74192N Problems - Won't count

Discussion in 'The Projects Forum' started by Yoctogram, Apr 21, 2013.

  1. Yoctogram

    Thread Starter New Member

    Apr 14, 2013
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    When I pre-set the inputs to a number and engage the clock (represented by the switches), the 7 segment display will not count up or down. Still pretty new to this so help would be greatly appreciated.

    A screenshot is attached of the circuit I constructed.
     
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    Last edited: Apr 21, 2013
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,347
    1,029
    You are dealing with a lot of old eyes here. I can't read the skiz. Plus, red and blue against white is very hard to read in any case.

    That said, it looks like you are allowing CLR to float high in normal operation. That will keep the counters from counting. If you use the circuit as is, put a 150ohm resistor from CLR to ground. When the switch is open, the resistor will pull CLR down and allow it to count.

    A better circuit would have CLR held to ground with a NC switch and a 10K-68K pullup. Pressing the switch would open it and the resistor would pull the CLR inputs up to logic 1.

    I also don't know about S1-S3 but keep in mind that a TTL input floats to a logic 1 when its not connected to anything i.e. switches are open.

    There may be other problems but I can't tell.
     
  3. takao21203

    Distinguished Member

    Apr 28, 2012
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    463
    Readable schem. pls.
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
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    If I could decipher that properly, it would seem you've neglected to route your load pin.
     
  5. Yoctogram

    Thread Starter New Member

    Apr 14, 2013
    3
    0
    Sorry for the quality; i tried to zoom a bit more but i can't zoom in too far without losing some of the circuit. Here's a (hopeuflly) better screenshot.

    [​IMG]

    I tried the fix with tying the fix with the 150ohm resistor but it didn't work.

    Also, when routing the LOAD to VCC it presets the 7-segment display to a "9" and will actively count up/down - this is a problem because it ignores any pre-set inputs. When i tie the LOAD to ground it will acknowledge the preset but will not count up or down.

    inputlsb, S1, S2, S3 are switches used to preset the number i want to display in the 7-segment*
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
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    A flip flop will come up in a indeterminate state, your software is assuming 9 for the sake of your simulation.

    This sounds like another case of needing a power-on reset.

    A capacitor from LOAD to ground ensures the initial value is loaded into the counters, and a resistor pulls it high as the voltage on the capacitor comes up. A bleeder resistor in parallel with the capacitor can help mitigate any issues of a capacitor remaining charged after power is disconnected, but probably not a big deal here...
     
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  7. Yoctogram

    Thread Starter New Member

    Apr 14, 2013
    3
    0
    Thanks tshuck! Got it working now :)
     
  8. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Glad to hear it!
     
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