USB host->dev format: 4 bytes, LSB first

register write:
  17:00	data
  30:18 addr
  31    '0'

register block read:
  03:00 count-1
  17:16 '00'
  30:18 base addr
  31    '1'

set destination mask:
  04:00 mask for chan A
  09:05 mask for chan B
  14:10 mask for chan C
  17:16 '01'
  31    '1'

send to LED display:
  07:00 data to send
  09:08 load mask
  11:10 chan sel
  17:16 '10'
  31    '1'

set IRQ mask:
  04:00 mask for chan A
  09:05 mask for chan B
  14:10 mask for chan C
  17:16 '11'
  31    '1'

USB dev->host format: 3 bytes per packet (1-16 packets in block), LSB first

register read:
  17:00 data
  21:18 source (0-4: A, 5-9: B, 10-14: C)
  22	'1' if first in block, '0' otherwise
  23	'0'

interrupt buffer:
  17:00 data
  21:18 source (0-4: A, 5-9: B, 10-14: C)
  22	'1' if first in block, '0' otherwise
  23	'1'