In a direct Rx configuration the received data is directly output to an IC pin that in turn directly feeds another MCU pin for further processing. Typically in such applications all the signal detection (in form of preamble and / or sync word detection algorithms) are done by the processing MCU as opposed to the receiver’s own frame controller (FRC) block.

In the RAIL library there exist a function that places the receiver into this mode: RAIL_DirectModeConfig. When enabled the RX_DATA will be output to EFR32_PC11. This output data has the following properties:

• It is changing at the demodulator’s clock rate, which is typically 5-10x higher than the DR itself.
• It only works in 2FSK mode by simply outputting the sign of the frequency with regards to the nominal frequency, i.e., if the frequency is higher than the nominal it will output logical ones, if it is lower that the nominal frequency it will output zeroes.
• There is no frequency error compensation on it.
• It is not synchronized to any clock.

Note that at the writing of this article it is stated in RAIL documentation that parallel to RX_DATA an RX_CLK signal is also output. This is not correct, not the least because there is no RX_CLK signal RX_DATA is synchronized to.

Above direct mode operation can also be referred to as asynchronous direct mode operation. What about synchronous direct mode operation, you may quite rightly ask. Synchronous direct mode operation also provides an RX_CLK signal RX_DATA is synchronized to. Such a mode does not exist on the Wireless Gecko (EFR32).

The reason it does not exist is that a successful sync word detection gates the availability of the RX_CLK signal. So without sync word detection there is no RX_CLK, if you have sync word detection, however you may as well carry on and use packet mode. If for some reason RX_CLK and RX_DATA (placed on pins) still come useful in your packet mode application keep an eye on the radio related PRS signals (see below the current offering) in the RAIL documentation, sooner or later RX_DATA and RX_CLK will show up.