S2-LPS datasheet(30/73 Pages) STMICROELECTRONICS

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4.4.5.8.1

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).

4.4.5.8.2

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.

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 39. 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

4.4.5.8.3

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, while it is decreased by 4 every time a bit

repetition occurs. The running peak PQI is compared to a threshold value and the preamble valid IRQ is asserted

as soon as the threshold is passed. The preamble quality indicator threshold is 4 x PQI_TH (with PQI_TH = 0, 1,

…15).

4.4.5.8.4

SQI

The synchronization quality indicator (SQI) is a measurement of the best correlation between the received SYNC

word and the expected one. This indicator is calculated as the peak cross-correlation between the received data

stream and the expected SYNC word. If the SQI_EN = 1b, the running peak SQI is compared to a threshold

value and the SYNC valid IRQ is asserted as soon as the threshold is passed. The SYNC quality threshold is

equal to SYNC_LEN – 2 x SQI_TH (with SQI_TH = 0, 1, …7). When SQI_TH = 0b, perfect match is required. It

is recommended the SQI check always enabled. The peak SQI value can be read from the register SQI[5:0] and

represents the peak value from 0 to 32, while the bit SQI[6], when equal to ‘1’ indicates that the SQI peak value

refers to the secondary SYNC word.

S2-LPS

Digital modulator

DS13831 - Rev 1

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