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SN74LVCH8T245: SN74LVCH8T245 - Bus hold option and pull-up / pull-down resistors

Sunset
Genius 13965 points
Part Number: SN74LVCH8T245
Other Parts Discussed in Thread: SN74LVC8T245, , SN74LXCH8T245, SN74LXC8T245

Hello Team,

following up a similar topic as the following thread, I would like to add comments and question.

Going from from SN74LVC8T245 to SN74LVCH8T245.

Pull-up and pull down are not recommended.

We want to make sure that on the input either a 1 or 0 is fixed, some inputs need 1 and other 0.

So the output would have a fixed known value.

  • What would be the best way to do that with the bus hold option?

Today design is based on SN74LVC8T245, and there are some inputs that are with 5K pull up while other that have 5K pull down.

From the thread above, is written that resistors should be below 1K.

  • Would 5K still work or must be changed/removed?
  • Having 1K pull up or pull down on the input , will increase the current in the IC (as input)?

rephrasing the last question with the schematic below

The 1K resistor must be driven, so as load it will let the current of the driver be increased.

If I use the pin of the SN74LVCH8T245 in one direction, does the IC current increase if I do have 1K resistor on the input?

If it works in one direction, I would not think so.

Thanks,

SunSet

  • 0
    TI__Mastermind 37795 points

    Hi SunSet,

    Based on Emrys' answer in the linked thread, in the end, he still recommends to not use pull-down/up resistors. By adding resistors to a bus-hold input, this will add extra current drain unless the resistance is low enough.

    You can also read through section 6 of this Application Note: 

  • 0
    TI__Genius 13965 points

    Hello Albert,

    thanks for the feedback.

    Unfortunately there are not too many options.

    The PCB exists, the SN74LVC8T245 is out of stock, the SN74LVCH8T245  is an alternative.

    The value on the bits must be known.

    So the only option with the bus hold is to go lower than 1K pull-up or down.

    What I still do not understand:

    By adding resistors to a bus-hold input, this will add extra current drain unless the resistance is low enough.

    If I do have one bus direction, and I have the driver with push-pull output, driving the SN74LVCH8T245 as the previous thread, there is an increase of current going from 5K down to 1K, but this should flow in both directions (depending if pull-up or pull down is used)  from and into the driver, not the  SN74LVCH8T245 .

    Am I right?

    Or the current on the Vcc pin of the SN74LVCH8T245  also increases?

    Thanks,

    SunSet

  • 0 in reply to Sunset
    TI__Genius 16735 points

    Hi Sunset,

    Firstly, I want to offer our newly released SN74LXCH8T245 or SN74LXC8T245 whichever device you are looking to choose.

    Secondly, yes having very low resistance at the output will not allow your output to switch because all the current will be drawn into the resistor. Also having a very low resistance at the input will allow your device to switch but you will see an increase in current consumption on your power supply. This is the dilemma with using external pullups or pulldowns with bushold circuits. Is there no way for them to DNP the resistors at the I/O?

    Regards,

    Karan