Rainbow-electronics ATmega128RFA1 Manual de usuario descargar pdf (Pagina 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

≤ -90 dBm. For an ED value in the

range of 0 to 84, the RF input power can be calculated as follows:

1 2 ... 64 65 66 67 68 69 70 71 72 73 74 ... 523 524

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