AX5043 supports several framing modes including “HDLC” mode, “Raw - Pattern Match” mode, and “Raw” mode. For HDLC and pattern matching there is dedicated hardware to perform the framing, timing, and packet handling. These are configured in software, and then handle packet reception automatically, interrupting the MCU core after a packet has been received.
In contrast, Raw mode bypasses all of this advanced functionality and places the raw bits directly into the FIFO to be read by the MCU. in the case of “Raw, Soft Bits” mode, rather than a hard 1/0 bit decision being reported, a range of bits from 0-255 is reported. This value represents the “confidence” of the demodulator on the bit value. For example, a value of 255 would represent a confident 1, 0 would represent a confident 0, and 64 would represent a semi-confident 0.
This raw data can be useful when additional signal processing is required to extract a signal from an otherwise unreliable communication channel. One example of where the Raw, Soft Bits mode has been useful is to allow the use of a Decision Feedback Equalization algorithm to mitigate inter-symbol interference in a 4-FSK signal.
Raw mode is not a universal solution, however. One major drawback to it is that the framing modes perform critical functions such, as Automatic Frequency Correction and Bit Timing Recovery. With these modes disabled, the frequency and timing offsets that occur naturally due to things like TCXO tolerances can cause failure of the communication system. For this reason, it is recommended to use a hybrid approach if raw mode is desired.
The recommended hybrid approach consists of using “Raw, Pattern Match” mode to receive the preamble and sync word. This is done in the hardware, and ensures that the receiver can lock on to the incoming signal. The radio can be configured to generate an interrupt to the MCU once the preamble has been detected. This interrupt can trigger the MCU to then issue a command to the radio to switch to raw mode. The raw bits for the remainder of the packet can then be read out by the microcontroller and processed as desired.