BQ27541 Datasheet, PDF(30/38 Page) Texas Instruments – Single Cell Li-Ion Battery Fuel Gauge for Battery Pack Integration

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bq27541

SLUS861 â DECEMBER 2008 ........................................................................................................................................................................................... www.ti.com

COMMUNICATIONS

AUTHENTICATION

The bq27541 can act as a SHA-1/HMAC authentication slave by using its internal engine. Sending a 160-bit

SHA-1 challenge message to the bq27541 will cause the gauge to return a 160-bit digest, based upon the

challenge message and a hidden, 128-bit plain-text authentication key. If this digest matches an identical one

generated by a host or dedicated authentication master, and when operating on the same challenge message

and using the same plain text keys, the authentication process is successful.

KEY PROGRAMMING (DATA FLASH KEY)

By default, the bq27541 contains a default plain-text authentication key of

0x0123456789ABCDEFFEDCBA9876543210. This default key is intended for development purposes. It should

be changed to a secret key and the part immediately sealed, before putting a pack into operation. Once written, a

new plain-text key cannot be read again from the fuel gauge while in SEALED mode.

Once the bq27541 is UNSEALED, the authentication key can be changed from its default value by writing to the

Authentication( ) Extended Data Command locations. A 0x00 is written to BlockDataControl( ) to enable the

authentication data commands. The bq27541 is now prepared to receive the 16-byte plain-text key, which must

begin at command location 0x40 and ending at 0x4f. Once written, the key is accepted when a successful

checksum for the key has been written to AuthenticateChecksum( ). The gauge can then be SEALED again.

KEY PROGRAMMING (THE SECURE MEMORY KEY)

As the name suggests, the bq27541 secure-memory authentication key is stored in the secure memory of the

bq27541. If a secure-memory key has been established, only this key can be used for authentication challenges

(the programmable data flash key is not available). The selected key can only be established/programmed by

special arrangements with TI, using the TIâs Secure B-to-B Protocol. The secure-memory key can never be

changed or read from the bq27541.

EXECUTING AN AUTHENTICATION QUERY

To execute an authentication query in UNSEALED mode, a host must first write 0x01 to the BlockDataControl( )

command, to enable the authentication data commands. If in SEALED mode, 0x00 must be written to

DataFlashBlock( ), instead.

Next, the host writes a 20-byte authentication challenge to the Authenticate( ) address locations (0x40 through

0x53). After a valid checksum for the challenge is written to AuthenticateChecksum( ), the bq27541 uses the

challenge to perform it own the SHA-1/HMAC computation, in conjunction with its programmed keys. The

resulting digest is written to AuthenticateData( ), overwriting the pre-existing challenge. The host may then read

this response and compare it against the result created by its own parallel computation.

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