NRF52811-QCAA-R7 datasheet(200/444 Pages) NORDIC

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Peripherals

In addition to the already available shortcuts, one is provided between READY event and CCASTART task so

that a CCA can automatically start when the receiver is ready. A second shortcut has been added between

CCAIDLE event and the TXEN task, so that upon detecting a clear channel the RADIO can immediately

enter transmit mode.

6.14.13.7 Receive sequence

The reception is started by first putting the RADIO in receive mode. After writing to the RXEN task, the

RADIO will start ramping up and enter the RXRU state.

When the READY event is generated, the RADIO enters the RXIDLE mode. For the baseband processing to

be enabled, the START task must be written. An outline of the IEEE 802.15.4 reception can be found in the

figure below.

RXRU

RXIDLE

SHR

PAYLOAD

CRC

RXIDLE

RXDISABLE

’X’

Figure 82: IEEE 802.15.4 receive sequence

When a valid SHR is received the RADIO will start storing future octets (starting with PHR) to the data

memory pointed to by PACKETPTR. After the SFD octet is received the FRAMESTART event is generated.

If the CRC module is enabled it will start updating with the second byte received (first byte in payload)

and run for the full frame length. The two last bytes in the frame are not written to RAM when CRC is

configured. However, if the result of the CRC after running the full frame is zero, the CRCOK event will

be generated. The END event is generated when the last octet has been received and is available in data

memory.

When a packet is received a link quality indicator (LQI) is also generated and appended immediately after

the last received octet. When using an IEEE 802.15.4 compliant frame, this will be just after the MSDU

since the FCS is not reported. In the case of a non-complient frame it will be appended after the full

frame. The LQI reported by hardware must be converted to IEEE 802.15.4 range by an 8-bit saturating

multiplication by 4, as shown in IEEE 802.15.4 ED measurement example on page 196. The LQI is

only valid for frames equal to or longer than three octets. When receiving a frame the RSSI (reported as

negative dB) will be measured at three points during the reception. These three values will be sorted and

the middle one selected (median 3) to be remapped within the LQI range. The following figure illustrates

the LQI measurement and how the data is arranged in data memory.

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