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BQ76925EVM: I2C pull-up resistor on bq76925EVM

Beginner
Intellectual 2491 points

Part Number: BQ76925EVM
Other Parts Discussed in Thread: BQ76925

According to the schematic for bq76925EVM, we place 3.3K pull-up resistor for I2C comm. 

Normally, we will recommend to place 4.7k to 10kohm. May I learn from you if there is a specified reason for this value?

  • 0
    TI__Guru 54715 points

    Hi Beginner,

    The I2C bus is an open drain bus with the high level and rise time typically provided by a pull up resistor.  The pull up voltage will set the high level.  The resistor will set the current needed when low and along with the capacitance of the bus, the rise time of the signal.  See the I2C specification available at https://www.i2c-bus.org/ for details on limits, it has a section 7.1 on sizing pull up resistors.

    A smaller pull up resistor is typically used for a heavily loaded bus, or long distance, or high capacitance, or faster speed.  In a battery the bus is often very short with light load, and being a battery with limited capacity, low current is desired.  So the pull up resistors are often large, but may be adjusted for the individual design.

    I don't know why the BQ76925 recommended the smaller pull up resistors. 3.3V and 3.3k will allow 1 mA of current per pull up.  Construction of the addressing for the BQ76925 avoided register addresses for faster transfer, but the clock speed is limited to 100kHz.  The system engineers may have needed the smaller value to meet the rise time specification, or wanted a fast edge for better margin.