Writing to a AT28 EEPROM from a modern environment

The Arduino Uno is a very popular platform based on the
ATMega328p. While Collapse OS doesn't run on AVR MCUs (yet?),
the Arduino can be a handy tool, which is why we have recipes
for it here.

In this recipe, we'll build ourselves an ad-hoc EEPROM holder
which is designed to be driven from an Arduino Uno.

Gathering parts

* An Arduino Uno
* A AT28C64B
* 2 '164 shift registers
* Sockets, header pins, proto board, etc.
* AVRA[1] (will some day rewrite to Collapse OS' ASM)
* avrdude to send program to Arduino

Schema

Schema is at img/at28wr.jpg.

This is a rather simple circuit which uses 2 chained '164 shift
register to drive the AT28 address pins and connects CE, WE, OE
and the data pins directly to the Arduino. Pins have been chosen
so that the protoboard can plug directly on the Arduino's right
side (except for VCC, which needs to be wired).

PD0 and PD1 are not used because they're used for the UART.

AT28 selection pins are pulled up to avoid accidental writes due
to their line floating before Arduino's initialization.

I've put 1uf decoupling caps next to each IC.

Software

The software in code/at28wr.asm listens to the UART and writes
every byte it receives to the AT28, starting at address 0. It
expects tty-escaped content (see /tools/ttysafe).

After having written the byte, it re-reads it from the EEPROM
and spits it back to the UART, tty-escaped.

Usage

After you've build and sent your binary to the Arduino with
"make send", you can send your (tty-safe!) content to your
EEPROM using /tools/pingpong.

[1]: http://avra.sourceforge.net/