Rainbow Electronics ATmega128RFA1 - Manual (Page 69)

69
8266A-MCU Wireless-12/09
ATmega128RFA1
For High Data Rate Modes the automated ED measurement duration is reduced to
32 µs as described in
"High Data Rate Modes" on page 86 . The measurement period in
these modes is still 128 µs for manually initiated ED measurements as long as the
receiver is in RX_ON state.
9.5.4.2 Measurement Description
There are two ways to initiate an ED measurement:
• Manually, by writing an arbitrary value to register 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. The end of the ED measurement is indicated by a
TRX24_CCA_ED_DONE interrupt.
The automatic ED measurement is started if a SHR is detected. The end of the
automatic measurement is not signaled by an interrupt.
The measurement result is stored after t
TR26
= 140 µs (128 µs measurement duration
and processing delay) in register PHY_ED_LEVEL.
Thus by using Basic Operating Mode a valid ED value from the currently received frame
is accessible 108 µs after the TRX24_RX_START interrupt and remains valid until the
next incoming frame generates a new TRX24_RX_START interrupt or until another ED
measurement is initiated.
When using the Extended Operating Mode it is recommended to mask the
TRX24_RX_START interrupt. Hence the interrupt cannot be used as timing reference.
A successful frame reception is signalized by the TRX24_RX_END interrupt. The
minimum time span between a TRX24_RX_END interrupt and a following SFD
detection is t
TR27
= 96 µs due to the length of the SHR. The ED value needs to be read
within 224 µs including the ED measurement time after the TRX24_RX_END interrupt.
Otherwise it could be overwritten by the result of the next measurement cycle. This is
important for time critical applications or if the TRX24_RX_START interrupt is not used
to indicate the reception of a frame.
The values of the register PHY_ED_LEVEL are:
Table 9-20. Register Bit PHY_ED_LEVEL Interpretation
PHY_ED_LEVEL Description
0xFF Reset value
0x00 … 0x54 ED measurement result of the last ED measurement
Note: 1. It is not recommended to manually initiate an ED measurement when using the
Extended Operating Mode.
9.5.4.3 Data Interpretation
The PHY_ED_LEVEL is an 8-bit register. The ED value of the radio transceiver has a
valid range from 0x00 to 0x54 with a resolution of 1 dB. All other values do not occur. A
value of 0xFF indicates the reset value. A value of PHY_ED_LEVEL = 0 indicates that
the measured energy is less than -90 dBm (see parameter RSSI_BASE_VAL in section
"Receiver Characteristics" on page 508 ). Due to environmental conditions (temperature,
voltage, semiconductor parameters etc.) the calculated ED value has a maximum
tolerance of ±5 dB, this is to be considered as constant offset over the measurement
range.
An ED value of 0 indicates an RF input power of P
RF
≤ -90 dBm. For an ED value in the
range of 0 to 84, the RF input power can be calculated as follows: