unified_retro_keyboard/hardware/interface-ascii-atmega2560/README.md

# 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.

<img alt="Assembly Rendering" src="images/PCB-assembly-rendering.png" height=75% width=75%>

## Features
- Parallel or serial output
- Up to 8 configuration settings via an up-to-8 position DIP switch
- Apple 1, Apple 2, and SOL-20 compatible outputs. Other configurations can be
  supported by making a custom cable.
- Can decode arbitrary keyboards up 16 rows by 8 columns.
- Supports up to 3 keyboard LEDs
- Supports up to 3 "special" host outputs, such as RESET, SCREEN_CLEAR, BREAK, etc.

## 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.
- 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.

- 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
   dot on the silk-screen layer.
1. Next, install the surface mount crysta, Y1.  The orientation is not important.
1. Next, solder in the Diodes D1-D3 and D4-D11
1. Do not populate diodes D12-D27. These diodes are jumpered short. If you are
   using a key matrix with no diodes, then cut the jumpers with an x-acto knife
   and populate the diodes.
1. Install all the capacitors. These are all 0.1 uF ceramic capacitors with 0.1"
   lead spacing. Many capacitors with 0.2" lead spacing are actually 0.1"
   emerging from the body, bent to 0.2", and can be straightened back to 0.1".
1. Next, solder in the resitor R1 and Resistor network RN1.
1. Install DIP switch SW1.
1. Install connector J7 
1. Install the In-circuit Serial Programming (ISP) header, J6. (right-angle, 2x3
   0.100")
1. (Optional) If you will be the digital outputs, or if you will be attaching
   switches or other sources of open-collector signals (such as RESET), then
   install connector J4 (right-angle, 1x5 0.100")
1. (Optional) If you will be using the serial port, install the serial header
   J3. (right angle, 1x4 0.100")
1. If you will be using the Apple 1 connector, install the DIP socket J1 (16-pin
   dip). If you plan to insert and remove the cable many times, a dual-wipe
   socket may be preferable to machine-pin, since it has a lower insertion
   force, and you will be less likely to bend pins. If you plan to insert the
   cable once and leave it forever, I suggest a machine-pin DIP socket.
1. If you will be using the Apple 2 connector, install the DIP socket J2 (16-pin
   dip).  The note for the Apple 1 connector selection applies here as well.
1. If you will be using the SOL-20 connector, install connector J5 (vertical
   2x10 0.100")

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 prevent conflicts between high and low keyboard
driver outputs. They allow the row drivers to pull rows low, but not high,
emulating open collector outputs.

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.

If you are attaching a keyboard with a diode per key, then the diodes on the keys perform the same function, in addition to preventing "ghosting", so the per-row diodes do not need to be installed.

### 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 (including the DIP switch).

### The 74LS07 hex buffer (U5) and pullup R6
The 74LS07 is 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.