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The I2C accelerometer does not work.

Juan Andreu
Prodigy 170 points
Other Parts Discussed in Thread: TXB0108, TXS0108E

Hello,

I am using the accelerometer MMA7455, this works to 2.4 to 3.6V, while the entire board works at 5V.
To maintain the same voltages of the circuit board and accelerometer, I put the TXB0108 that is able to raise the voltage to 5V (3.3V to 5V).
By putting up the circuit I do not receive the coordinates X, Y, Z in the Display.

If not because of TXB0108 or otherwise, or should also have put other pull-up resistors on the lines between the MMA7455 and TXB0108 (B_SDA_A and B_SCL_A).

Where you could be wrong?

attached circuit diagram.

THX.

  • 0
    TI__Expert 6335 points

    Juan,

    The TXB family of voltage translators do not support open drain interfaces such as I2C (see section 8.1 of the TXB0108 datasheet, which is reproduced below). Our TXS family ofvoltage translators support both open drain and push-pull interfaces.

    My recommendation is to replace the TXB0108PW device with a TXS0108EPW device. It is the same package and pinout.

  • 0 in reply to Nirav Patel
    Prodigy 170 points
    Hey , thanks you.
    One question....
    Once I replaced by TXS0108EPW not have to put pull-up resistors between the accelerometer and TXS0108EPW (in B_SCL_A and B_SDA_A lines).
    In the datasheet it makes carrying the integrated pull-up resistor that value are or have nothing to do with the pull-up resistors that are placed on the I2C bus .

    DATASHEET 8.1
    The pull-up resistors, commonly used in open-drain applications, have been conveniently integrated so
    that an external resistor is not needed. While this device is designed for open-drain applications, the device can
    also translate push-pull CMOS logic outputs.

    THX.
  • 0 in reply to Juan Andreu
    TI__Expert 6335 points

    Juan - Sorry for the late response. My feedback is below.

    Juan Andreu said:

    Once I replaced by TXS0108EPW not have to put pull-up resistors between the accelerometer and TXS0108EPW (in B_SCL_A and B_SDA_A lines).
    In the datasheet it makes carrying the integrated pull-up resistor that value are or have nothing to do with the pull-up resistors that are placed on the I2C bus .

    Most likely the external pullup resistors (R1 and R2 in your schematic - 1kohm) will not be required.

    1. External pullup resistors are required for open-drain interfaces like I2C --> TXS0108E has integrated pullup resistors

    2. Strong pullup resistors might be necessary to improve signal rise time --> TXS0108E has rise time accelerators to improve signal rise time.

    My recommendation is to verify your setup with the TXS0108E by uninstalling R1 and R2. Can you let me know the results? Thanks.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 170 points
    Hello,
    Once placed TXS0108E, everything works perfectly. Without having to put pull-up resistors between the accelerometer and TXS0108E.
    THX.