Gathering parts
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* A RC2014 Classic that could install the base recipe
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* A PS/2 keyboard. A USB keyboard + PS/2 adapter should work, but I haven't
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tried it yet.
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* A PS/2 female connector. Not so readily available, at least not on digikey. I
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de-soldered mine from an old motherboard I had laying around.
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* ATtiny85/45/25 (main MCU for the device)
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* 74xx595 (shift register)
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* 40106 inverter gates
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* Diodes for `A*`, `IORQ`, `RO`.
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* Proto board, RC2014 header pins, wires, IC sockets, etc.
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* [AVRA][avra]
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## Building the PS/2 interface
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Let's start with the PS/2 connector, which has two pins:
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Both are connected to the ATtiny45, `CLK` being on `PB2` to have `INT0` on it.
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The `DATA` line is multi-use. That is, `PB1` is connected both to the PS/2 data
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line and to the 595's `SER`. This saves us a precious pin.
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The ATtiny 45 hooks everything together. `CE` comes from the z80 bus, see below.
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This allows us to supply the z80 bus with data within its 375ns limits. `SRCLR`
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is hooked to the `CE` line so that whenever a byte is read, the 595 is zeroed
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out as fast as possible so that the z80 doesn't read "false doubles".
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The 595, to have its `SRCLR` becoming effective, needs a `RCLK` trigger, which
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doesn't happen immediately. It's the ATtiny45, in its `PCINT` interrupt, that
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takes care of doing that trigger (as fast as possible).
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Our device is read only, on one port. That makes the "Chip Enable" (`CE`)
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selection rather simple. In my design, I chose the IO port 8, so I inverted
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`A3`. I chose a 40106 inverter to do that, do as you please for your own design.
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I wanted to hook `CE` to a flip flop so that the MCU could relax a bit more
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w.r.t. reacting to its `PB4` pin changes, but I didn't have NAND gates that are
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fast enough in stock, so I went with this design. But otherwise, I would
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probably have gone the flip-flop way. Seems more solid.
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## Using the PS/2 interface
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To use this interface, you have to build a new Collapse OS binary. We'll use
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the xcomp unit from the base recipe and modify it.
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First, we need a `(ps2kc)` routine. In this case, it's easy, it's
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`: (ps2kc) 8 PC@ ;`. Add this after ACIA loading. Then, we can load PS/2
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subsystem. You add `411 414 LOADR`. Then, at initialization, you add `PS2$`
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after `ACIA$`. You also need to define `PS2_MEM` at the top. You can probably
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use `RAMSTART + 0x7a`.
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Rebuild, reflash, should work. For debugging purposes, you might not want to
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go straight to plugging PS/2 `(key)` into the system. What I did myself was
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to load the PS/2 subsystem *before* ACIA (which overrides with its own `(key)`)
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and added a dummy word in between to access PS/2's key.
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Also (and this is a TODO: investigate), I had a problem where the break key I
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got from `(ps2kc)` was 0x70 instead of 0xf0 which had the effect of duplicating
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all my keystrokes. I added a 0x70 -> 0xf0 replacement in my version of
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`(ps2kc)`. Does the trick (at the cost of a non-functional numpad 0).
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[avra]: https://github.com/hsoft/avra
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