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SN74LVC8T245: output was wrong

qiao zhang
Prodigy 10 points
Part Number: SN74LVC8T245
Other Parts Discussed in Thread: SN74LXC8T245

hi,

I encountered some problems when using SN74LVC8T245PW. When my board was just turned on and the MCU main program was not running, the output pin of SN74LVC8T245PW would have some PWMs output, some of which were 50HZ at a fixed frequency, and some were very high frequency, not fixed. And not every output pin is present, some output pins are and some are not, is there any explanation for this phenomenon? And how can I do to solve this problem?

The following is my circuit schematic diagram and test waveform for your reference.

Thanks in advance!

Qiao Zhang

  • 0
    TI__Guru 52636 points

    Hi Qiao,

    If I understand correctly (by board turning on and MCU off), the LVC is powered when board is on.

    Hence, if observing an unknown output, this would mean the LVC device is powered on with the inputs floating (i.e an unknown input for an unknown output).

    The recommendation is to ensure the inputs of the LVC devices are at known states (high or low).

    If not feasible, the recommendation would be to use SN74LXC8T245 with internal pull-downs to help avoid floating inputs, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    Hi Michael,

    thank you for your answer.your understanding is correct, we already know that adding a pull-down resistor to the input will solve this problem. The following are some of my doubts about this problem: 1. Why do some output pins of the same chip, under the same situation, have this problem, and some do not (such as STEP1 has this problem, DIR1 does not)? 2. We know that this chip is designed by CMOS process, so it is prone to interference when the input is floating. Since our company uses many TI chips with similar process, is this problem possible?

    have a good day.

    Best Regards

    Qiao

  • 0 in reply to qiao zhang
    TI__Guru 52636 points

    Hi Qiao,

    Please note that, floating would be referring to either high, low, or in-between. Hence, it would make sense that all floating pins could observe any unknown state. I.e some may be low or high, observed as no problem while others may be in an in-between state observed as a problem, thanks.

    Best Regards,

    Michael.