Cascaded D Type Flip-Flops

Discussion in 'General Electronics Chat' started by carpboy, Nov 28, 2010.

  1. carpboy

    Thread Starter New Member

    Sep 11, 2010
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    0
    I'm working on a project where I am cascading flip-flops on some LS74's. Q' is tied back to the data pin. The scope on the first stage one has a nice output, all as expected w/ 1 kHz input (NMOS). When I add in the 2nd device of the IC my output waveform of the 2nd device is not 50% duty time but actually 2/3.

    If I cascade to a device on a different IC the waverform of that output is a perfect divide-by-4. Trying both devices on a different IC resulted in the 2/3 waveform again.

    Can anyone give some clues as to what may be going on here? I've tried various sized caps across the 5 VDC and ground, 0.03u though 0.6u.

    Thanks.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Have you tied all inputs to one of the power supplies? TTL is unusual in that it will assume a high on an open input, but generally it is a good idea to make sure all inputs to gates are connected to one logic level or another.

    If you post a schematic we could be of more help.
     
  3. carpboy

    Thread Starter New Member

    Sep 11, 2010
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    0
    Hi Bill,

    Attached is a sim screen cap of what I am doing.

    5V from an LM3805 powers an NMOS amp. 1 kHz square wave from my PC into this amp gives a real solid 0-5 V square wave. The PC soundcard output is coupled though a 0.1u cap. The NMOS output has a 90 nF cap across it and ground as I was getting unseen transients.

    My goal is to make a stupid simple 4 stage cascade using D type FF's. I have several LS74's on hand.

    Input to FF1 and output perfect. I then take that output and put it into FF2 (on the same IC) and the output duty cycle is not 50/50.

    If I take FF1 output and cascade to an FF on a different IC the output is perfect.

    I did get it to work but it was strange:

    IC1 FF1 to IC2 FF1 to IC2 FF2 to IC1 FF2.

    When I hooked up the 3rd stage (IC2 FF2) I had the bad output. I noticed that the ground of this IC (pin 7) was connected to the other IC pin 7 (which of course itself was jumpered to the ground bus).

    I moved the ground jumper on IC2 pin 7 from IC1 pin 7 right to the ground bus. Voila, the output was perfect.

    Finishing off and adding stage 4, all still good. If I put the IC2 pin 7 jumper back to IC1 pin 7 bad output again.

    So I have no idea what is going on here. Everything sits in a typical proto board.

    Thanks.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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  5. carpboy

    Thread Starter New Member

    Sep 11, 2010
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    Thanks. They happen to be TI's. I was looking at a Fairchild sheet and didn't notice the two flavors. However, these are N's and the pins are as expected.
     
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