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SN74AVCH16T245: IO with bus hold random status at power up

Silvia Ferreira1
Prodigy 110 points
Part Number: SN74AVCH16T245
Other Parts Discussed in Thread: SN74HCS125, TLV840, TLV809E

During the device power-up we’ve notice the IO-with bus hold-status is random, when the IO is at high impedance.

We would like to know if, setting the IO at the same voltage than the level shifter, through a pull-up or pull-down, can guarantee a known state for the IO after powering-up the device.

For instance, if we have the following diagram:

We place a pull up/down of 4k75 P3V3 & P1V8 rise at the same time.

- Could we ensure that the bus hold is going to be ALWAYS the following?

   *2A6: 0V

   *2A7: 0V

   *2A8: 3V3

   *2B6, 2B7 and 2B8: random (0V or 3V3)

-Will this guaranteed by design (the fact of having the described bus hold status) ? If so, which are the restrictions to meet this situation?


  • 0
    Guru 260930 points

    The pull-up/-down resistor must have a value of less than 6.6 kΩ to be able to override the bus hold (see the IBHLO/IBHHO specifications). But such a strong resistor makes the bus hold meaningless, i.e., when the signal is not otherwise driven, it will not stay at its old value. Would this be acceptable in your application? (Do you actually need bus hold?)

    If you need bus hold on these signals, then you must drive the desired state on the line for a short time after power up, but not afterwards. Use buffers like the SN74HCS125 (with a series resistor of less than 6.6 kΩ), and connect their /OE to /RESET. (If you have no reset signal, generate it with something like the TLV809E/TLV840, or use an R/C circuit.)

  • 0
    Prodigy 110 points

    Thanks a lot for the reply. When we calculate the value for that resistor to skip the 3.3V bus hold, we obtain 1.6kohm (Vil_max/Ibusholdmax=0.8V/500uA = 1.6kohm). Why then the 6.6kohm value? Thanks

  • 0 in reply to Silvia Ferreira1
    Guru 260930 points

    Sorry, I was using 3.3 V. The correct calculation is shown in section 6.3 of the bus-hold application report, and results in about 3 kΩ.

    In any case, that resistor is active only for a short time, so power consumption is less important, and you can simply use something like 1 kΩ.