2 configuration and cca request, 3 data interpretation, 4 interrupt handling – Rainbow Electronics ATmega128RFA1 User Manual
Page 71: Configuration and
71
8266A-MCU Wireless-12/09
ATmega128RFA1
CCA Mode
Description
0, 3
Carrier sense with energy above threshold
.
CCA shall report a busy medium using a logical combination of
-
Detection of a signal with the modulation and spreading characteristics of
this standard and
-
Energy above the ED threshold.
Where the logical operator may be configured as either OR (mode 0) or
AND (mode 3).
9.5.5.2 Configuration and CCA Request
The CCA modes are configurable via register PHY_CC_CCA.
Usimg the Basic Operating Mode, a CCA request can be initiated manually by setting
CCA_REQUEST = 1 of register PHY_CC_CCA, if the radio transceiver is in any RX
state. The current channel status (CCA_STATUS) and the CCA completion status
(CCA_DONE) are accessible in register TRX_STATUS.
The CCA evaluation is done over eight symbol periods and the result is accessible
t
TR28
= 140 µs (128 µs measurement duration and processing delay) after the request.
The end of a manually initiated CCA measurement is indicated by a
TRX24_CCA_ED_DONE interrupt.
The sub-register CCA_ED_THRES of register CCA_THRES defines the received power
threshold of the “energy above threshold” algorithm. The threshold is calculated by
RSSI_BASE_VAL + 2 • CCA_ED_THRES [dBm]. Any received power above this level
is interpreted as a busy channel.
Note that it is not recommended to manually initiate a CCA measurement when using
the Extended Operating Mode.
9.5.5.3 Data Interpretation
The current channel status (CCA_STATUS) and the CCA completion status
(CCA_DONE) are accessible in register TRX_STATUS. Note, register bits CCA_DONE
and CCA_STATUS are cleared in response to a CCA_REQUEST.
The completion of a measurement cycle is indicated by CCA_DONE = 1. If the radio
transceiver detected no signal (idle channel) during the measurement cycle, the
CCA_STATUS bit is set to 1.
When using the “energy above threshold” algorithm, any received power above
CCA_ED_THRES level is interpreted as a busy channel. The “carrier sense” algorithm
reports a busy channel when detecting an IEEE 802.15.4 signal above the
RSSI_BASE_VAL (see parameter RSSI_BASE_VAL in
). The radio transceiver is also able to detect signals below
this value, but the detection probability decreases with the signal power.
9.5.5.4 Interrupt Handling
The TRX24_CCA_ED_DONE interrupt is issued at the end of a manually initiated CCA
measurement.
Note:
A CCA request should only be initiated in the receive states of Basic Operating Mode.
Otherwise the radio transceiver generates a TRX24_CCA_ED_DONE interrupt and
sets the register bit CCA_DONE = 1 even if no CCA measurement was performed.