mirror of https://github.com/hsoft/collapseos.git
80 lines
2.9 KiB
Plaintext
80 lines
2.9 KiB
Plaintext
# Interfacing a PS/2 keyboard
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Collapse OS needs a way to input commands and keyboards are one
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of the most straightforward ways to proceed. The PS/2 protocol
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is very widespread and relatively simple.
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We explain here how to interface a PS/2 keyboard with a RC2014.
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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 + adapter also works, if it's
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not too recent (if it still speaks PS/2).
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* A PS/2 female connector.
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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 (https://github.com/hsoft/avra). The code for this recipe
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hasn't been translated to Collapse OS' AVR assembler yet.
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# Building the PS/2 interface
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Let's start with the PS/2 connector (see img/ps2-conn.png),
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which has two pins.
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Both are connected to the ATtiny45, CLK being on PB2 to have
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INT0 on it.
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The DATA line is multi-use. That is, PB1 is connected both to
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the PS/2 data line and to the 595's SER. This saves us a
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precious pin.
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The ATtiny 45 (img/ps2-t45.png) hooks everything together. CE
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comes from the z80 bus (img/ps2-z80.png).
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The 595 (img/ps2-595.png) allows us to supply the z80 bus with
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data within its 375ns limits. SRCLR is hooked to the CE line so
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that whenever a byte is read, the 595 is zeroed out as fast as
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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
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trigger, which doesn't happen immediately. It's the ATtiny45, in
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its PCINT interrupt, that takes care of doing that trigger (as
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fast as possible).
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Our device is read only, on one port. That makes the "Chip
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Enable" (CE) selection rather simple. In my design, I chose the
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IO port 8, so I inverted A3. I chose a 40106 inverter to do
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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
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bit more w.r.t. reacting to its PB4 pin changes, but I didn't
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have NAND gates that are fast enough in stock, so I went with
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this design. But otherwise, I would probably have gone the
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flip-flop way. Seems more solid.
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Then, all you need to do is to assemble code/ps2ctl.asm and load
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it onto your ATtiny.
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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
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binary. This binary needs two things.
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First, we need a "(ps2kc)" routine (see doc/protocol.txt). In
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this case, it's easy, it's ": (ps2kc) 8 PC@ ;". Then, we can
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load PS/2 subsystem. You add "411 414 LOADR". Then, at
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initialization, you add "PS2$". You also need to define PS2_MEM
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at the top. You can probably use "SYSVARS + 0xaa".
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The PS/2 subsystem provides "(key)" from "(ps2kc)".
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For debugging purposes, you might not want to go straight to
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plugging PS/2 "(key)" into the system. What I did myself was to
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load the PS/2 subsystem *before* ACIA (which overrides with its
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own "(key)") and added a dummy word in between to access PS/2's
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key.
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