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SN74LVC1T45 input

Doug Vetter
Prodigy 200 points
Other Parts Discussed in Thread: SN74LVC1T45, TXS0104E, TXB0104

I'm considering the use of the SN74LVC1T45 for voltage translation for a SPI bus as well as some other control signals between a 3.3V microcontroller and a 5V sensor.  As these are all unidirectional signals I am planning to tie the direction pin high or low accordingly to turn these into unidirectional translators.

Other than the dual power supplies the SN74LVC1T45 appears be largely similar to other 74 series logic.  In the past working with 74-series logic without a hold feature I have been instructed by TI to use pullups or pulldowns of sufficient strength on the inputs to ensure the outputs are always in a known / default state, even at power up, to ensure power consumption is minimized and also to prevent damage to the device.

Are pullups / pulldowns on the inputs to the SN74LVC1T45 required for this purpose or is is okay to leave the inputs floating on power up and during periods between communication activity (i.e. microcontroller GPIO driver is high Z)?

-Doug

  • 0
    TI__Expert 6335 points

    Doug,

    The SN74LVC1T45 requires external pullups/pulldowns. Our TXB0104 and TXS0104E can also be implemented for SPI. Would this work for your application? You would not require external pullups/pulldowns.
     
    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 200 points
    Okay, thanks. I didn't see the H version but I'll check it out.

    I looked at the TXB and TXS series earlier today and they were my first choice. Problem is:

    TXB requires other devices to have > 2mA drive to overcome the internal 4K pullups and while my sensor's IOs can source 5mA they can only sink 0.8mA.

    TXS also appears to require pullups on the B side even on push-pull signals like SPI (correct me if I'm wrong) and I was hoping to minimize the parts count.

    So I suppose I can go with either the SN74LVCH1T45 or the TXS series.

    Thanks for the response.
  • 0 in reply to Doug Vetter
    TI__Expert 6335 points
    Doug,

    1. I misspoke on the SN74LVCH1T45. That part does not exist. My apologies.
    2. The TXS translators have integrated 10k pullup resistors on both 'A' and 'B' ports. You would not require any external resistors.

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    Prodigy 200 points
    With regard to:

    1) I discovered that pretty quickly when I tried to find it on Digikey.

    I have decided to go ahead with the part that does exist (1 bit, no bus hold) mostly because I have only 9 signals bridging the power domains, they're a blend of push-pull and open drain, TX and RX, and some need to be tied high or low simply to ensure the circuits are in the right state for my application when the micro outputs go High Z.

    As it turns out bus hold doesn't really solve the application-specific initial state problem -- it simply ensures the buffers won't chatter / draw current. So I need pullups / pulldowns anyway, and while they can be added to the input of a device with bus hold, from what I've read that is possible, but discouraged.

    So it's the SN74LVC1T45 for me.

    2) I don't see how the TXS would be compatible with push-pull signals that need to be tied down by default, as the pullups are in fact required to get the B side to reach the rail when the A side is driven high and the switch turns off. A nice part for open-drain, though.

    Thanks for the followup.