BQ27541-V200_17 Datasheet, PDF (32/46 Pages) Texas Instruments – Single Cell Li-Ion Battery Fuel Gauge for Battery Pack Integration

bq27541-V200
Not Recommended for New Designs
SLUSA11B – FEBRUARY 2010 – REVISED SEPTEMBER 2013
www.ti.com
The fuel gauge also performs a single offset calibration when (1) the condition of AverageCurrent( ) ≤≤ 100mA
and (2) {voltage change since last offset calibration ≥ 256mV} or {temperature change since last offset calibration
is greater than 8°C for ≥ 60s}.
Capacity and current measurements will continue at the last measured rate during the offset calibration when
these measurements cannot be performed. If the battery voltage drops more than 32mV during the offset
calibration, the load current has likely increased considerably; hence, the offset calibration will be aborted.
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
Authenticate( ) Extended Data Command locations. A 0x00 is written to BlockDataControl( ) to enable the
authentication data commands. The DataFlashClass() is issued 112 (0x70) to set the Security class. Up to 32
bytes of data can be read directly from the BlockData() (0x40...0x5f) and the authentication key is located at
0x48 (0x40 + 0x08 offset) to 0x57 (0x40 + 0x17 offset). The new authentication key can be written to the
corresponding locations (0x48 to 0x57) using the BlockData() command. The data is transferred to the data flash
when the correct checksum for the whole block (0x40 to 0x5f) is written to BlockDataChecksum() (0x60). The
checksum is (255- x) where x is the 8-bit summation of the BlockData() (0x40 to 0x5F) on a byte-by-byte basis .
Once the authentication key is written, 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 TI’s 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 the SHA-1/HMAC computation, in conjunction with the programmed key. The resulting
digest is written to Authenticate( ), 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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