NRF52820-QDAA-R datasheet(100/437 Pages) NORDIC

100 / 437 page

Peripherals

For long packets (MODE.LENGTH = Extended), the keystream generation needs to start earlier, such as

when the TXEN task in RADIO is triggered.

Refer to Timing specification on page 109 for information about the time needed for generating a

keystream.

RFU

EPL

MIC

CRC

READY

TXEN

END

keystream

generation

KSGEN

encryption

ENDCRYPT

READY

START

SHORTCUT

RADIO

AES CCM

RU: Ramp-up of RADIO

P: Preamble

A: Address

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

EPL: encrypted payload

CRYPT

ENDKSGEN

SHORTCUT

PPI

Figure 33: On-the-fly encryption of short packets (MODE.LENGTH = Default) using a PPI connection

6.4.6 Decrypting packets on-the-fly in RADIO receive mode

When the AES CCM peripheral decrypts a packet on-the-fly while RADIO is receiving it, the AES CCM

peripheral must read the encrypted packet from the same memory location that RADIO is writing to.

The INPTR on page 107 pointer in the AES CCM must point to the same memory location as the

PACKETPTR pointer in RADIO, see Configuration of on-the-fly decryption on page 100.

AES CCM

RFU

EPL

L+4 RFU

Unencrypted packet

Encrypted packet

MIC

Scratch area

CCM data

structure

MODE = DECRYPTION

RADIO

RXEN

From remote

transmitter

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

PL: unencrypted payload

EPL: encrypted payload

OUTPTR

INPTR

PACKETPTR

SCRATCHPTR

CNFPTR

Figure 34: Configuration of on-the-fly decryption

In order to match RADIO’s timing, the KSGEN task must be triggered early enough to allow the keystream

generation to complete before the decryption of the packet shall start.

For short packets (MODE.LENGTH = Default) the KSGEN task must be triggered no later than when the

START task in RADIO is triggered. In addition, the CRYPT task must be triggered no earlier than when the

ADDRESS event is generated by RADIO.

If the CRYPT task is triggered exactly at the same time as the ADDRESS event is generated by RADIO, the

AES CCM peripheral will guarantee that the decryption is completed no later than when the END event in

RADIO is generated.

This use-case is illustrated in On-the-fly decryption of short packets (MODE.LENGTH = Default) using a PPI

connection on page 101 using a PPI connection between the ADDRESS event in RADIO and the CRYPT

4463_156 v1.0

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NRF52820-QDAA-R Datasheet(PDF) 100 Page - Nordic Semiconductor

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