This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74HC165: Data output being driven high when not supposed to

Part Number: SN74HC165
Other Parts Discussed in Thread: SN74HCT165, SN74HCS165

Hi team,

Hope you're well!

My customer recently swapped out a SN74LS165AD for a SN74HC165DR on their board and are now experiencing failures where they weren't before. Swapping the HC out for the LS fixes things.

It appears the HC165 isn't driving the lines.

You can see the HC165 start to drive the line low, but the signal would go back high prior to being clocked in (Green is data out of the HC165, Red is CLK to the HC165, and Blue is the Data Load signal to the HC165).

In this case they expect the bit on the first clock cycle to be low, but it goes back high prior to being clocked in.


Output with the LS165 as expected



I asked for a schematic but they don't keep their old schematics (this is for an old production design). Do you forsee any issues with the swap that may be causing this?



  • Hi Lauren,

    There are two possible culprits here -- input voltage or input transition rate. You managed to give me scope shots with no voltage or timing information, so I can't tell either.

    I can't tell if the clock signal is going up to 5V or not (it's common for old TTL logic to output at 3.5V). If it's not making it high enough, it's possible that the 'HC165 just isn't being triggered properly. This would be the first thing I'd check. The fix if this is the case is to switch to an HCT family device -- SN74HCT165. The "T" is for "TTL-Compatible" - which is just a fancy way of saying the input threshold is reduced.

    The other possibility is the input transition rate. It looks like the clock signal has a pretty rounded edge, so it may be too slow. This isn't an issue for bipolar logic (LS) however it is for modern CMOS logic. The inputs need fast transitions. If the transition is too slow, it can result in internal oscillations and issues like what you're seeing here. If this is the case, I'd recommend to try the SN74HCS165 instead - this device has Schmitt-trigger inputs that can handle the slow transition rate and should fix the issue.

    I should note that it could also be a combination of both of those. Hopefully one of the two options above will help.

  • Thanks Emrys, that helps a lot!