linux/Documentation/spi/butterfly.rst
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   2spi_butterfly - parport-to-butterfly adapter driver
   3===================================================
   4
   5This is a hardware and software project that includes building and using
   6a parallel port adapter cable, together with an "AVR Butterfly" to run
   7firmware for user interfacing and/or sensors.  A Butterfly is a $US20
   8battery powered card with an AVR microcontroller and lots of goodies:
   9sensors, LCD, flash, toggle stick, and more.  You can use AVR-GCC to
  10develop firmware for this, and flash it using this adapter cable.
  11
  12You can make this adapter from an old printer cable and solder things
  13directly to the Butterfly.  Or (if you have the parts and skills) you
  14can come up with something fancier, providing circuit protection to the
  15Butterfly and the printer port, or with a better power supply than two
  16signal pins from the printer port.  Or for that matter, you can use
  17similar cables to talk to many AVR boards, even a breadboard.
  18
  19This is more powerful than "ISP programming" cables since it lets kernel
  20SPI protocol drivers interact with the AVR, and could even let the AVR
  21issue interrupts to them.  Later, your protocol driver should work
  22easily with a "real SPI controller", instead of this bitbanger.
  23
  24
  25The first cable connections will hook Linux up to one SPI bus, with the
  26AVR and a DataFlash chip; and to the AVR reset line.  This is all you
  27need to reflash the firmware, and the pins are the standard Atmel "ISP"
  28connector pins (used also on non-Butterfly AVR boards).  On the parport
  29side this is like "sp12" programming cables.
  30
  31        ======    =============   ===================
  32        Signal    Butterfly       Parport (DB-25)
  33        ======    =============   ===================
  34        SCK       J403.PB1/SCK    pin 2/D0
  35        RESET     J403.nRST       pin 3/D1
  36        VCC       J403.VCC_EXT    pin 8/D6
  37        MOSI      J403.PB2/MOSI   pin 9/D7
  38        MISO      J403.PB3/MISO   pin 11/S7,nBUSY
  39        GND       J403.GND        pin 23/GND
  40        ======    =============   ===================
  41
  42Then to let Linux master that bus to talk to the DataFlash chip, you must
  43(a) flash new firmware that disables SPI (set PRR.2, and disable pullups
  44by clearing PORTB.[0-3]); (b) configure the mtd_dataflash driver; and
  45(c) cable in the chipselect.
  46
  47        ======    ============    ===================
  48        Signal    Butterfly       Parport (DB-25)
  49        ======    ============    ===================
  50        VCC       J400.VCC_EXT    pin 7/D5
  51        SELECT    J400.PB0/nSS    pin 17/C3,nSELECT
  52        GND       J400.GND        pin 24/GND
  53        ======    ============    ===================
  54
  55Or you could flash firmware making the AVR into an SPI slave (keeping the
  56DataFlash in reset) and tweak the spi_butterfly driver to make it bind to
  57the driver for your custom SPI-based protocol.
  58
  59The "USI" controller, using J405, can also be used for a second SPI bus.
  60That would let you talk to the AVR using custom SPI-with-USI firmware,
  61while letting either Linux or the AVR use the DataFlash.  There are plenty
  62of spare parport pins to wire this one up, such as:
  63
  64        ======    =============   ===================
  65        Signal    Butterfly       Parport (DB-25)
  66        ======    =============   ===================
  67        SCK       J403.PE4/USCK   pin 5/D3
  68        MOSI      J403.PE5/DI     pin 6/D4
  69        MISO      J403.PE6/DO     pin 12/S5,nPAPEROUT
  70        GND       J403.GND        pin 22/GND
  71
  72        IRQ       J402.PF4        pin 10/S6,ACK
  73        GND       J402.GND(P2)    pin 25/GND
  74        ======    =============   ===================
  75