Q: What does the TMGRXPREAMBLE register do?

A: Please have a look at the “Receiver State Diagram” in the programming manual on page 19, as well as the description of the individual registers.

The way the chip normally operates is that RX starts with parameter set 0 (RADIOSTATE = 0x0C). If the MATCH1 unit matches preamble then the radio changes to parameter set 1 (RADIOSTATE = 0x0D). If the MATCH0 unit matches the sync word then the radio changes to parameter set 3 (RADIOSTATE = 0x0D).

With the help of the TMXRXPREAMBLE registers you can allow/forbid the various state transitions and define timeouts after which RADIOSTATE falls back if the MATCH conditions goes away.