74LS390 Question

Discussion in 'The Projects Forum' started by bertz, Nov 11, 2013.

  1. bertz

    Thread Starter Member

    Nov 11, 2013
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    I am using a pair of 74LS390 in a timer circuit configured to divide by 100. I will be using the 1Qd and 2Qd outputs. Do I ground the Qb and Qc outputs or leave them floating? Also should I ground pins 2 & 14 (CLR) since the oscillator will run all of the time?
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    The unused outputs (Qn) must be left unconnected. The two CLEAR lines should be ground (as CLEAR is active high and you are not using it).

    Depending on the application, you might want to incorporate a power-on clear to initialize the counters.

    Welcome to AAC and thanks for including the schematic with your question. That helps.
     
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  3. bertz

    Thread Starter Member

    Nov 11, 2013
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    Thanks John!
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Never leave digital inputs open(not connected). As the result may be unpredictable. For TTL logic like your counter IC. Open inputs will take high level as a rule of thumb. But "noise" may change that. So any open input may behave unpredictable
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Unused INPUTS, as a general rule, should always have a current path to either Vcc/Vdd or GND/Vss, or unpredictable oscillations may occur, resulting in difficult-to-troubleshoot problems.

    On the other hand, unused OUTPUTS must always be left disconnected, or you will likely have high current flow, high power dissipation in the IC, and may very well burn it up.
     
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  6. bertz

    Thread Starter Member

    Nov 11, 2013
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    Thanks Sarge

    Getting back up to speed- thanks for the refresher

    Bert
     
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