555 Timer Driving Counter Problem

Discussion in 'General Electronics Chat' started by paw, Apr 29, 2012.

  1. paw

    Thread Starter New Member

    Jan 11, 2012
    18
    0
    Hey All,

    Just a quick thank you in advance!

    Ok, so I have a circuit as shown - a 555 timer set up with a period of 0.5 seconds. Now, this is driving an 8 bit counter, which I am using to trigger other things.

    So, I am using Bit 4, which should go high @ 8 seconds, and bit 5, which should go high @ 16 seconds.

    The problem I am having is that bit 5 is actually going high at about 27 seconds as shown in the output waveform.

    Now I know the 555 isnt outputting a perfect square wave, but nearly a square wave. Is this the reason why? Just cant get my head around it!

    Cheers,

    PP
     
  2. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    I see that the time difference between Bit4 and Bit5 is a 3 to 1 ratio. My first problem is that I don't know the part number of the 8 bit counter, so I can't look it up.
     
  3. paw

    Thread Starter New Member

    Jan 11, 2012
    18
    0
    Its the generic counter from Multisim...
     
  4. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    Sorry. I don't sim.

    Next helper.
     
  5. w2aew

    Member

    Jan 3, 2012
    219
    64
    From your picture, it looks like the PERIOD is closer to 1 second, not 0.5 seconds. At ~50% duty cycle, the pulse WIDTH is about 0.5s. However, the counter looks at only rising edges (or falling edges, depending on counter), thus it "counts" every cycle, not the width of the pulses.
     
  6. paw

    Thread Starter New Member

    Jan 11, 2012
    18
    0
    Morning Bump
     
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