ZAURA Datasheet, PDF (25/99 Pages) –

ZAURA RF Wireless Library
Programmer’s Reference Manual
13
node is transmitting at any given moment, it is unlikely that a receiver in
range of the transmitters will be able to successfully receive any frames.
To perform a carrier sense before transmission, the
ZAURA_RF_CsAttempts configuration variable must be set to a nonzero
value. In this instance, the radio is placed into receive mode long enough to
capture the current Receive Signal Strength Indicator (RSSI). The higher
the RSSI value, the more RF energy the radio is detecting, and the more
likely another station is already transmitting. If ZAURA_RF_CsAttempts
is set to zero, RSSI is not sampled prior to transmission (i.e., no carrier
sense is performed).
If the sampled RSSI is below ZAURA_RF_RssiThresh, the node will initi-
ate transmission of the DATA or SDATA frame. If the RSSI value is greater
than or equal to ZAURA_RF_RssiThresh, the transmitting station deter-
mines that the channel is in use. In this instance, the node will increment a
ChBusy counter (initialized to 0 each time the ZAURA_RF_Transmit API
is called) and will wait for a pseudo-random back-off period. After this
pseudorandom delay, the node will again sample RSSI if ChBusy is less
than ZARA_RF_CsAttempts. This process continues until the frame is
transmitted (RSSI < ZAURA_RF_RssiThresh) or the transmission is
aborted (ChBusy reaches the value of ZARA_RF_CsAttempts) and a sta-
tus of ZAURA_RF_TX_CHANNEL_BUSY is returned to the caller.
The back-off period is chosen at random on an interval of 164 to 419 ticks
of the external 32.768 kHz oscillator corresponding to a delay of approxi-
mately 5.0 ms to 12.8 ms.
Prior to sending an ACK frame, the ZAURA RF node will sample RSSI if
ZAURA_RF_CsAttempts is nonzero. However, unlike sending a DATA
or SDATA frame, there is no random back-off, and a ChBusy counter is
not used. Instead, the node attempting to send an ACK will initiate trans-
mission as soon as RSSI < ZAURA_RF_RssiThresh. If the channel
remains busy for a period of approximately 8 ms, then the node will abort
transmission of the ACK. If ZAURA_RF_CsAttempts is zero, then the
RM006003-1011
Channel Access Rules