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TXB0104: TXB0104 sometimes after power up not working

Karsten Kankowski
Intellectual 270 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXS0206A

Dear Experts,

I have an issue with TXB0104 used to translate 5V  <-> 3.3V to connect an MCU ATMEGA1284P to a SD-card. Please see attache part of schematic.
After power up the TXB0104 often do not work. I control the OE using a small transistor to switch to HIGH ( 3.3V ) 500ms after Power is up which seemed long enought to get a stable VCCA and VCCB. I think it should be possible to drive OE directly by 5V logic of MCU because the OE input is 5V tolerance but I´m not sure. The OE is pulled down by 3.9K resistor. The SPI interface is initialized before OE is set to HIGH

The question is why TXB0104 often do not work after power up ??

Thanks a lot for all your input.

Best Regards
Karsten

  • 0
    Intellectual 270 points

    Just a notet about OE:

    In the Datasheet chapter 10:
    "To ensure the high-impedance state of the outputs during power up or power down, the OE input pin must be tied to GND through a pulldown resistor and must not be enabled until VCCA and VCCB are fully ramped and stable."
    - This means I have to ensure that VCCA and VCCB are stabel befor OE can be set to HIGH therefore a timing is necessary.

    In Application Report SCEA043–March 2010 chapter 4:
    "If the application does not require output enable control, The OE pin should be tied to the VCCA supply."
    - This means a simple wiring to VCCA will work!

  • 0
    Guru 316780 points

    T1 should have been a PNP. But you can simply connect OE directly to the MCU.

    But are you sure that a TXB translator is compatible with the electrical characteristics of SD cards? TI makes specialized SD card translators (e.g., TXS0206A), but not for 5V.

  • 0 in reply to Clemens Ladisch
    Intellectual 270 points
    Hi Clemens,

    Thanks for your input.

    I saw the TXS0206A but it´s hard to get, only up to 3.3v ( I need the 5V for other logic parts ) and DSBGA size wich I can´t Implement on the PCB. I will try the TXS0104 which is pin-compatible to the TXB0104.

    At least I do not want to do drive OE explicite. The best case will be to tie this to VCCA. But I´m confused about the Datasheet and Application report which tell complete different things how to handel OE for my point of view.

    To connect SD-Cards it´s not necessary to have a automatic bidirectional level shifter in general. But the TXB0104 has a nice form factor and my first approach was that I do ne care about the directions and therefore the pin-layout at PCB routing....

    So I wonder what is the exact reason that the TXB0104 is sometimes not working. It looks like that the automatic direction switch do not correctly work in my schematic. But I do not know what to do...

    An option can be to fix the direction of the driver, but how to do this?


    Best regards
    Karsten
  • 0 in reply to Karsten Kankowski
    Guru 316780 points

    You can connect OE directly to VCC. You need to drive it from the MCU only if you actually need all pins to be high impedance at power up.

    The TXB implements automatic direction sensing by having deliberately weak output drivers, which are active all the time. This means that the other devices need to have enough drive strength to override the TXB, and that the TXB is not able to override most pull-up/-down resistors.

    If your signals are unidirectional, use unidirectional or direction-controlled translators.

  • 0 in reply to Karsten Kankowski
    TI__Guru 62080 points

    Clemens,

    As always, appreciate your valuable inputs.

     Karsten,

    OE can be directly connected to the Vcca supply. Since they are over voltage tolerant, it can also be connected to Vccb to have the device always enabled once its powered up.

    The datasheet recommends the pulldown resistor only in case when the IO ports are to be in Hi-Z during power up/down.

  • 0 in reply to ShreyasRao
    Intellectual 270 points
    Dear Shreyas,
    Thanks for clarification that OE can be directly tied to VCCB. This make things quite easy.
    I will try the TXB again in hope the SD-Card´s can drive the weak buffer. If not I will try the TXS or Unidirectional translators.
    Thanks a lot!
  • 0 in reply to Karsten Kankowski
    TI__Guru 62080 points
    Karsten,

    Please let us know how it goes.