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125 lines
4.8 KiB
Plaintext
125 lines
4.8 KiB
Plaintext
# 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
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at all! We're going to build an adapter for a PS/2 keyboard to
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plug as a SMS controller.
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The PS/2 logic will be the same as the regular PS/2 adapter (see
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doc/hw/ps2.txt) but instead of interfacing directly with the
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bus, we interface with the SMS' controller subsystem (that is,
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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
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ourselves to 7bit ASCII and put TH, TR and TL as inputs". That
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could work, except that the SMS will have no way reliable way
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(except timers) of knowing whether polling two identical values
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is the result of a repeat character or because there is no new
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value yet.
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On the AVR side, there's not way to know whether the value has
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been read, so we can't to like on the RC2014 and reset the value
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to zero when a RO request is made.
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We need communication between the SMS and the PS/2 adapter to be
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bi-directional. That bring the number of usable pins down to 6,
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a bit low for a proper character range. So we'll fetch each
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character in two 4bit nibbles. TH is used to select which nibble
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we want.
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TH going up also tells the AVR MCU that we're done reading the
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character and 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
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keep up with the rapid z80 polling pace. In the regular adapter,
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I hooked CE directly on the AVR, but that was a bit tight
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because the MCU is barely fast enough to handle this signal
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properly. I did that because I had no proper IC on hand to build
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a SR latch.
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In this recipe, I do have a SR latch on hand, so I'll use it. TH
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triggering will also trigger that latch, indicating to the MCU
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that it can load the next character in the '164. When it's done,
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we signal the SMS that the next char is ready by resetting the
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latch. That means that we have to hook the latch's output to TR.
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Nibble selection on TH doesn't involve the AVR at all. All 8
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bits are pre-loaded on the '164. We use a 4-channel multiplexer
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to make TH select 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,
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but I haven't tried it yet.
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* A PS/2 female connector.
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* A SMS controller you can cannibalize for the DB-9 connection.
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A stock DB-9 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
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debug it. I could obviously not hope for it to work as a
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keyboard adapter on the first time, right on port A, driving the
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shell. I braced myself mentally for a logic analyzer session and
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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
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shell with the D-pad isn't fun at all, but it's possible. So I
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hacked myself a temporary debug kernel with a "a" command doing
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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
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poking-around and crossing-fingers was involved, but overall, I
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think it was much less effort than creating a full test setup.
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There's a certain satisfaction to debug a device entirely on
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your target machine...
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# Building the PS/2 interface
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See schematic at img/ps2-to-sms.png. The PS/2-to-AVR part is
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identical to doc/hw/ps2.txt.
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We control the '164 from the AVR in a similar way to what we did
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in rc2014/ps2, that is, sharing the DATA line with PS/2 (PB1).
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We clock the '164 with PB3. Because the '164, unlike the '595,
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is unbuffered, no need for special RCLK provisions.
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Most of the wiring is between the '164 and the '157. Place them
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close. The 4 outputs on the '157 are hooked to the first 4 lines
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on the DB-9 (Up, Down, Left, Right).
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In my prototype, I placed a 1uf decoupling cap next to the AVR.
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I used a 10K resistor as a pull-down for the TH line (it's not
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always driven).
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If you use a 4043, don't forget to wire EN. On the '157, don't
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forget to wire ~G.
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The code expects a SR-latch that works like a 4043, that is, S
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and R are triggered high, S makes Q high, R makes Q low. R is
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hooked to PB4. S is hooked to TH (and also the A/B on the '157).
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Q is hooked to PB0 and TL.
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The code for the ATtiny is in code/smsps2ctl.fs. You can build
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it with /cvm/avra.sh.
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# Building the binary
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We start with the base SMS xcomp and add a few things:
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1. at the top: "SYSVARS 0x72 + CONSTANT PS2_MEM"
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2. After VDP load: "621 LOAD : (ps2kc) (ps2kcB) ;" (that binds
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us to port B)
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3. Right after: "411 414 LOADR" (that gives us "(key)")
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4. After "VDP$": "PS2$".
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Rebuild, send to SMS, then run with your keyboard interface
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plugged to PortB. It should mostly work. There are still a few
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glitches to iron out...
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