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TXB0104: TXB0104 problem

ROGER BORZUMATI
Prodigy 170 points
Part Number: TXB0104
Other Parts Discussed in Thread: TMS320F28034, SN74AVC4T774, SN74LVC2T45, SN74LV4T125, SN74LV1T126, SN74LVC1T45

Good morning, i am using TXB0104 for shifting SPI signal of 3.3V from a DSP (in particular TMS320F28034) to 5V SPI for interfacing with a slave SPI device.

I read of possible problem of TXB0104 to drive some loads in its I/O pin in DC. For example, CS signal remains low for enough time during the transition: it is a problem for the TXB0104 to mantain this DC low level? How much is its load current and capacitance capability? Another example could be when MISO or MOSI (for example in a 8 bit SPI communication) have all bits to 0 or all bits to 1 for the entire time. Could this example be another problem for the peripheral?

Is it necessary to use a buffer?

Thank you.

  • 0
    TI__Guru 57390 points

    Roger,

    The TXB is a weak driver (20uA loading and ~70pF load cap).
    The weak driver is due to the fact that it needs to be auto bidirectional.
    As long as the loading on the IOs are not beyond 20uA, it can be supported well with TXB.
    You could also consider using SN74AVC4T774 for SPI communications with DIR. control for each lines.

  • 0 in reply to ShreyasRao
    Prodigy 170 points

    Good morning, thank for your answer.

    I can't use SN74AVC4T774 because I have to translate a 3.3V SPI to 5V SPI so the component you recommended me is not suitable for the function.

    Other similar component?

    Thank you.

  • 0 in reply to ROGER BORZUMATI
    TI__Guru 57390 points
    Roger,

    Sorry about that. You are right. SN74AVC4T774 cannot be used for 5V conversion.
    The LVC family , SN74LVC2T45 could be considered but you would require 2 of them.
    TXB0104 seems to be the natural choice as a small one chip solution currently assuming the loading is tolerable.
  • 0 in reply to ShreyasRao
    Prodigy 170 points

    Good morning,

    thank for your reply. Since SN74LVC2T45 has a DIR direction for both the inputs, If I consider SPI I have CS,MOSI,SCLK(in a direction) and MISO in another direction; so I could not use the component you recommended me. Rather I have thought to use SN74LV4T125 for the first three signals with 3.3V to 5V translation, whereas for MISO the single SN74LV1T126 with 5V to 3.3V translation.

    In you opinion is this component choicewill work ?

    Thank you

  • 0 in reply to ROGER BORZUMATI
    TI__Guru 57390 points
    Yes, agreed. This will work as well, Roger.
    Usually, the SN74LVC2T45*1 and 2*sn74LVC1T45 is suggested but this suggestion being a 2 chip solution would also work.