User Manual

94
8111A–AVR–05/08
AT86RF231
8.5 Clear Channel Assessment (CCA)
The main features of the Clear Channel Assessment (CCA) module are:
• All 4 modes are available as defined by IEEE 802.15.4-2006 in section 6.9.9
• Adjustable threshold for energy detection algorithm
8.5.1 Overview
A CCA measurement is used to detect a clear channel. Four modes are specified by
IEEE 802.15.4 - 2006:
8.5.2 Configuration and Request
The CCA modes are configurable via register 0x08 (PHY_CC_CCA).
Using the Basic Operating Mode, a CCA request can be initiated manually by setting
CCA_REQUEST = 1 (register 0x08, PHY_CC_CCA), if the AT86RF231 is in any RX state. The
current channel status (CCA_STATUS) and the CCA completion status (CCA_DONE) are
accessible in register 0x01 (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 an interrupt
IRQ_4 (CCA_ED_READY).
The sub-register CCA_ED_THRES of register 0x09 (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 inter-
preted as a busy channel.
Note, it is not recommended to manually initiate a CCA measurement when using the Extended
Operating Mode.
Table 8-8. CCA Mode Overview
CCA Mode Description
1
Energy above threshold.
CCA shall report a busy medium upon detecting any energy above the ED threshold.
2 Carrier sense only.
CCA shall report a busy medium only upon the detection of a signal with the modulation
and spreading characteristics of an IEEE 802.15.4 compliant signal. The signal strength
may be above or below the ED threshold.
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).