BQ27350 Datasheet, PDF (27/33 Pages) Texas Instruments – Single Cell Li-Ion Battery Manager With Impedance Track Fuel Gauge Technology

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bq27350
Single Cell Li-Ion Battery Manager
With Impedance Track Fuel Gauge Technology
SLUS754 – MARCH 2007
6.2 AUTHENTICATION
The bq27350 can act as a SHA-1/HMAC authentication slave, either by using its internal engine or by
using an external bq26100 (contains secure memory). In both implementations, sending a 160-bit SHA-1
challenge message to the bq27350 will cause the IC to return a 160-bit digest, based upon the challenge
message and hidden plain-text authentication keys. When this digest matches an identical one, generated
by a host or dedicated authenication master operating on the same challenge message and using the
same plain text keys, the authentication process is successful.
The
bq27350
contains
a
default
plain-text
authentication
key
of
0x0123456789ABCDEFFEDCBA987654321. If using the bq27350's internal authentication engine, the
default key can be used for development purposes, but should be changed to a secret key and the part
immediately sealed, before putting a pack into operation. If using the a bq26100 for external
authentication, the key must be changed to 16 bytes of "0x00".
6.2.1 KEY PROGRAMMING
When the bq27350's SHA-1/HMAC internal engine is used, authentication keys are stored as plain-text in
memory. A plain-text authentication key can only be written to the bq27350 while the IC is in UNSEALED
mode. Once the IC is UNSEALED, a 0x00 is written to BlockDataControl( ) to enable the authentication
data commands. Next, subclass ID and offset are specified by writing 0x70 and 0x00 to DataFlashClass( )
and DataFlashBlock( ), respectively. The bq27350 is now prepared to receive the 16-byte plain-text key,
which must begin at command location 0x4C. The key is accepted once a successful checksum has been
written to BlockDataChecksum( ), FOR THE ENTIRE 32-BYTE BLOCK (0x40 through 0x5f), NOT JUST
THE 16-BYTE KEY.
When using an external bq26100 authentication IC, authentication keys are stored within the bq26100's
secure memory. The bq26100 uses a split-key system of two 64-bit keys, instead of a single 128-bit key.
Details on programming these keys can be found in the bq26100 data sheet.
6.2.2 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 AuthenticateData( ) address locations (0x40 through 0x53). After a valid checksum for the challenge is
written to AuthenticateChecksum( ), the bq27350 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.
7 REFERENCE SCHEMATIC
The reference circuit on the next page shows an I²C communication implementation of the bq27350. D1
and associated resistors provide added protection against ESD appearing on the communication lines.
Capacitors C4/C8 and C15/C16 are also used for ESD protection and are paired together for increased
voltage handling ability and redundancy protection, should one capacitor fail in a shorted state.
Optional circuits for external authentication, using the bq26100 (U1), and external temperature
measurements (R12, R14, and C10) are also shown.
Common-mode and differential filters (R7-19 and C12-14) remove noise at input of the coulomb counter
input. U4, Q4, and associated passives form the Li+ cell protector, while U3, C5, and C9 comprise the
LDO IC and its input/output capacitors. R5 and C7 provide a delayed "power good" signal to the MRST
input.
Q3, R13, and D2 act as an LDO disable circuit (LDO is off when PACK- and SE pin are both "high").
Finally, Q1, Q2, R1, R2 and R4 form a precision voltage divider that reduces cell voltage to levels suitable
for the bq27350.
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REFERENCE SCHEMATIC
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