BQ26100_15 Datasheet, PDF (8/29 Pages) Texas Instruments – Based Security and Authentication IC

bq26100
SLUS696B – JUNE 2006 – REVISED AUGUST 2015
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7.3 Feature Description
7.3.1 Non-Volatile Memory
The bq26100 device has a bq2022 compatible memory and command structure with new commands to access
added memory. The bq26100 device uses a combination of non-volatile OTP and non-volatile EEPROM. The
OTP should be programmed in the factory as an external voltage is required to program the bits; the EEPROM
can be programmed in the field, with the programming voltage generated automatically by an internal-charge
pump.
Four pages of 32x8-bits OTP are accessed with the bq2022 compatible command set, while a fifth page of 32x8-
bits are accessed with a new command set. Each page of OTP can be locked once programmed, blocking
further writes to the page. There is an additional provision to allow for page redirection at the host in the event
that a page is programmed incorrectly. The redirection is not automatic, but a host system can determine where
a page redirection is occurring and read the appropriate page for uncorrupted data.
The EEPROM consists of 16x8-bits that can be written in the same way as for RAM-based volatile memory. The
timing of the writes is different than writing to RAM to allow for the internal charge pump to create the voltage
necessary to set the bit values.
7.3.2 Authentication
The bq26100 device contains a SHA-1 engine to generate a modified version of the FIPS 180 HMAC. The
authentication uses a challenge or public message transmitted from the host and a secret key stored on the
bq26100 device to generate a 160-bit hash that will be unique. The contents of the challenge are unimportant,
but each challenge should be generated randomly to improve the security of the authentication.
To compute the HMAC, let H designate the SHA-1 hash function, M designate the message transmitted to the
bq26100 device, and KD designate the unique 128 bit device key of the device. HMAC(M) is defined as:
H[KD || H(KD || M)]
where
• || symbolizes an append operation
(1)
The message, M, is appended to the device key, KD, and padded to become the input to the SHA-1 hash. The
output of this first calculation is then appended to the device key, KD, padded again, and cycled through the
SHA-1 hash a second time. The output is the HMAC digest value.
The secret key is stored in separate OTP available in bq26100. The key space is split into two 64-bit spaces that
can be programmed and locked at separate times, providing an opportunity to split the key between two different
programming entities to ensure that no key leak can occur from a single source.
7.3.3 Communication and Power
The bq26100 device uses a single-wire communication protocol, SDQ, that allows for broadcast or targeted
communication to a number of devices on the one-wire bus. Each device is programmed with a unique 64-bit
address and the protocol consists of an automatic arbitration scheme that allows the host to determine the ID of
every device on the bus.
The bq26100 device takes advantage of the pullup on the SDQ line to power a capacitor connected to the PWR
pin and the charge on this capacitor is used parasitically when the SDQ line is low. As a result, there is no need
for additional power to be supplied to the device.
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