S2-LP datasheet(68/90 Pages) STMICROELECTRONICS

68 / 90 page

Link layer protocol

S2-LP

68/90

DocID029944 Rev 1

programmable threshold. If the channel is busy, a back-off procedure may be activated to

repeat the process a certain number of times, until the channel is found to be idle. When

the limit is reached, an interrupt notifies that the channel has been repeatedly found busy

and so the transmission has not been performed. While in back-off, the S2-LP stays in

SLEEP state in order to reduce power consumption. CCA may optionally be persistent

continuing until the channel becomes idle or until the MCU stops it. The thinking behind

using this option is to give the MCU the possibility of managing the CCA by itself, for

instance, with the allocation of a transmission timer: this timer would start when MCU

finishes sending out data to be transmitted, and would end when MCU expects that its

transmission take place, which would occur after a period of CCA.

The overall CSMA/CA flowchart is shown in Figure 19: "Flowchart of the S2-LP CSMA

procedure", where Tcca and Tlisten are two of the parameters controlling the clear channel

assessment procedure. Design practice recommends that these parameters average the

channel energy over a certain period expressed as a multiple of the bit period (Tcca) and

repeat such measurement several times covering longer periods (Tlisten). The measurement

is performed directly by checking the carrier sense (CS) generated by the receiver module.

Figure 19: Flowchart of the S2-LP CSMA procedure

To avoid any wait synchronization between different channel contenders, which may cause

successive failing CCA operations, the back-off wait time is calculated randomly inside a

contention window. The back-off time BO is expressed as a multiple of back-off time units

(BU). The contention window is calculated on the basis of the binary exponential back-off

(BEB) technique, which doubles the size of the window at each back-off retry (stored in the

NB counter):

�� = �� ∗ (6 + rand(0: 2(��+1)) ∗ (�� + 1))

During this time, the S2-LP is kept in the SLEEP state. If this CSMA mode is used, the user

must set the SLEEP_MODE_SEL bit to 1 in order to guarantee the FIFO retention during

the SLEEP phase.

S2-LP 数据表(PDF) 68 Page - STMicroelectronics