I2C interface to the firmware
-----------------------------

To control the operation of the keyboard's firmware, the firmware
exposes some "registers" that the user can read/write using the
two kinds of I2C transfers.

1) set values of consecutive block of registers starting from REG_ADDR:
  
  START TX(DEV_ADDR+WR REG_ADDR [REG_DATA REG+1_DATA REG+2_DATA ...]) STOP

2) read values of consecutive block of registers starting from REG_ADDR:

  START TX(DEV_ADDR+WR REG_ADDR) STOP
  START TX(DEV_ADDR+RD) RX(REG_ADDR [REG_DATA REG+1_DATA REG+2_DATA ...]) STOP

Device address is 0x15.


Registers
---------

0x00: Device ID1 (0x4b)
0x01: Device ID2 (0x42)
0x02: Firmware revision
0x03: System configuration
   bit 0: disable KB scanning (1: scanning disabled, 0: scanning enabled)
   bit 1: poll mode
          1: don't rely on row change detection, poll the matrix periodically
             (prevents MCU power down)
          0: power down the MCU when no key is pressed, and rely on change
             detection on row inupts to wake the MCU up
   bit 2: enable USB debug interface
          1: enabled
          0: disabled

0x10: Keyboard data for column 1
...
0x0b: Keyboard data for column 12
0x0c: CRC8 of keyboard data from 0x10-0x0b


0x70: Flashing mode unlock key
      (writing 0x46 to this register unlocks the flashing mode.)
0x71: Flashing control
      Writing various commands to this register makes the MCU execute them,
      if the MCU is not executing the previous command. Available commands:
      0x52 - read a block of data from code ROM
      0x57 - write a block of data to code ROM
      0x45 - erase the code rom memory block (128B)
      0x43 - confirm the validity of the firmware and enable redirection
             to it from the main app (this redirection is automatically
             disabled before executing the 0x57 command)
0x7d: target address low byte
0x7e: target address high byte
0x7f: CRC8 calculated for the 128B block of data from 0x80-0xff
      - this must be written by the user when preparing data for write
        operation
      - this is update by the MCU after reading the data from flash memory
0x80: 128B block of EEPROM data (either read from code memory or to be
...   written)
0xff