Not Recommended for New Designs

bq28550-R1

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SLUSAS4A – OCTOBER 2012 – REVISED SEPTEMBER 2014

The communication format and protocol complies with the SMBus.

7.6.3 Programming

7.6.3.1 General Purpose Input-Output

7.6.3.1.1

CFET Control

The GPIO can be configured to provide a signal called Charge FET control (CFET) using firmware. This output

controls external circuitry to change the state of the external CHG FET. A low signal on this pin in association

with recommended external components turns OFF the CHG FET. A high output maintains or turns ON the CHG

FET after a valid turn OFF activity. The state of the GPIO on power-up will be low until the system is initialized

and an internal power supply is active. The CFET control bit is located in the Control status register. If this output

is not used, then connect this pin to VREG externally.

The following parameters can be programmed in firmware for monitoring and protection using CFET control

circuitry, providing the limits are set within the boundaries of the hardware thresholds:

•

Cell Overvoltage Condition

•

Overcurrent Charging Condition

•

Overtemperature Charging Condition

7.6.3.1.2

ALERT Output

Another option for the GPIO is to be configured as an ALERT output to indicate any one of the following fault

conditions, and to provide an interrupt signal to the host microprocessor:

•

Overcurrent Charging Condition

•

Overcurrent Discharging Condition

•

Overtemperature Condition

•

Overvoltage Condition

•

Undervoltage Condition

The threshold trigger for these conditions is set through firmware registers, with each fault condition to be

selectable for detection. The output is an open drain output with an external pull-up (through a 3.3k resistor). The

ALERT output can be reset after the fault condition is removed OR can be latched and reset only by the host

MCU resetting the ALERT register value to zeroes. If this output is not used, then connect this pin to VREG

externally.

7.6.4 Communications

7.6.4.1 Authentication

The bq28550-R1 device can act as a SHA-1/HMAC authentication slave by using its internal engine. Refer to the

Application Note SLUA359 for SHA-1/HMAC for information.

By sending a 160-bit SHA-1 challenge message to the bq28550-R1 device, it causes 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.

7.6.4.1.1

Key Programming (Data Flash Key)

By

default,

the

bq28550-R1

device

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 gas gauge while in SEALED mode.

Once the bq28550-R1 device is FULL ACCESS, 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 bq28550-R1 device is now prepared to receive the 16-byte plain-

text key, which must begin at the 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.

Copyright © 2012–2014, Texas Instruments Incorporated

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