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TXS0108E: OE pin setting

Wen Li
Expert 5251 points
Part Number: TXS0108E
Other Parts Discussed in Thread: USB2ANY

In the TXS0108E data sheet, OE pin is pulled down to ground permanently via a resistor. Will this work for the normal operation after the device powered up?

Based on my understanding (by reading the datasheet), during the normal operation, the OE pin should be high. So after power up, OE pin should be pulled up externally, or not.

Thanks and regards

Wen

  • 0
    Guru 297030 points

    The OE pin is also connected to the controller, which can force it high:

    This is done to ensure that the TXS is disabled when the system is powered up.

    If you do not care about random I/O states during power up, you can connect OE directly to VCC.

  • 0 in reply to Clemens Ladisch
    TI__Expert 5251 points

    Great, thanks!

    I created a schematics as below. The OE is connected to a GPIO pin of a micro-controller. During power-up, the GPIOs will be at the input mode, so the Resistor will pull the OE low. After power up, the micro-controller will take over the control of OE. The controller should drive the GPIO to high, if it wants to enable this interface.

    This is a great device for this application. It should be used for all boards that will be connected to TI's "USB2ANY" adapter; which enable an external USB to SPI/I2C communication. Please confirm if the schematics below is correct. Based on the datasheet, I assume no pull-up Rs will be needed for I2C on A-side.

    Best regards

    Wen Li      

  • 0 in reply to Wen Li
    TI__Genius 16735 points

    Hi,

    The TXS has internal pullups so you won't require external pullups unless you want a stronger pullup.

    Do note that the VCCA < VCCB so your external device will need to have a higher Vcc.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Expert 5251 points

    Thanks for the quick reply.

    The nominal value of VCCB (from USB2ANY) is at 3.3V. And my local digital digital supply is also 3.3V (Norminal). What if the VCCB is slightly lower than VCCA, e.g. VCCB=3.25V and VCCA=3.3V ; what will happen?

    Thanks and regards

    Wen

  • 0 in reply to Wen Li
    Guru 297030 points

    VCCA and VCCB Bias Requirements for Bidirectional Translators (SCEA060) says:

    TXS and TXB-type translators require that VCCA is less than or equal to VCCB. This is due to an internal protection diode that can become forward biased when the voltage on VCCA exceeds the voltage on VCCB. […] VCCB fixed at 2.5 V and VCCA swept from 2.5 V to 3.5 V:

    As shown in Figure 4, … when the voltage on VCCA exceeds the voltage on VCCB by roughly 0.6 V, the diode begins to conduct and the ICCA current begins to drastically increase as VCCA increases.

  • 0 in reply to Clemens Ladisch
    TI__Expert 5251 points

    Thanks to all for the great!