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SN74AVCBH164245: SN74AVCBH164245 BUS HOLD Feature Question

Jade Goo
Intellectual 1940 points
Part Number: SN74AVCBH164245

Hi Team,

In SN74AVCBH16245 datasheet page1, "Active bus-hold circuitry is provided to hold unused or floating data inputs at a valid logic level. Use of pullup or pulldown resistors with the bus-hold circuitry is not recommended.".

1. Does "Control Signal" also support BUS HOLD feature?

2. Unused input pin floating is recommended for bus hold buffer case?

3. If customer place pull-up resistor to input/output signal for initial setting when buffer or driver output siable case. I think recommended pull-up resistor value exisitng for use bus hold and that related with IBHLO and IBHHO spec, would you let me know recommended pull-up resistor value for input/output?

Thank you.

Best Regards,
Jade

Active bus-hold circuitry is provided to hold unused or floating data inputs at a valid logic level. Use of pullup
or pulldown resistors with the bus-hold circuitry is not recommended.Active bus-hold circuitry is provided to hold unused or floating data inputs at a valid logic level. Use of pullup

or pulldown resistors with the bus-hold circuitry is not recommended.

  • 0
    TI__Mastermind 24665 points
    Hello Jade,

    1. No, the control signal does not have bus hold.

    2. It is recommended to keep bus hold inputs floating, yes.

    3. Pullup/pulldowns are allowed on the control inputs, do not place pullups/pulldowns on the IO pins.

    Here is an app note on bus hold circuits: www.ti.com/.../scla015.pdf

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    TI__Intellectual 1940 points

    Hi Michael,

    In application note, page14, 6.3 mentioned about pull-up/pull-down resistor.
    Following application note, can use pullup/pulldown resistor with suitable value for bus hold.
    Would you check again and let me know which spec should be check when select resistor value?
    Thank you.

    Best Regards,
    Jade

  • 0 in reply to Jade Goo
    TI__Guru* 96661 points
    Hi Jade,
    What initial value is the customer trying to set?

    In the application report, it mentions that bus-hold circuits default to 0V on startup assuming there is some capacitance at the input. If you want to be absolutely certain of this, you can add a small capacitor to ground (10pF should be plenty) on the lines that you need to start low.

    The application report you referred to has detailed instructions on how to select a pull-up resistor value, including equations, if that's what you need to do. Is there something confusing in the app report?
  • 0 in reply to Emrys Maier
    TI__Intellectual 1940 points

    Hi Emrys,

    They want to set some output to "high" and some output to "low" thru pull-up and pull-down resistor in intial stage without input.
    Depend on receiver side, "high" thershold is different and they think to adjust using pull-up resistor.
    Would you let me know is there any confuse about pull-up/pull-down resistor with bus-hold?
    Thank you.

    Best Regards,
    Jade

  • 0 in reply to Jade Goo
    TI__Guru* 96661 points

    Hi Jade,

    This is section 6.3 from the previously mentioned application report. It describes how to select pull-up resistors for bus hold circuits in detail.  I'm not sure what else I can do to help.

  • 0 in reply to Emrys Maier
    TI__Intellectual 1940 points
    Hi Emrys,

    Rp = (Vccmin - Vt) / II(hold)max

    However I cannot match with SN74AVCBH164245 parameter.
    Can you guide which spec shold be consider for this formula with SN74AVCBH164245?
    If you provide the feedback with example, it is helpful to understand.
    Thank you.

    Best Regards,
    Jade
  • 0 in reply to Jade Goo
    TI__Guru* 96661 points
    Ah, I understand the confusion now.

    The Vccmin is easy - just the minimum supply value that is to be used. In this example, I am going to use 3.3V assuming a very stable 3.3V source.

    The electrical parameters for the SN74AVCBH164245 are extremely detailed and have more than one spec for the II(hold)max value. These are listed as I_BHLO (bus hold low overdrive current) and I_BHHO (bus hold high overdrive current). Assuming 3.3V operation, we can use 525 uA for the calculation.

    Vt is the threshold voltage, which is a little harder to get. From the VIH/VIL values in the recommended operating conditions, we know that the device will switch somewhere between 0.8V and 2V. We can use these values to calculate our answer, then take the worst case as the final solution.

    Plugging in the values yields:
    Rp = (3.3 - 2)/525E-6 = 2476.19 ohms
    Rp = (3.3 - 0.8)/525E-6 = 4761.91 ohms

    Since a smaller pull-up is stronger and will be more likely to work, we can say that the pull-up resistor should be less than or equal to 2476 ohms, so a 2.2kohm resistor would do the job.


    Note that adding a 2.2kohm resistor to the input will cause added current draw, since the device will now always be driving current when in the low state. Additionally, the 2.2kohm resistor will set-up a voltage divider with the internal pull-up and the CMOS input will be held at ~31% of Vcc (Vin = Vcc * 1k/(2.2k+1k)) if the external driver is turned off. This will cause shoot-through current well in excess of deltaIcc in the datasheet. While it is unlikely to damage the device, it will certainly draw more current than usually expected from this type of device.

    This is why Michael recommends against adding pull-up resistors to bus-hold circuits. It can technically be done, but it is usually not the best solution. We highly recommend changing to a part that is not bus-hold if this is a requirement.