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Bad batch of shift registers?

tinyspud
Intellectual 360 points
Other Parts Discussed in Thread: SN74LV165A

Hi everyone

I bought a bunch of shift registers (SN74LS164N) locally and I am trying to figure out of it's just a bunch of bad shift registers or if I'm doing something stupid.  I'm bit banging the output from my RTOS task with GIO on a RM42 Launchpad and this is what the inputs from the launchpad and the outputs from the register looks like:

The A channel is held high and B is used to shift in the data; I was wiggling the clock to see if I could output anything.  The shift register is powered by the 5V rail off the launchpad and the logic is 3.3V; output is 0.2V low and 4.8V high.  Right now the outputs are only loaded by the logic analizer I'm using.

I think I have bad chips but I just wanted to make sure that I'm not doing something stupid before I go buy more chips (from a different source).  Nominal clock frequency is ~2.5kHz

  • 0
    TI__Guru 62280 points
    Hi Tinyspud,

    do you have scope shots of the Vcc rail and gnd rail ? Is there any droop in those which is causing 4.8V output high ?
    in any case , what are the top side marking on the device to confirm if anything is missing here?
  • 0 in reply to ShreyasRao
    Intellectual 360 points

    Hi Shreyas (I think?  Sorry)

    Here Channel 4 is the input signal, Channel 5 is the clock, Channel 6 is the /reset line and channel 8 and 9 are the outputs; only sampling two of them because of RAM (logic analizer on my PC)

    Channel 3 is the ground and channel 7 is the supply; ground stays around 0V and VCC is at 5.25V

    The two outputs are at 0.125 and 4.590.

  • 0 in reply to tinyspud
    TI__Guru 62280 points
    Hi ,
    yes , it is Shreyas.
    The datasheet specifies the Voh is min of 2.4V with the IOH of 400uA . This device has unbalanced drive with IOH max of 400uA and IOL of 8mA .
    The outputs you have mentioned are within the spec . Maybe the loading by the probes are causing the voltage droop you are seeing .
    any way to measure the current at the output to confirm the case ?
    You might consider other balanced drive devices with significant IOH drive capability . e.g SN74LV165A has 8ma drive?