S2-LP datasheet(42/90 Pages) STMICROELECTRONICS

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Block description

S2-LP

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DocID029944 Rev 1

while it is decreased by 4 every time a bit repetition occurs. If the PQI_EN = 1b, 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).

5.5.8.4

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.

5.5.9

CS blanking

The CS blanking feature prevents data to be received if the RSSI level on the air is below

the RSSI threshold (set by the RSSI_TH field). The feature can be enabled through the

CS_BLANKING bit in the ANT_SELECT_CONF register.

5.5.10

Antenna switching

The device implements a switching based antenna diversity algorithm. The antenna

switching function allows controlling an external switch in order to select the antenna

providing the highest measured RSSI. The switching decision is based on a comparison

between the received power level on antenna 1 and antenna 2 during the preamble

reception controlling through GPIO an external RF switch in order to select the antenna

providing the highest measured RSSI.

When antenna switching is enabled, the two antennas are repeatedly switched during the

reception of the preamble of each packet, until the carrier sense threshold is reached

(static CS mode must be used). From this point on, the antenna with highest power is

selected and switching is frozen. The switch control signal is available on GPIO and in the

MC_STATE[1] register.

The algorithm is controlled by the following parameters:



AS_MEAS_TIME: this register/parameter controls the time interval for RSSI

measurement. The actual measurement time is done with the following formula.

Equation - Antenna switching measurement time

�� =

24 ∙ 2��_�� ∙ 2��_��_��

��

In case of FSK modulation, the whole

�� is used to let the signal level settle after the

antenna switch and one single measurement is taken at the end of such period.

In case of OOK modulation, after one first interval equal to

�� again used to let the

signal level settle after the antenna switch, one second interval still equal to

��, is used

to perform a peak power measurement and select the best antenna.



AS_ENABLE: this parameter enables the antenna switching function.

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