SLUUC54C February 2020 – March 2024 BQ27Z558 , BQ27Z561 , BQ27Z561-R2
The HDQ interface is an asynchronous return-to-one protocol where a processor sends the command code to the fuel gauge. With HDQ, the least significant bit (LSB) of a data byte (command) or word (data) is transmitted first.
The HDQ pin (A3) is open-drain and requires an external pullup resistor.
The 8-bit command code consists of two fields: the 7-bit HDQ command code (bits 0:6) and the 1-bit RW field (MSB bit?7). The RW field directs the fuel gauge either to:
The HDQ peripheral can transmit and receive data as either an HDQ master or slave.
The host processor initiates HDQ serial communication by sending a break command to the fuel gauge. A break is detected when the DATA pin is driven to a logic-low state for a time t(B) or greater. The DATA pin should then be returned to its normal ready high logic state for a time t(BR). The fuel gauge is now ready to receive information from the host processor.
The HDQ Communication Basics Application Report (SLUA408) provides details of HDQ communication basics, including an alternative method to use a standard two-wire UART for single-wire HDQ communication.
The BQ27Z561-R2 gauge is shipped in I2C mode, but TI provides a method to switch to HDQ mode via SwitchToHDQ() (see AltManufacturerAccess() 0x7C40 for details).