Serial Out (Qs) vs Q8 - shift register

Discussion in 'General Electronics Chat' started by Darren Midkiff, Jan 7, 2016.

  1. Darren Midkiff

    Thread Starter New Member

    Jan 5, 2016
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    What is the difference in the last output and the serial output of a shift register (ie. 74HC595)? Logically, it seems like they would be the same, but I hooked up 3 595s tying Q8 to to the next register's data pin and it did not work properly. When I switched it to Qs it did.
     
  2. Darren Midkiff

    Thread Starter New Member

    Jan 5, 2016
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    0
    Update: It works fine for a slow arduino program (clock every 200 ms), but seems to break when using arduino shiftOut()
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Here's what the datasheet has to say:



    The serial out pin updates on SHCP, and the outputs update on STCP. Look at the 2nd paragraph to see what happens of you clock both with the same clock input.

    source: http://www.nxp.com/documents/data_sheet/74HC_HCT595.pdf
     
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,544
    1,251
    Since you chose to keep your schematic secret, it's impossible to give accurate advice. However, if you look at figures 2 and 4 in the above datasheet, you will see that there are two reasons why Q8 and Qs can have different states. What are you doing with the latch and output enable controls?

    ak
     
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