mirror of
https://github.com/hsoft/collapseos.git
synced 2024-11-14 14:58:05 +11:00
116 lines
4.9 KiB
Markdown
116 lines
4.9 KiB
Markdown
# PS/2 keyboard on the SMS
|
|
|
|
Using the shell with a D-pad on the SMS is doable, but not fun at all! We're
|
|
going to build an adapter for a PS/2 keyboard to plug as a SMS controller.
|
|
|
|
The PS/2 logic will be the same as the [RC2014's PS/2 adapter][rc2014-ps2] but
|
|
instead of interfacing directly with the bus, we interface with the SMS'
|
|
controller subsystem (that is, what we poke on ports `0x3f` and `0xdc`).
|
|
|
|
How will we achieve that? A naive approach would be "let's limit ourselves to
|
|
7bit ASCII and put `TH`, `TR` and `TL` as inputs". That could work, except that
|
|
the SMS will have no way reliable way (except timers) of knowing whether polling
|
|
two identical values is the result of a repeat character or because there is no
|
|
new value yet.
|
|
|
|
On the AVR side, there's not way to know whether the value has been read, so we
|
|
can't to like on the RC2014 and reset the value to zero when a `RO` request is
|
|
made.
|
|
|
|
We need communication between the SMS and the PS/2 adapter to be bi-directional.
|
|
That bring the number of usable pins down to 6, a bit low for a proper character
|
|
range. So we'll fetch each character in two 4bit nibbles. `TH` is used to select
|
|
which nibble we want.
|
|
|
|
`TH` going up also tells the AVR MCU that we're done reading the character and
|
|
that the next one can come up.
|
|
|
|
As always, the main problem is that the AVR MCU is too slow to keep up with the
|
|
rapid z80 polling pace. In the RC2014 adapter, I hooked `CE` directly on the
|
|
AVR, but that was a bit tight because the MCU is barely fast enough to handle
|
|
this signal properly. I did that because I had no proper IC on hand to build a
|
|
SR latch.
|
|
|
|
In this recipe, I do have a SR latch on hand, so I'll use it. `TH` triggering
|
|
will also trigger that latch, indicating to the MCU that it can load the next
|
|
character in the '164. When it's done, we signal the SMS that the next char is
|
|
ready by resetting the latch. That means that we have to hook the latch's output
|
|
to `TR`.
|
|
|
|
Nibble selection on `TH` doesn't involve the AVR at all. All 8 bits are
|
|
pre-loaded on the '164. We use a 4-channel multiplexer to make `TH` select
|
|
either the low or high bits.
|
|
|
|
## Gathering parts
|
|
|
|
* A SMS that can run Collapse OS
|
|
* A PS/2 keyboard. A USB keyboard + PS/2 adapter should work, but I haven't
|
|
tried it yet.
|
|
* A PS/2 female connector. Not so readily available, at least not on digikey. I
|
|
de-soldered mine from an old motherboard I had laying around.
|
|
* A SMS controller you can cannibalize for the DB-9 connection. A stock DB-9
|
|
connector isn't deep enough.
|
|
* ATtiny85/45/25 (main MCU for the device)
|
|
* 74xx164 (shift register)
|
|
* 74xx157 (multiplexer)
|
|
* A NOR SR-latch. I used a 4043.
|
|
* Proto board, wires, IC sockets, etc.
|
|
|
|
## Historical note
|
|
|
|
As I was building this prototype, I was wondering how I would debug it. I could
|
|
obviously not hope for it to work as a keyboard adapter on the first time, right
|
|
on port A, driving the shell. I braced myself mentally for a logic analyzer
|
|
session and some kind of arduino-based probe to test bit banging results.
|
|
|
|
And then I thought "why not use the genesis?". Sure, driving the shell with the
|
|
D-pad isn't fun at all, but it's possible. So I hacked myself a temporary debug
|
|
kernel with a "a" command doing a probe on port B. It worked really well!
|
|
|
|
It was a bit less precise than logic analyzers and a bit of poking-around and
|
|
crossing-fingers was involved, but overall, I think it was much less effort
|
|
than creating a full test setup.
|
|
|
|
There's a certain satisfaction to debug a device entirely on your target
|
|
machine...
|
|
|
|
## Building the PS/2 interface
|
|
|
|
|
|
|
|
The PS/2-to-AVR part is identical to the rc2014/ps2 recipe. Refer to this
|
|
recipe.
|
|
|
|
We control the '164 from the AVR in a similar way to what we did in rc2014/ps2,
|
|
that is, sharing the DATA line with PS/2 (PB1). We clock the '164 with PB3.
|
|
Because the '164, unlike the '595, is unbuffered, no need for special RCLK
|
|
provisions.
|
|
|
|
Most of the wiring is between the '164 and the '157. Place them close. The 4
|
|
outputs on the '157 are hooked to the first 4 lines on the DB-9 (Up, Down, Left,
|
|
Right).
|
|
|
|
In my prototype, I placed a 1uf decoupling cap next to the AVR. I used a 10K
|
|
resistor as a pull-down for the TH line (it's not always driven).
|
|
|
|
If you use a 4043, don't forget to wire EN. On the '157, don't forget to wire
|
|
~G.
|
|
|
|
The code expects a SR-latch that works like a 4043, that is, S and R are
|
|
triggered high, S makes Q high, R makes Q low. R is hooked to PB4. S is hooked
|
|
to TH (and also the A/B on the '157). Q is hooked to PB0 and TL.
|
|
|
|
## Usage
|
|
|
|
The code in this recipe is set up to listen to the keyboard on port B, leaving
|
|
port A to drive, for example, an Everdrive with a D-pad. Unlike the generic
|
|
SMS recipe, this kernel has no character selection mechanism. It acts like a
|
|
regular shell, taking input from the keyboard.
|
|
|
|
`kernel/sms/kbd.asm` also has a FetchKC implementation for port A if you prefer.
|
|
Just hook it on. I've tried it, it works.
|
|
|
|
Did you get there? Feels pretty cool huh?
|
|
|
|
[rc2014-ps2]: ../../rc2014/ps2
|