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Original question:

TXB0108: Level Translator functionality

TXB0108: Logic forum

song.y
Prodigy 10 points
Part Number: TXB0108

Our current design is shown in the figure.

Before the FPGA is started and the configuration is completed, the GPIO of the FPGA has a built-in 75K pull-up resistor.

If the pull-down resistor is not added to the B port, then the B port will be high. However, we want the B port to be low before the FPGA configuration is complete.

Therefore, we added a pull-down resistor on the B port.
In debugging, we found that. When the pull-down resistor is 100K, the B port may also be high when the system is powered on. When the pull-down resistance is continuously reduced to about 22K. When the system is powered on, the B port is always kept low. I would like to ask, is there any theoretical support for the value of the pull-down resistor?

  • 0
    Guru 316690 points

    You cannot force the active A and B pins to have different values without breaking the normal functionality.

    75 kΩ resistors would be OK (see section 8.3.5). To disable the TXB during FPGA power up, pull OE low.

  • 0
    TI__Guru 50496 points

    Hi Song,

    Per Clemens feedback, could you help clarify if any additional concerns? Thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    Sorry for the late reply.
    Let me rephrase the problem I'm having.
    The A port of TXB0108 is connected to the GPIO of the FPGA, and the FPGA has a built-in 75K pull-up resistor by default. The B port of TXB0108 is connected to the chip select control signal of QSPI FLASH. When the system is powered on, before the FPGA completes boot, its GPIO port causes the B port of the TXB0108 to be at high level due to the pull-up resistor. We do not want this situation.
    So, I tried to add a pull-down resistor to the B port of TXB0108 to give the B port a default level state. According to the TXB datasheet, I initially chose a 100K pull-down resistor. However, testing with an oscilloscope shows that when the system is powered on, the B port is sometimes at a high level and sometimes at a low level. So, I lowered the resistance of the pull-down resistor on port B from 100K to 22K. After many tests, when the system is powered on, the B port is stable and low.
    Because this 22K is a test result, I want to have its theoretical support.

  • 0 in reply to song.y
    Guru 316690 points

    22 kΩ works because it is stronger than the TXB output that tries to go high initially. But you must not use a pull-down resistor with less than 50 kΩ. With such a resistor, the output might not be able to output a proper high-level signal when you actually want it to work.

    The only way to solve your problem is by pulling OE low during power up.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    1. When using a 22K pull-down resistor, the high level of port B is 2.8V, no problem.
    2, 22 kΩ works because it is stronger than the TXB output that tries to go high initially//Is this statement supported by theoretical calculations?
    Thank you very much for your reply, looking forward to your reply again.

  • 0 in reply to song.y
    Guru 316690 points

    1. It is not guaranteed; temperature or manufacturing variation might change this.

    2. No. You're lucky that it works. There is no guarantee.