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SN74AVCBH164245: External pull-up resistor on bus line

Alice57
Intellectual 1730 points
Part Number: SN74AVCBH164245
Other Parts Discussed in Thread: SN74AVCB164245

Hello Team,

Though this device is not recommend to use of pull-up or pull-down resistors with bus line, customer's design specification is required pull-up resistor on input bus line.

Current design is VCCA = 2.5V, VCCB = 3.3V and the direction of data is from B to A. There are pull-up resistor on B bus. When customer increase the pull-up resistor value, data transition is not properly occurred(high to low or low to high). But when customer decrease the pull-up resistor value like 2.2kohm, data transition is smoothly occurred. How we calculate the external pull-up resistor value when we expecting smooth data transition? 

Best regards,

Alice

  • 0
    TI__Guru 59120 points

    Hi Alice,

    Would the customer consider using the non-bushold version of the same device SN74AVCB164245?

    These do not have bus hold circuitry internally and hence can have external pull up resistors.

    For more information on bus hold devices and design considerations, please refer to the app note here:

    www.ti.com/.../scla015b.pdf

  • 0 in reply to ShreyasRao
    TI__Intellectual 1730 points
    Hi Rao,

    Thank you for the comment. I already read that app note(System Considerations For Using Bus-hold Circuits To Avoid Floating Inputs).

    If customer follow the pull-up resistor guideline in app note section 6.3, could we expect smooth data transition?

    Best regards,
    Alice
  • 0 in reply to Alice57
    TI__Guru 59120 points
    Hi Alice,

    I would highly recommend switching to the device which can take external pull-up resistor without bus-hold circuitry. The reason is that bus-hold circuitry has internal feedback resistors to keep the last known state of the device. having an external pullup resistor can be detrimental to the state of the output. It could form a voltage divider or could conflict from the last known state of the device(device wants to stay low but there is external pullup resistor).
    However, if the customer wants to use the bus-hold device, following the app note is highly recommended.
    what was the earlier higher pullup resistance used? Do you have the scopeshot of the transition?