RSSI_DET_SEL<1:0>. Users can read the register directly to obtain the final RSSI value.
In RSSI comparison procedure, the real-time RSSI value measured is compared to the threshold in dBm set by users in
RSSI_TRIG_TH<7:0>-128 and it outputs 1when it is greater than the threshold and outputs 0 when it is less. This output is used
as one of the RSSI_VLD signal sources.
The method of selecting smoothing filter order
The filtering order is equivalent to how many symbols passing through to obtained one correct RSSI value. The higher the order,
the longer the filtering time, and the more smooth the result is. However, the order is limited by the measurement condition as
well. For example, when RX_PREAM_SIZE is set as the length of 16 symbols, and RSSI_DET_SEL<1:0> is set to 1 (other words,
active PREAM_OK is set as the RSSI measurement condition), it can output a correct RSSI value only if the filtering order is set
not greater than 16. That is because, it receives PREAM_OK after receiving 16 symbols in a preamble, if the filtering order is set
as 16, it can exactly obtain one correct average value based on the 16 symbols received. Surely, a more stable measurement
result can be obtained if the order is set smaller than RX_PREAM_SIZE. Similarly, when the measurement condition is
SYNC_OK or PKT_OK, it needs to ensure the number of symbols received before the two interrupts occurring is greater than the
filtering order.
To accommodate different using habits of users, the CMT2219B provides both RSSI values in dBm and simple 8-bit code values.
RSSI values in dBm is recommended in general.
1.3 RSSI Measurement and Comparison in OOK Mode
Figure 3. Structure Diagram of RSSI Measurement and Comparison in OOK Mode
Since OOK is based on amplitude modulation, RSSI will show high and low transitions. However, users need a constant RSSI
value representing signal 1, therefore the PEAK DETECTOR is used in the circuit to pick up peak values (the RSSI value
representing 1) in the signal and output them to register.
Moreover, 2 RSSI comparison methods are available in OOK mode. One is the same as that of FSK mode, the other is called
automatic absolute threshold which is enabled by setting AUTO_ABS_EN. When it is enabled, the chip will generate a threshold
automatically for noise shielding and compared it to the output of PEAK DETECTOR with support of comparison result output.
