Documentation/spi/butterfly.rst
This is a hardware and software project that includes building and using a parallel port adapter cable, together with an "AVR Butterfly" to run firmware for user interfacing and/or sensors. A Butterfly is a $US20 battery powered card with an AVR microcontroller and lots of goodies: sensors, LCD, flash, toggle stick, and more. You can use AVR-GCC to develop firmware for this, and flash it using this adapter cable.
You can make this adapter from an old printer cable and solder things directly to the Butterfly. Or (if you have the parts and skills) you can come up with something fancier, providing circuit protection to the Butterfly and the printer port, or with a better power supply than two signal pins from the printer port. Or for that matter, you can use similar cables to talk to many AVR boards, even a breadboard.
This is more powerful than "ISP programming" cables since it lets kernel SPI protocol drivers interact with the AVR, and could even let the AVR issue interrupts to them. Later, your protocol driver should work easily with a "real SPI controller", instead of this bitbanger.
The first cable connections will hook Linux up to one SPI bus, with the AVR and a DataFlash chip; and to the AVR reset line. This is all you need to reflash the firmware, and the pins are the standard Atmel "ISP" connector pins (used also on non-Butterfly AVR boards). On the parport side this is like "sp12" programming cables.
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Signal Butterfly Parport (DB-25)
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SCK J403.PB1/SCK pin 2/D0
RESET J403.nRST pin 3/D1
VCC J403.VCC_EXT pin 8/D6
MOSI J403.PB2/MOSI pin 9/D7
MISO J403.PB3/MISO pin 11/S7,nBUSY
GND J403.GND pin 23/GND
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Then to let Linux master that bus to talk to the DataFlash chip, you must (a) flash new firmware that disables SPI (set PRR.2, and disable pullups by clearing PORTB.[0-3]); (b) configure the mtd_dataflash driver; and (c) cable in the chipselect.
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Signal Butterfly Parport (DB-25)
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VCC J400.VCC_EXT pin 7/D5
SELECT J400.PB0/nSS pin 17/C3,nSELECT
GND J400.GND pin 24/GND
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Or you could flash firmware making the AVR into an SPI slave (keeping the DataFlash in reset) and tweak the spi_butterfly driver to make it bind to the driver for your custom SPI-based protocol.
The "USI" controller, using J405, can also be used for a second SPI bus. That would let you talk to the AVR using custom SPI-with-USI firmware, while letting either Linux or the AVR use the DataFlash. There are plenty of spare parport pins to wire this one up, such as:
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Signal Butterfly Parport (DB-25)
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SCK J403.PE4/USCK pin 5/D3
MOSI J403.PE5/DI pin 6/D4
MISO J403.PE6/DO pin 12/S5,nPAPEROUT
GND J403.GND pin 22/GND
IRQ J402.PF4 pin 10/S6,ACK
GND J402.GND(P2) pin 25/GND
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