ATECC508A datasheet(61/109 Pages) MICROCHIP

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execution is in progress. There is no way to reset the counter other than to put the device into sleep or

idle mode and then wake it up again.

The watchdog timer is implemented as a fail-safe mechanism where no matter what happens on either

the system side or inside the device, including any I/O synchronization issue, power consumption will fall

to the ultra-low sleep level automatically.

The device resets the values stored in the SRAM and internal status registers when it transitions to the

sleep mode; however, if the device is explicitly put into the idle mode through the appropriate I/O

sequence, then the device retains the contents of the two SRAM registers (TempKey and RNG Seed).

Normally, all command sequences must complete within tWATCHDOG if they require a state that is stored in

the SRAM registers. If there is a need to implement a command sequence that is longer than the

watchdog interval, the system software can use this idle mode mechanism to ensure that the sequence

can complete successfully..

If a command is attempted when insufficient time remains prior to watchdog timer execution, the device

will return the Watchdog Timeout error code without attempting to execute the command. This feature

prevents situations in which the command may only be partially executed at the time the watchdog timer

resets the device. In particular, the limited use counter is always decremented prior to execution of the

crypto computation; therefore, an aborted command might result in fewer counts remaining without the

result being available to the system. The device will never be left in an unusable state after an aborted

command.

9.2

CheckMac Command

The CheckMAC command calculates a MAC response that would have been generated on an

ATECC508A, ATECC108A, or ATSHA204A device and then compares the result with the input value. It

returns a boolean result to indicate the success or failure of the comparison.

Prior to running this command, the Nonce and/or GenDig commands may have been optionally run to

create a key or nonce value in TempKey. The input mode parameter determines the source of the key

(the first 32 bytes of SHA message) and challenge/nonce (the second 32 bytes of SHA message). If

KeyConfig.ReqRandom is one, the RNG must have been used during the execution of the Nonce

command, or else this command will return an error.

If authorization is required by the KeyConfig before use of a key, this authorization function can be

accomplished by executing this command with Mode<1> set to zero. TempKey should have been

previously loaded with a nonce via the Nonce command. If KeyConfig.ReqRandom is one, the RNG

should have been used during the execution of that Nonce command. If the CheckMac succeeds, then

an internal AuthValid flag will be set and KeyID retained internally in AuthKeyID. See Section Authorized

Keys for more details.

If the comparison matches, then the target EEPROM slot value may be copied into TempKey. If KeyID is

even, then the target slot is KeyID+1, or else the target slot is KeyID. For the copy to take place the mode

parameter to CheckMac must have a value of 0x01 or 0x05 and SlotConfig.ReadKey for the target key

must be zero. This copy will take place regardless of the value of SlotConfig.LimitedUse and/or

LastKeyUse for the target slot.

ATECC508A

Security Commands

Datasheet Complete

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