spi_butterfly - parport-to-butterfly adapter driver
  ===================================================
  
  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 ciruit 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.

  
  	Signal	  Butterfly	  Parport (DB-25)
  	------	  ---------	  ---------------
  	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
  
  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.
  
  	Signal	  Butterfly	  Parport (DB-25)
  	------	  ---------	  ---------------
  	VCC	= J400.VCC_EXT	= pin 7/D5
  	SELECT	= J400.PB0/nSS	= pin 17/C3,nSELECT
  	GND	= J400.GND	= pin 24/GND

  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:

  
  	Signal	  Butterfly	  Parport (DB-25)
  	------	  ---------	  ---------------
  	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