8251
Programmable Communication Interface
register usage
19880619/wj van ganswijk

0  rw data
1  w  command
      .......1  transmit enable
      ......1.  data terminal ready (dtr)
      .....1..  receive enable
      ....1...  send break character
      ...1....  reset error flags (pe,oe,fe)
      ..1.....  request to send (rts)
      .1......  internal reset
      1.......  enter hunt mode (enable search for sync characters)
   r  status
      .......1  transmitter ready
      ......1.  receiver ready
      .....1..  transmitter empty
      ....1...  parity error (pe)
      ...1....  overrun error (oe)
      ..1.....  framing error (fe), async only
      .1......  sync detect, sync only
      1.......  data set ready (dsr)

After internal reset, set mode with following sequence:

0  w  mode instruction
      ......00  sync mode
      ......01  async, baud rate factor is 1
      ......10  async, baud rate factor is 16
      ......11  async, baud rate factor is 64
      ....00..  character length is 5 bits
      ....01..  character length is 6 bits
      ....10..  character length is 7 bits
      ....11..  character length is 8 bits
      ...1....  parity enable
      ..1.....  even/odd parity
      .1......  sync:  external sync detect (syndet) is an input/output
      1.......  sync:  single/double character sync
      00......  async: invalid
      01......  async: 1   stop bit
      10......  async: 1.5 stop bits
      11......  async: 2   stop bits
1  w  xxxxxxxx  sync character in sync mode
2  w  xxxxxxxx  2nd sync character in sync mode with double character sync

*end