BQ27541-V200_1007 Datasheet, PDF (32/43 Pages) Texas Instruments – Single Cell Li-Ion Battery Fuel Gauge for Battery Pack Integration

bq27541-V200
SLUSA11 – FEBRUARY 2010
www.ti.com
The fuel gauge also performs a single offset 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 80°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
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 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 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.
HDQ SINGLE-PIN SERIAL INTERFACE
The HDQ interface is an asynchronous return-to-one protocol where a processor sends the command code to
the bq27541. With HDQ, the least significant bit (LSB) of a data byte (command) or word (data) is transmitted
first. Note that the DATA signal on pin 12 is open-drain and requires an external pull-up resistor. The 8-bit
command code consists of two fields: the 7-bit HDQ command code (bits 0–6) and the 1-bit R/W field (MSB
bit 7). The R/W field directs the bq27541 either to
• Store the next 8 or 16 bits of data to a specified register or
• Output 8 bits of data from the specified register
The HDQ peripheral can transmit and receive data as either an HDQ master or slave.
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