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hardware/interface-ascii/README.md

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+# ASCII Interface - Atmega328p
+
+This is is a key scanner module with parallel output, and optional serial output, supporting keyboards
+of up to 16 rows X 8 columns.
+
+The ATMega 328P was selected for the sole reason that it is an architecture familiar to many hobbyists,
+with an accessible programming environment and ecosystem, in a DIP format that fits the retro look and is
+easy to solder for most hobbyists.  All of the ICs, other than the microcontroller, are only present to
+compensate for the limited number of GPIO lines on this small 28-pin microcontroller.  Using a bigger chip
+would greatly simplify the hardware and even slightly simplify the hardware layer of the firmware.
+
+## Overview
+- The keyboard rows are driven by a pair of 74LS138 decoders, allowing 4 GPIO lines to drive 16 rows.
+- The columns are read in via an 8-bit shift register, controlled by 3 GPIO lines.
+- One 8-bit port is used for the parallel ASCII output.
+- Two GPIO lines are used to generate special outputs to the host.  These could be RESET, BREAK, CLEAR, etc.
+- One GPIO line is used to control a keyboard LED.
+- Three of the ISP lines are also used as outputs to controll two more Keyboard LEDs and one more "special" output line.
+- The top two parallel I/O bits can also be configured as UART I/O.  This could be used to provide serial output instead
+or parallel output, to support a bootloader, or even to accept a serial input stream from another computer to send to the
+host as parallel ASCII data.  This last application would require careful timing to avoid conflicts.
+
+## Assembly Notes
+
+The minimum functioning circuit includes 
+1. the Microcontroller (U1)
+1. the 74LS166 shift register (U2)
+1. one 74LS138 decoder (U3).
+1. The resistor network RN1
+1. The two resistors R3 and R4.  If you are using the keyboard in only a parallel configuration or only a
+serial configuration, then you can jumper these resistors with a piece of wire.
+1. Any connectors required.
+
+## Optional components
+### Diodes D1-D16
+These diodes are intended to mitigate ghosting when used with a keyboard matrix that does not have
+any diodes installed.  These provide one diode per row.  If you are attaching a keyboard with no diodes, then you only need
+to populate the diodes corresponding to rows on the keyboard.  If the keyboard has 8 rows, then you may want to install
+8 diodes corresponding to those rows.
+
+Note that the footprints for these diodes include a copper jumper on the TOP copper layer.  If you install any of these
+diodes, you should cut the jumpers for those diodes.  Otherwise the diodes do nothing.
+
+### DIP switch and associated diodes
+The DIP switch is optional.  If you don't want to be able to set options via the DIP switches, you can set all your 
+preferences in the firmware, or just accept the default behavior, and skip the DIP switch and Diodes D17-D20 and D24-D27.
+
+### Second 74LS138 multiplexer (U4)
+Only needed if you have more than 8 rows.
+
+### The 74LS07 hex buffer (U5)
+Only needed if you are using LED2 or LED3 on the keyboard, or the OUT2 open collector output.
+
+### Diodes D21, D22, D23
+Some keyboards may not wire all keys into the matrix. For example, some other interface modules may
+assign special hardware functions to certain keys.  Those keys are brought directly to the keyboard
+connector.  For the classic keyboard, the POWER key, the '@' key, and the RUBOUT key are not directly wired.
+This module wires them into the matrix via D21, D22, and D23, respectively.  For keyboards with no separately wired
+keys, these diodes may be omitted.
+
+### Resistors R1, R2, R5
+These resistors provide current limiting for up to three keyboard LEDs.  If no keyboard LEDs are installed, then these
+resistors can be ommitted.