SPIRIT1 datasheet(35/94 Pages) STMICROELECTRONICS

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SPIRIT1

Operating modes

Doc ID 022758 Rev 3

35/94

units (BU). The contention window is calculated on the basis of the binary exponential

backoff (BEB) technique, which doubles the size of the window at each backoff retry (stored

in the NB counter):

BO= rand(0,2NB)×BU

The CSMA procedure is then controlled by the following parameters:

SEED_RELOAD: enables/disables the reload of the seed used by the backoff random

generator at the start of each CSMA procedure (at the time when the counter is reset, i.e.

NB=0). If this functionality is not enabled, the seed is automatically generated and updated

by the generator circuit itself.

CSMA_ON: enables/disables the CSMA procedure (11th bit of the PROTOCOL[1] register);

this bit is checked at each packet transmission.

CSMA_PERS_ON: makes the carrier sense persistent, i.e. the channel is continuously

monitored until it becomes free again, skipping the backoff waiting steps (9th bit of the

PROTOCOL[1] register); the MCU can stop the procedure with an SABORT command.

BU_COUNTER_SEED_MSByte/LSByte: these bytes are used to set the seed of the

pseudo-random number generator when the CSMA cycle starts (CSMA_CONFIG[3:2]

registers), provided that the SEED_RELOAD bit is enabled. Value 0 is not allowed, because

the pseudo-random generator does not work in that case.

BU_PRESCALER[5:0]: prescaler which is used to configure the backoff time unit (b)

BU=BU_PRESCALER in Figure 6 (field of the CSMA_CONFIG[1] register).

CCA_PERIOD[1:0]: code which programs the Tcca time (expressed as a multiple of Tbit

samples) between two successive CS samplings (field of the CSMA_CONFIG[1] register),

as follows:

●

00 64×Tbit

●

01 128×Tbit

●

10 256×Tbit

●

11 512×Tbit.

CCA_LENGTH[3:0]: configuration of Tlisten = [1..15] x Tcca

NBACKOFF_MAX[2:0]: max. number of backoff cycles.

Note that the backoff timer is clocked on the 34.7 kHz clock, because, in this case, the SPIRIT1 is in SLEEP

state, in order to reduce power consumption.

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