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In the beginning of Collapse OS' Forth version, the readline sub- system was optional. This is why we had this separate RDLN$ routine and that the input buffer was allocated at boot time. It's been a while since the RDLN system has been made mandatory, but we still paid the complexity tax of this separation. Not anymore.
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 0xa2 + 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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