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Is it okay to interconnect HDQ and I2C clock line on the BQ27541-G1 ?

Scott Brigham
Prodigy 30 points
Other Parts Discussed in Thread: BQ27541-G1

We want to lay out our circuit board with this fuel gauge chip and an MSP430 so we could change from interfacing to it from HDQ initially to I2C later by changing the software in the MSP430. We would like to use the same MSP430 I/O line to run the HDQ line or the I2C clock line, just because the TI-provided software for these happen to use the same line.


My question is, if we are running in HDQ mode, is there any problem with the I2C clock line being wiggled, and conversely, if we are running in I2C mode, is there any problem with the HDQ line being wiggled?  We could always put two zero ohm resistors in the layout and populate the one that would connect either the I2C clock or the HDQ line to the I/O line but it would be easier just to have them both connected.

  • 0
    TI__Expert 8230 points

    Hello Scott,

    Couple things to notice: the bq27541-G1 is shipped in I2C mode and can be converted to HDQ later by a one-time process where you send a series of commands to the gauge. Once the device is in HDQ, it cannot be converted back to I2C. Reason I mention this is because I noticed you mentioned the design wanted to change from interfacing initially from HDQ to I2C later. 

    Having said that, assuming the gauge is in HDQ mode, having an input on the SCL line shouldn't cause any issues. However, while in I2C mode, the HDQ pin is still active and used for other functionality in the gauge, having SCL and HDQ connected may cause communication errors in the gauge, I verified this on an EVM and I would get NACKs from the gauge.

    I recommend using the zero ohm resistor option or if your GPIO budget allows it, have two separate pins on the MSP430 for HDQ and I2C, and modify the sample code on the MSP430 to reflect the port assignment change.

    Thanks.

  • 0 in reply to Fernando Lopez
    Prodigy 30 points
    Fernando, Thanks for the quick, detailed and helpful answer.