S2-LPQTR datasheet(41/90 Pages) STMICROELECTRONICS

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S2-LP

Block description

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by the device on the falling edge of such clock signal so the MCU must read it during

falling edge of CLK.

5.5.8

Receiver quality indicators

5.5.8.1

RSSI

The received signal strength indicator (RSSI) is a measurement of the received signal

power at the antenna measured in the channel filter bandwidth. The measured RSSI is in

steps of 1 dB, from 0 to 255 (1 byte value) and it is offset in such a way that the number 0

corresponds to -146 dBm, so the register value can be converted in dBm by subtracting

146. Laboratory calibration may be needed for accurate absolute power measurements.

The RSSI value can be read through two registers: RSSI_LEVEL_CAPTURE and

RSSI_LEVEL_RUN. In particular RSSI_LEVEL_CAPTURE reports the RSSI value

captured at the end of the SYNC word detection, exit from RX state by SABORT command

or RX timeout expiration, while RSSI_LEVEL_RUN is the continuous output of the RSSI

filter. The last mode supports the continuous fast SPI reading that means if the CSn signal,

of the SPI interface, is kept low, after the first 16 bits (S2-LP status register), then a new

RSSI value will be available every 8 SPI clock cycles (this mode is the same of the SPI

burst mode, but no automatic address increment).

5.5.8.2

Carrier sense

The carrier sense functionality can be used to detect if any RF signal is being received, the

detection is based on the measured RSSI value. There are two operational modes for

carrier sensing: static and dynamic carrier sensing.

When static carrier sensing is used (CS_MODE = 0), the carrier sense signal is asserted

when the measured RSSI is above the value specified in the RSSI_TH register and is de-

asserted when the RSSI falls 3 dB below the same threshold.

When dynamic carrier sense is used (CS_MODE = 1, 2, 3), the carrier sense signal is

asserted if the signal is above the threshold and a fast power increase of 6, 12 or 18 dB is

detected; it is de-asserted if a power fall of the same amplitude is detected.

The carrier sense signal is also used internally to the demodulator to start the automatic

frequency compensation and timing recovery algorithms and for the CSMA procedure (for

this usage in should be set CS_MODE = 0).

The carrier sense function is controlled by the following parameters:



RSSI threshold: this parameter sets the minimum signal power above which the

carrier sense signal is asserted (RSSI_TH register).



CS mode: this parameter controls the carrier sense operational modes.

Table 43: CS mode description

CS_MODE

Description

Static carrier sensing

Dynamic carrier sensing with 6 dB dynamic threshold

Dynamic carrier sensing with 12 dB dynamic threshold

Dynamic carrier sensing with 18 dB dynamic threshold

5.5.8.3

PQI

The preamble quality indicator (PQI) is intended to provide a measurement of the reliability

of the preamble detected. The PQI is increased by 1 every time a bit inversion occurs,

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