ATSHA204A datasheet(17/93 Pages) MICROCHIP

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Table 2-9. Configuration Zone Locking

Lock State

Read Access

Write Access

LockConfig == 0x55 (unlocked)

Read

Write

LockConfig != 0x55 (locked)

Read

<never>

2.1.4.2

Data and OTP Zone Locking

Throughout this document, if LockValue is 0x55, then both the Data and OTP zones are said to be

unlocked; otherwise they are locked.

There is neither read nor write access to the Data and OTP zones prior to locking of the Configuration

zone.

Table 2-10. Data and OTP Zone Access Restrictions

Lock State

Read Access

Write Access

LockValue == 0x55 (unlocked)

<never>

Write

LockValue != 0x55 (locked)

Read(1)

Write(1)

Note:

Based on Slot Configuration for a given slot.

2.1.4.3

OTP Zone Locking

Reads and writes of the OTP zone depend upon the state of the LockConfig, LockValue and OTP mode

bytes in the Configuration zone.

2.2

Static RAM (SRAM)

The device includes an SRAM array that is used to store the input command or output result,

intermediate computation values and/or an ephemeral key. The entire contents of this memory are always

invalidated whenever the device goes into sleep mode or the power is removed. The ephemeral key is

named TempKey and can be used as an input to the MAC, HMAC, CheckMac, GenDig and DeriveKey

commands. It is also used as the Data protection (Encryption or Decryption) key by the Read and Write

commands. See Section TempKey.

2.2.1

TempKey

TempKey is a storage register in the SRAM array that can be used to store an ephemeral result value

from the Nonce, GenDig, CheckMac, or SHA commands. The contents of this register can never be read

from the device (although the device itself can read and use the contents internally).

This register contains the elements shown in the table below.

Table 2-11. TempKey Storage Register

Name

Bit Length Description

TempKey

256

(32-bytes)

Nonce (from Nonce command) or Digest (from GenDig command).

SlotID

If TempKey was generated by GenDig (see the GenData and CheckFlag bits), these

bits indicate which key was used in its computation. The four bits represent one of the

slots of the Data zone.

ATSHA204A

Device Organization

DS40002025A-page 17

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