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Original question:

SN74AHC1G14: The customer uses the 3.3v VCC and 5v Vi, does it have problems

SN74AHC1G14: Issues on recognizing logical low

Ricardo Padilla
Prodigy 70 points
Part Number: SN74AHC1G14

Hi:

There's a product, that uses a couple of SN74AHC1G14, with a 3 volts VCC, I Think  SN74AHC1G14 are being used kind of a buffer since is the interface between a serial comport and a MSP430FG47x, to the SIMO and SOMI pins.

So, pin P2.4/UCA0TXD/UCA0SIMO on the MSP430 is in "high state" (3 volts), but after a command (received through another SN74AHC1G14 to the UCA0SOMI pin @ the MSP430 )  goes to low (measuring milivolts with an oscilloscope) so the problem is that after that signal there is a SN74AHC1G14 (wiith Vcc 3 volts), and when UCA0SIMO  is in high state the output @ SN74AHC1G14 is on low, but when UCA0SIMO switches to low (milivolts) SN74AHC1G14 remains always in low.

So, in order to verify it was working, we grounded input and switched, Also tried to apply external voltage to verify Vt+ and Vt- parameters and also switches ok @1.55 volts (aproximately), but somehow is not able to detect the "low state" coming from MSP430.

Do you have an idea what can be causing this?

Thanks :)

  • 0
    TI__Guru* 96862 points
    Hi Ricardo,
    Do you have a schematic that shows all of the connections for the SN74AHC1G14? With text alone it's quite difficult to know what's really going on.

    It would also be helpful to see an oscope shot of the input and output while they are operating normally and when you see the error.
  • 0 in reply to Emrys Maier
    Prodigy 70 points

    Hi Emrys, this is the schem:

    TX and RX as I said go to a MSP430. Te issue is on TX, I have a 0, but not getting a 1 @ u1 output.
     Below measurement is in P13 (measure avg voltage):

    And this is at P14 (btw i've measured straight at u1 pin4 and result is the same), (I've also isolated removing r32):

    And when we grounded on purpose (connecting u1 pin2 to ground) we got a 1:

    So, I can't figure why the low sent by MSP430 is not being read as a low, even when we have milivolts...

    Thanks!

  • 0 in reply to Ricardo Padilla
    Prodigy 70 points

    Hi Emrys, this is the schem:

    TX and RX as I said go to a MSP430. Te issue is on TX, I have a 0, but not getting a 1 @ u1 output.
     Below measurement is in P13 (measure avg voltage):

    And this is at P14 (btw i've measured straight at u1 pin4 and result is the same), (I've also isolated removing r32):

    And when we grounded on purpose (connecting u1 pin2 to ground) we got a 1:

    So, I can't figure why the low sent by MSP430 is not being read as a low, even when we have milivolts...

    Thanks!

  • 0 in reply to Ricardo Padilla
    TI__Guru* 96862 points
    There only possibility I can think of for what's happening would be that the ground for the SN74AHC1G14 is not at the same voltage as the ground for the incoming signals. In this case, the device's threshold would be referenced to the wrong ground and wouldn't switch properly.

    The SN74AHC1G14 is a very simple analog device - unless it has been damaged, I wouldn't expect it to give you any troubles.
  • 0 in reply to Emrys Maier
    Prodigy 70 points
    The grounds are both (masp430 and SN74AHC1G14) the same, I know it is a very simple device, that's why I can't understand whys is this happening, this is a mass production product, and I know the device is working, but many times it behaves like this...
  • 0 in reply to Ricardo Padilla
    TI__Guru* 96862 points
    When you see this behavior, have you tried swapping to a new SN74AHC1G14?

    What about swapping to a pin-to-pin compatible "14" function (Schmitt-trigger inverter)?
  • 0 in reply to Emrys Maier
    Prodigy 70 points
    The issue fixes by heating up microcontroller or putting soldering iron on the output, however I'm positive that there is not solder issues, and also I'm measuring (with oscilloscope) at the input of SN74AHC1G14 having a 0, so I'm sure SN74AHC1G14 is not damaged as you can see in schem we have 2 of them, and the other is working fine.
  • 0 in reply to Ricardo Padilla
    TI__Guru* 96862 points
    So, my recommendation would be to do an A-B-A swap to determine if the problem follows the SN74AHC1G14 device or if it stays with the board - or if it clears up entirely.