Description of the interface to the Freetronics Dot Matrix Display (DMD)
========================================================================

Pins:
    D6: A - interleave phase select LSB
    D7: B - interleave phase select MSB (set the MSB first)
    D8: SCLK - this is the "latch" pin that copies all previously transferred
        data from the shift register to the actual display.  Normally low,
        then flip to high and then low again to latch the data.
    D9: nOE - Output enable (when low).
    D10: Conflict detect (if it goes low, something else is using SPI)
         This is the standard SPI SS - Slave select - pin.
    D11: R - this is the standard SPI MOSI - Master out, slave in - pin.
    D12: MISO - master in, slave out - unused
    D13: CLK - standard Serial Clock pin for SPI

Initialisation:
    SPI: bit order = MSB-first, data mode = 0, clock divider = 128
        (use a smaller clock divider, such as 2, for better performance
        when driving multiple displays)

    Set A, B, CLK, SCLK, and nOE to LOW outputs
    Set R to HIGH output

    phase = 0

Every 5ms, do the following to refresh the display:
    Send the bytes for 4 interleaved rows based on phase, via SPI:
        phase == 0: 0, 4, 8, 12 (actually sent in the order 12, 8, 4, 0)
        phase == 1: 1, 5, 9, 13
        phase == 2: 2, 6, 10, 14
        phase == 3: 3, 7, 11, 15
    Then latch the data for the rows as follows:
        Set nOE to LOW
        Set SCLK to HIGH and then back to LOW
        Set A and B to indicate the current phase
        Set nOE to HIGH
    Finally, increment the phase, wrapping around from 3 to 0
    The above code assumes the following memory organisation:
        The display is W panels wide and H panels high.
        Each panel is 32 pixels wide (4 bytes) and 16 pixels high.
        Memory is organised as W * H * 4 bytes for each row, and 16 rows.
        That is, the 2D arrangement of panels is flattened into 1D for
        clocking the data out.
        In addition, a 1 bit in memory is a LED that is *off*.

Other things:
    Multiple panels need to be daisy-chained in the order N, ..., 1 with N
    plugged into the connector board on the Arduino and 1 at the end of
    the daisy-chain.  Because the shipped cables are short, multiple rows
    can be oriented in alternating order with odd-numbered rows
    (counting from 1) upside-down and reversed.  More details here:
    http://www.adebenham.com/category/arduino/dmd/