Two 74595 shift registers don't shift in sequence

Discussion in 'The Projects Forum' started by IodinePit, Mar 24, 2014.

  1. IodinePit

    Thread Starter New Member

    Mar 24, 2014
    I tried a basic Arduino project involving pushing bits to a 74595 shift register. It works great. I then tried to connect another 74595, with its serial input coming from the first chip's serial output. Both running on the same clock input. The downstream chip doesn't function correctly, sometimes bits shift, sometimes not, sometimes things randomly clear. It is not a defective unit, because if I swap chips, it's still the downstream chip that has the issue.

    The only thing I can think of is that one is HC and one is HCT. Is that supported?

    Can you give me some pointers on what could be going wrong here?
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    HC parts and HCT parts have different input voltage ranges (that's what the T gets you), and this could be the problem. Can you post a schematic?

  3. Papabravo


    Feb 24, 2006
    The HC part has CMOS thresholds which are basically one-half of Vcc. The HCT parts has an input low threshold of less than 0.8V, and an input high threshold of greater than 2.0V. What is your Vcc? If you are trying to run them at 3.3V that might be a problem. You also need to look at the output high and output low specifications for both chips.

    Also if the socket for the downstream chip has ANY floating inputs that could also be a problem.