linux/Documentation/spi/butterfly
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   1spi_butterfly - parport-to-butterfly adapter driver
   2===================================================
   3
   4This is a hardware and software project that includes building and using
   5a parallel port adapter cable, together with an "AVR Butterfly" to run
   6firmware for user interfacing and/or sensors.  A Butterfly is a $US20
   7battery powered card with an AVR microcontroller and lots of goodies:
   8sensors, LCD, flash, toggle stick, and more.  You can use AVR-GCC to
   9develop firmware for this, and flash it using this adapter cable.
  10
  11You can make this adapter from an old printer cable and solder things
  12directly to the Butterfly.  Or (if you have the parts and skills) you
  13can come up with something fancier, providing ciruit protection to the
  14Butterfly and the printer port, or with a better power supply than two
  15signal pins from the printer port.  Or for that matter, you can use
  16similar cables to talk to many AVR boards, even a breadboard.
  17
  18This is more powerful than "ISP programming" cables since it lets kernel
  19SPI protocol drivers interact with the AVR, and could even let the AVR
  20issue interrupts to them.  Later, your protocol driver should work
  21easily with a "real SPI controller", instead of this bitbanger.
  22
  23
  24The first cable connections will hook Linux up to one SPI bus, with the
  25AVR and a DataFlash chip; and to the AVR reset line.  This is all you
  26need to reflash the firmware, and the pins are the standard Atmel "ISP"
  27connector pins (used also on non-Butterfly AVR boards).  On the parport
  28side this is like "sp12" programming cables.
  29
  30        Signal    Butterfly       Parport (DB-25)
  31        ------    ---------       ---------------
  32        SCK     = J403.PB1/SCK  = pin 2/D0
  33        RESET   = J403.nRST     = pin 3/D1
  34        VCC     = J403.VCC_EXT  = pin 8/D6
  35        MOSI    = J403.PB2/MOSI = pin 9/D7
  36        MISO    = J403.PB3/MISO = pin 11/S7,nBUSY
  37        GND     = J403.GND      = pin 23/GND
  38
  39Then to let Linux master that bus to talk to the DataFlash chip, you must
  40(a) flash new firmware that disables SPI (set PRR.2, and disable pullups
  41by clearing PORTB.[0-3]); (b) configure the mtd_dataflash driver; and
  42(c) cable in the chipselect.
  43
  44        Signal    Butterfly       Parport (DB-25)
  45        ------    ---------       ---------------
  46        VCC     = J400.VCC_EXT  = pin 7/D5
  47        SELECT  = J400.PB0/nSS  = pin 17/C3,nSELECT
  48        GND     = J400.GND      = pin 24/GND
  49
  50Or you could flash firmware making the AVR into an SPI slave (keeping the
  51DataFlash in reset) and tweak the spi_butterfly driver to make it bind to
  52the driver for your custom SPI-based protocol.
  53
  54The "USI" controller, using J405, can also be used for a second SPI bus.
  55That would let you talk to the AVR using custom SPI-with-USI firmware,
  56while letting either Linux or the AVR use the DataFlash.  There are plenty
  57of spare parport pins to wire this one up, such as:
  58
  59        Signal    Butterfly       Parport (DB-25)
  60        ------    ---------       ---------------
  61        SCK     = J403.PE4/USCK = pin 5/D3
  62        MOSI    = J403.PE5/DI   = pin 6/D4
  63        MISO    = J403.PE6/DO   = pin 12/S5,nPAPEROUT
  64        GND     = J403.GND      = pin 22/GND
  65
  66        IRQ     = J402.PF4      = pin 10/S6,ACK
  67        GND     = J402.GND(P2)  = pin 25/GND
  68
  69
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