0=No action
1=DART: Reserved, SIO: Reset Receive CRC Checker
2=DART: Reserved, SIO: Reset Transmit CRC Generator
3=DART: Reserved, SIO: Reset Tx Underrun/End of Message latch
2=SDLC Mode (0111 1110 Flag) (SIO only, not DART)
3=External SYNC Mode (SIO only, not DART)
6-7 Rx/Tx DART clock mode (0..3=X1, X16, X32, X64)