Spit bytes through SPI from an Arduino Uno

This recipe programs an Arduino Uno to spit data from its Flash
memory through its SPI pins. This allows you to get data into
systems with as little as 2 input pins available.

The Arduino acts as an SPI master and assumes the presence of
an activated slave.

The use case for which this project was created (Apple IIe
reading SPI through its game port) needed SPI to be quite slow
because it couldn't keep up otherwise.

This is why we apply a general 16x clock divider through CLKPR
and set the SPI speed to f_osc/128. This gives us a rate of
about 8KHz, which is plenty slow for just about anything.

It transmits data in chunks of $100 bytes, beginning at address
$100. The number of chunks it transmit is read from address $ff.

The program begins transmitting on startup. To control the
moment of transmission, you use the Reset button.

While transmitting, it reads the result of the SPI exchange in
its SPI data register and spits it to UART. This way, if your
destination echoes anything and your arduino's UART is plugged
to something, you can control that echo.

Of course, due to the nature of SPI, your first byte will be
garbage and you won't get the last byte.

Gathering parts

* An Arduino Uno
* avrdude to send program to Arduino

Programming the Arduino

The program lives in arch/avr/blk.fs and is built using Collapse
OS' AVR assembler. A Makefile exists in arch/avr/spispit that
takes care of doing this automatically. "make" will yield
"spispit.bin"

Data to spit has to be placed in a file named "data" and
"make send" will combine spispit.bin and data and place the
proper number of blocks at address $ff. It will then send that
to the Arduino using avrdude.

At this point, it's ready to use.

Ignore the first 3 SCK toggles

On an Arduino Uno that has its bootloader enabled, SCK is going
to toggle 3 times before it begins spitting its payload. The
logic reading this payload has to ignore those first 3 toggles.

Check the LED

Because SCK is wired to the builtin LED on the Arduino Uno, you
can check whether we're still transmitting by looking at the
LED. At 8KHz, its blinking is visible to the naked eye.