NRF52820-QDAA-R datasheet(181/437 Pages) NORDIC

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Peripherals

• To automatically disable the RADIO whenever the CCA reports a busy medium, the shortcut between

CCABUSY and DISABLE can be used.

• To immediately start a CCA after ramping up into RX mode, the shortcut between RXREADY and

CCASTART can be used.

Conversion

The conversion from a CCAEDTHRES, LQI, or EDSAMPLE value to dBm can be done with the following

equation, where VALHARDWARE is either CCAEDTHRES, LQI, or EDSAMPLE. LQI and EDSAMPLE are hardware-

reported values, while CCAEDTHRES is set by software. Constants ED_RSSISCALE and ED_RSSIOFFS are

from electrical specifications.

PRF[dBm] = ED_RSSIOFFS + VALHARDWARE

Figure 70: Conversion between hardware value and dBm

The ED_RSSISCALE constant is used to calculate power in 802.15.4 units (0-255):

PRF[802.15.4 units] = MIN( ED_RSSISCALE x VALHARDWARE, 255 )

Figure 71: Conversion between hardware value and 802.15.4 units (0-255)

6.12.13.5 Cyclic redundancy check (CRC)

IEEE 802.15.4 uses a 16-bit ITU-T cyclic redundancy check (CRC) calculated over the MAC header (MHR)

and MAC service data unit (MSDU).

The standard defines the following generator polynomial:

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.

The following code shows how to configure 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.

6.12.13.6 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 following figure.

4463_156 v1.0

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