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SN74HC74: Different part variation - Possible timing issues

Kian Najarian
Prodigy 40 points
Part Number: SN74HC74
Other Parts Discussed in Thread: SN74HCS08

Hi,

We have been using SN74HC74DR for a number of years without issue. Recently our production team started to notice failures in the test jig for one of our boards, caused by timing issues. The code for the test jig was slightly changed to match the boards allowing it to pass but they have been failing when used in a product assembly.

At the moment it seems both working and non working boards have TI part SN74HC74DR fitted. The working boards however have the marking "13A3DGM G4 HC74" on SN74HC74DR while the non working boards have the marking "1AZ3C7JG3 HC74".
I have not been able to find any information on the difference between the markings, perhaps it is different manufacturing locations which also use different materials.

I have also not seen any information regarding any potential differences between the electrical specifications of the parts. If someone could help with identifying potential causes for timing issues between the parts or other important differences that would be great.

Thank you!

Kian

  • 0
    TI__Guru* 96947 points

    Hi Kian,

    Can you be more specific in the type of failure detected? What exactly is the issue you are seeing?  Scope shots of the failure and a schematic of the failing circuit would be best for us to help troubleshoot.

  • 0 in reply to Emrys Maier
    Prodigy 40 points

    Hi Emrys,

    I am not able to confirm the specifics of the issue right now but it is being suggested that one part is SN74HC74DR and the other SN74HC74DRG4. Do you know what the differences between them with the G4 suffix would be? Is it just a slight material difference or are the limits etc effected too.

    The part is U2 on the schematic. Thanks

  • 0 in reply to Kian Najarian
    TI__Guru* 96947 points

    Both of those devices will meet the datasheet specs, although there can be differences between batches of devices.

    I don't see any problems with the circuit surrounding the HC74, however this circuit seems like it may cause problems:

    It's generally not a good idea to put a slow rising edge into a standard CMOS input like this -- usually the input should have a rise time under 50ns, but it's closer to 1us here. This can cause oscillations at the output and unexpected behaviors from the gate.

    I'd recommend to switch to a Schmitt-trigger input gate such as SN74HCS08 to avoid that issue -- this would also slightly change the timing of the circuit, so R and C would have to be updated, but it would avoid the possibility of larger issues.