Rainbow Electronics AT86RF231 - Manual (Page 91)

91
8111A–AVR–05/08
AT86RF231
8.4 Energy Detection (ED)
The Energy Detection (ED) module is characterized by:
• 85 unique energy levels defined
• 1 dB resolution
8.4.1 Overview
The receiver ED measurement is used by the network layer as part of a channel selection algo-
rithm. It is an estimation of the received signal power within the bandwidth of an IEEE 802.15.4
channel. No attempt is made to identify or decode signals on the channel. The ED value is cal-
culated by averaging RSSI values over eight symbols (128 µs).
For High Data Rate Modes the automated ED measurement duration is reduced to 32 µs, refer
to
Section 11.3 “High Data Rate Modes” on page 137 . For manually initiated ED measurements
in these modes the measurement period is still 128 µs as long as the receiver is in RX_ON state.
8.4.2 Measurement Description
There are two ways to initiate an ED measurement:
• Manually, by writing an arbitrary value to register 0x07 (PHY_ED_LEVEL), or
• Automatically, after detection of a valid SHR of an incoming frame.
For manually initiated ED measurements the radio transceiver needs to be in one of the states
RX_ON or BUSY_RX state. The end of the ED measurement is indicated by an interrupt
IRQ_4 (CCA_ED_READY).
An automated ED measurement is started if an SHR is detected. The end of the automated
measurement is not signaled by an interrupt.
The measurement result is stored after t
TR26
= 140 µs (128 µs measurement duration and pro-
cessing delay) in register 0x07 (PHY_ED_LEVEL).
Thus by using Basic Operating Mode, a valid ED value from the currently received frame is
accessible 108 µs after IRQ_2 (RX_START) and remains valid until a new RX_START interrupt
is generated by the next incoming frame or until another ED measurement is initiated.
By using the Extended Operating Mode, it is recommended to mask IRQ_2 (RX_START), thus
the interrupt cannot be used as timing reference. A successful frame reception is signalized by
interrupt IRQ_3 (TRX_END). The minimum time span between a TRX_END interrupt and a fol-
lowing SFD detection is t
TR27
= 96 µs due to the length of the SHR. Including the ED
measurement time, the ED value needs to be read within 224 µs after the TRX_END interrupt;
otherwise, it could be overwritten by the result of the next measurement cycle. This is important
for time critical applications or if interrupt IRQ_2 (RX_START) is not used to indicate the recep-
tion of a frame.
Note, it is not recommended to manually initiate an ED measurement when using the Extended
Operating Mode.
The values of the register 0x07 (PHY_ED_LEVEL) are:
Table 8-7. Register Bit PHY_ED_LEVEL Interpretation
PHY_ED_LEVEL Description
0xFF Reset value
0x00.... 0x54 ED measurement result of the last ED measurement