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README cleanups.

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Dave 2020-04-14 00:49:06 -05:00
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37
hardware/interface-ascii-atmega2560/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.
output, supporting keyboards of up to 16 rows X 8 columns. This design uses the ATMega2560, since it's an easy port from the ATMega328P, which was initially selected for the sole reason of being familiar to hobbyists. The 2560 has much more I/O, which simplifies the design, assembly, and firmware, and opens up some additional possibilities.
<img alt="Assembly Rendering" src="images/PCB-assembly-rendering.png" height=75% width=75%>
@ -22,24 +14,33 @@ even slightly simplify the hardware layer of the firmware.
- Supports up to 3 keyboard LEDs
- Supports up to 3 "special" host outputs, such as RESET, SCREEN_CLEAR, BREAK, etc.
## Differences from ATMega328P version
- Backward compatible with the ATMega328P version, with following enhancements:
1. The Output2 can be a regualar TTL, open-collector, or open-emitter. In the
328P version, changing between regulary TTL output and open-collector on
Output2 required changing an IC.
1. The serial port no longer shares lines with the parallel output port, so both
serial I/O and parallel output can be used at the same time without risk of
conflict. However, serial I/O is not yet supported in the firmware.
1. Because the rows are no longer driven by a 4-to-16 decoder circuit, and the
columns are no longer read via a shift register, the row and column lines can
be used for both input and output, which allows this board to interface with
OSI keyboards directly (not yet supported in 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.
- Two GPIO ports are used to drive the row outputs, and one GPIO port is used to
read the columns.
- One 8-bit port is used for the parallel ASCII output.
- Three GPIO lines are used to generate special outputs to the host. These could
be RESET, BREAK, CLEAR, etc. These may be configured as open-collector (Hi-Z
for HI, GND for LO), or open-emitter (5V for high, Hi-Z for LO).
- Three GPIO lines are used to control keyboard LEDs.
- The DIP switch is wired into row 15 (last row) of the matrix. In the future,
the DIP switch will be moved to row 9 to reduce RAM usage and speed up key scanning.
- Three GPIO lines are used to control keyboard LED, or other keyboard functions.
- The DIP switch is wired into row 8 (of 0-15), to reduce RAM usage and speed up
key scanning.
- A serial (UART) port is provided. 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 source to send to the host as parallel ASCII data.
## Assembly Notes
1. Solder the surface-mount microcontroller first. Be sure to match pin 1 to the

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hardware/interface-ascii/README.md
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open-collector drivers.
- Three GPIO lines are used to control keyboard LEDs.
- The DIP switch is wired into row 15 (last row) of the matrix. In the future,
the DIP switch will be moved to row 9 to reduce RAM usage and speed up key scanning.
- The DIP switch is wired into row 8 (of 0-15) to reduce RAM usage and speed up
key scanning.
- 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