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