S2-LP datasheet(69/90 Pages) STMICROELECTRONICS

69 / 90 page

S2-LP

Link layer protocol

DocID029944 Rev 1

69/90

The CSMA procedure is controlled by the following parameters:

CSMA_ON: enable/disable the CSMA procedure, this bit will be checked at each packet

transmission.

CSMA_PERS_ON: makes the carrier sense persistent that means the channel is

continuously monitored until it becomes free again, skipping the back-off waiting steps. The

MCU can stop the procedure with a SABORT command.

CCA_PERIOD: code that programs the Tcca time (expressed as multiple of Tbit samples)

between two successive CS samplings, as follows:



00b: 64xTbit



01b: 128xTbit



10b: 256xTbit



11b: 512xTbit.

NUM._OF_CCA_PERIOD: configuration of Tlisten = [1..15] x Tcca.

SEED_RELOAD: enable/disable the reload of the seed used by the back-off random

generator at the start of each CSMA procedure (at the time when the counter is reset, for

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

updated by the generator circuit itself.

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

pseudo-random number generator when the CSMA cycle starts, if the SEED_RELOAD bit

is enabled. Value 0 is not allowed, because the pseudo-random generator is not working in

that case.

BU_PRESCALER: PRESCALER which is used to configure the back-off time unit

BU=BU_PRESCALER.

NBACKOFF_MAX: max number of back-off cycles.

Below the timelines of the main cases of transmission with CSMA.

If the channel is free, no matters the value of the persistent_mode bit, the device must

assert channel free for a number of NUM._OF_CCA_PERIOD (T. listen) before

transmitting:

Figure 20: CSMA if channel is free (timeline)

If the channel is busy and persistent_mode bit is 1, the device will check the channel

continuously in Tcca periods.

When the channel becomes free, it must assert channel free for a number of

NUM._OF_CCA_PERIOD (T. listen) before transmitting:

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