NRF52820-QDAA-R7 datasheet(99/437 Pages) NORDIC

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Peripherals

Empty packets (length field is set to 0) will not be decrypted but instead moved unmodified through the

AES CCM peripheral. These packets will always pass the MIC check.

CCM supports different widths of the LENGTH field in the data structure for decrypted packets. This is

configured in register MODE on page 106.

AES CCM

RFU

EPL

L+4 RFU

Unencrypted packet

Encrypted packet

MIC

Scratch area

CCM data

structure

MODE = DECRYPTION

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

PL: unencrypted payload

EPL: encrypted payload

OUTPTR

INPTR

SCRATCHPTR

CNFPTR

Figure 31: Decryption

6.4.4 AES CCM and RADIO concurrent operation

The CCM peripheral is able to encrypt/decrypt data synchronously to data being transmitted or received

on the radio.

In order for CCM to run synchronously with the radio, the data rate setting in register MODE on page

106 needs to match the radio data rate. The settings in this register apply whenever either the KSGEN or

CRYPT tasks are triggered.

The data rate setting of register MODE on page 106 can also be overridden on-the-fly during an ongoing

encrypt/decrypt operation by the contents of register RATEOVERRIDE on page 108. The data rate setting

in this register applies whenever the RATEOVERRIDE task is triggered. This feature can be useful in cases

where the radio data rate is changed during an ongoing packet transaction.

6.4.5 Encrypting packets on-the-fly in radio transmit mode

When the AES CCM peripheral encrypts a packet on-the-fly while RADIO is transmitting it, RADIO must

read the encrypted packet from the same memory location that the AES CCM peripheral is writing to.

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

PACKETPTR pointer in the radio, see Configuration of on-the-fly encryption on page 99.

AES CCM

RFU

EPL

L+4 RFU

Unencrypted packet

Encrypted packet

MIC

Scratch area

CCM data

structure

MODE = ENCRYPTION

RADIO

TXEN

To remote

receiver

H: Header (S0)

L: Length

RFU: reserved for future use (S1)

PL: unencrypted payload

EPL: encrypted payload

INPTR

OUTPTR

PACKETPTR

SCRATCHPTR

CNFPTR

Figure 32: Configuration of on-the-fly encryption

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

generation to complete before packet encryption begins.

For short packets (MODE.LENGTH = Default), the KSGEN task must be triggered before or at the same

time as the START task in RADIO is triggered. In addition, the shortcut between the ENDKSGEN event and

the CRYPT task must be enabled. This use-case is illustrated in On-the-fly encryption of short packets

(MODE.LENGTH = Default) using a PPI connection on page 100. It uses a PPI connection between the

READY event in RADIO and the KSGEN task in the AES CCM peripheral.

4463_156 v1.0

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