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SN74HC132 internal pulldown?

Yuting Zhang
Prodigy 30 points
Other Parts Discussed in Thread: SN74HC132

I built a PCB shield to work with Arduino. There is one NAND gate SN74HC132 on the PCB. The Arduino's digital output pins write HIGH or LOW to signal one NAND gate and other devices. When Arduino turned off, the pin connected to NAND input is still low (short with ground), but other pins are pulled up by pullups on other device. 

Based on such observations, I thought the NAND gate inputs have internal pulldown, but could not find it clearly in its datasheet. Is that right?

Thanks.

  • 0
    TI__Mastermind 46480 points

    Hi Yuting

    There is no pull down resistors and the pin should not float. If the inputs are turned off then you should use a pullup or pull down to insure they do not float.

  • 0 in reply to Chris Cockrill
    Prodigy 30 points

    Thank you, Chris.  A follow up question:

    One Arduino digital output pin is connected to the NAND input, with a pullup of 4.7k to 5V. The Arduino output is written low and then Arduino is turned off. From the Arduino datasheet and my observation, Arduino cannot keep output LOW after turned off. But the NAND input remain low, short with ground. 

    Does this due to the schmitt trigger in the NAND gate, which prevents the input pulled up?

  • 0 in reply to Yuting Zhang
    TI__Mastermind 46480 points

    The schmitt trigger should not be able to pull any direction. Are you able to disconect the Arduino and see if the low is on it or the HC132? I dont see how the HC132 could pull it low unless it is damaged.

  • 0 in reply to Chris Cockrill
    Prodigy 30 points

    Yes, you are right. When I disconnect the Arduino, the input pin pulled up with 4.7k to 5V gives 2.9v now. This 2.9v feeds to 3A and 3B, the 3Y gives LOW, following NAND's truth table. However, with a turned off Arduino connected, basically an Arduino output pin to the NAND input pin, pin voltage is about 1.7V, and 3Y can still keep HIGH .

    Do you think the NAND gate damaged? 

  • 0 in reply to Yuting Zhang
    TI__Expert 6380 points

    If you are physically turning the Arduino off, take a look on it's output specs for that I/O.  There is probably an ESD diode to the VCC rail, and when it is  off might be a diode/leakage path to ground or 4.7K pull up ath is trying to power the Arduino to some extent via the dioe path...

     

    You may want to look at putting a buffer in there that can support partial power off

    or even a configurable part like http://www.ti.com/product/sn74aup1t57 which supports power off, pin strappable logic and voltage translation