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 exists 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 changes at the demodulator’s clock rate, which is typically 5-10x higher than the DR itself.
• It "only" (see OOK bullet below) works in 2FSK mode, and contains the sign of the frequency relative to the nominal frequency - i.e., if the frequency is higher than nominal it will output logical ones, if it is lower than nominal it will output zeroes.
• There is no frequency error compensation on it.
• It is not synchronized to any clock.
• OOK is also supported, but with this modulation Rx Direct Mode exhibits about 10 dB worse sensitivity vs. OOK in Packet Mode (direct mode was designed and optimized for 2FSK modulation).  However, OOK direct mode Rx is functional, and is available to customers who can tolerate the sensitivity hit vs. packet mode.

Note that at the writing of this article the RAIL API documentation states that an RX_CLK signal is also output in parallel with RX_DATA. This is not correct, as there is in fact no RX_CLK signal that the RX_DATA is synchronized to.

Hence, the above direct mode operation can also be referred to as asynchronous direct mode operation. You may quite rightly ask: "What about synchronous direct mode operation?"  Synchronous direct mode operation does provide an RX_CLK signal to which the RX_DATA signal is synchronized, but 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) would still be useful in your packet mode application, keep an eye on the radio related PRS signals (see below the current offering) in the RAIL documentation - as sooner or later RX_DATA and RX_CLK will show up.