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@@ -0,0 +1,111 @@ +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 |