NRF52811-QCAA-R datasheet(199/444 Pages) NORDIC

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Peripherals

G(x) = x16 + x12 + x5 + 1

In receive mode the RADIO will trigger the CRC module when the first octet after the frame length (PHR)

is received. The CRC will then update on each consecutive octet received. When a complete frame

is received the CRCSTATUS register will be updated accordingly and the CRCOK or CRCERROR events

generated. When the CRC module is enabled it will not write the two last octets (CRC) to the frame Data

RAM. When transmitting, the CRC will be computed on the fly, starting with the first octet after PHR, and

inserted as the two last octets in the frame. The EasyDMA will fetch frame length minus 2 octets from

RAM and insert the CRC octets insitu.

Below is a code snippet for configuring the CRC module for correct operation when in IEEE 802.15.4 mode.

The CRCCNF is written to 16-bit CRC and the CRCPOLY is written to 0x11021. The start value used by IEEE

802.15.4 is zero and CRCINIT is configured to reflect this.

/* 16-bit CRC with ITU-T polynomial with 0 as start condition*/

NRF_RADIO->CRCCNF = ((RADIO_CRCCNF_SKIPADDR_Ieee802154 << RADIO_CRCCNF_SKIPADDR_Pos) |

(RADIO_CRCCNF_LEN_Two << RADIO_CRCCNF_LEN_Pos));

NRF_RADIO->CRCPOLY = 0x11021;

NRF_RADIO->CRCINIT = 0;

The ENDIANESS subregister must be set to little-endian since the FCS field is transmitted from left bit to

right.

6.14.13.6 Transmit sequence

The transmission is started by first putting the RADIO in receive mode and triggering the RXEN task.

An outline of the IEEE 802.15.4 transmission is illustrated in the figure below.

PAYLOAD

CRC

RXRU

Clear channel

TXRU

TXIDLE

SHR

TXDISABLE

RXIDLE

Figure 81: IEEE 802.15.4 transmit sequence

The receiver will ramp up and enter the RXIDLE state where the READY event is generated. Upon receiving

the ready event, the CCA is started by triggering the CCASTART task. The chosen mode of assessment

(CCACTRL.CCAMODE register) will be performed and signal the CCAIDLE or CCABUSY event 128 µs later. If

the CCABUSY event is received, the RADIO will have to retry the CCA after a specific back-off period. This is

outlined in the IEEE 802.15.4 standard, Figure 69 in section 7.5.1.4 The CSMA-CA algorithm.

If the CCAIDLE event is generated, a write to the TXEN task register enters the RADIO in TXRU state.

The READY event will be generated when the RADIO is in TXIDLE state and ready to transmit. With the

PACKETPTR pointing to the length (PHR) field of the frame, the START task can be written. The RADIO will

send the four octet preamble sequence followed by the start of frame delimiter (SFD register). The first

byte read from the Data RAM is the length field (PHR) followed by the transmission of the number of

bytes indicated as the frame length. If the CRC module is configured it will run for PHR-2 octets. The last

two octets will be substituted with the results from running the CRC. The necessary CRC parameters are

sampled on the START task. The FCS field of the frame is little endian.

4454_140 v1.0

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