Overview
--------

This is a small project to implement a connection such that binary files can
be downloaded to an Amstrad CPC from a Raspberry Pi via the joystick port.

The circuitry is such that GPIO pins are connected via transistors and mapped
to joystick directions:

	GPIO 2	-	Fire
	GPIO 3	-	Up
	GPIO 4 	-	Down
	GPIO 5	-	Left
	GPIO 6	-	Right

The idea is that a nibble is placed on the direction pins.  Once these are set
the fire pin is activated and held for a period.  Then the fire is released.
The next nibble is fired in the same way and this finishes the process for
sending a byte.

On the CPC side when reading the joystick via KM_GET_JOYSTICK(0xbb24) the
firmware docs reveal this bit layout:

Bit	Direction/Fire
0	Up
1	Down
2	Left
3	Right
4	Fire2
5	Fire1
6	Spare
7	Always zero

However KM_GET_JOYSTICK only reads the joystick every 1/50th of seconds, so this
is too slow for our purposes.  As such the joystick has to be read directly.
When read directly the bits are reversed (i.e. a bit is set if that direction
isn't pressed) so the results have to be xored with 255 to get the value we
want.

As the 4 directions are already arranged in the low nibble this layout is
maintained for the protocol, so that this is the mapping between the GPIO
pins and the info to send:

Bit	GPIO Pin
0	3
1	4
2	5
3	6
4	2


Z80 code
--------

The z80 directory contains code to read the information sent over the joystick
port.  xfer.z80 is for the basic file transfer.  copydisk.z80 is a more
complicated process used to transfer and write disk images.

Included is the text of a BASIC loader (loadxfer.bas) to poke in the results of
assembling xfer.z80.  This should be used to get the initial code across to the
CPC, which will involve some typing.  Needless to say, I'd save the code before
running it.


Pi code
-------

This comprises two command line tools.  cpcxfer is to transfer a file.
cpcdiskxfer is to transfer a disk image.  Usage is as follows:

cpcxfer file start_address milliseconds

    file is the file to send.

    start_address is the start address of the file.  Hex numbers can be
    preceded with 0x.

    milliseconds is the number of milliseconds to hold the fire button high
    and then the number of milliseconds to hold it off.  Hence one nibble is
    transferred every 2 * milliseconds.

cpcdiskxfer disk_image milliseconds

    disk_image is the DSK image to send.

    milliseconds is the number of milliseconds to hold the fire button high
    and then the number of milliseconds to hold it off.  Hence one nibble is
    transferred every 2 * milliseconds.


File transfer protocol
----------------------

This is a simple protocol using the following format:

Byte 0	-	Destination address low byte.
Byte 1	-	Destination address high byte.
Byte 2	-	File length low byte.
Byte 3	-	File length high byte.
Byte n	-	File length bytes.


Disk image transfer protocol
----------------------------

This protocol follows the following format.  Note that double sided disk
images are not supported.

First an overview packet is sent.

TODO