preset when power turns on

Discussion in 'General Electronics Chat' started by rafaltrus, Nov 27, 2012.

  1. rafaltrus

    Thread Starter New Member

    Nov 16, 2011
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    I am trying to build a "simplified keyboard encoding circuit" found in my textbook, but I cannot understand one part. Namely, "there are two 74HC195 4-bit shift registers connected as an 8-bit ring counter with fixed bit pattern of seven 1s and one 0 preset into it when the powers turns on."

    I know that the parallel-in/parallel-out register loads data when LOAD is HIGH, but how do I load the data only one time - when power is turned on? That is, when power is turned on, the counter loads data, but from that point on, it shift data indefinitely. I cannot think of any workable device that can be applied here.

    Any ideas?
    Thanks
     
  2. SPQR

    Member

    Nov 4, 2011
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    While waiting for the experts, I've looked at the datasheet and it looks like you can load data on the parallel lines,
    bring the shift/load line LOW (all of the parallel lines go to Q lines),
    then change the shift/load line HIGH, and clock the serial data.

    I think that's what you see in the function table, and in the Logic and Timing Diagrams.

    So to load it one time, at startup, you just hold shift/load LOW for X microseconds, then keep it high until you turn it off.

    Let's see what the experts say...
     
    Last edited: Nov 28, 2012
  3. rafaltrus

    Thread Starter New Member

    Nov 16, 2011
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    thanks for input

    this is exactly where I am stuck. When I power up the circuit, I don't know how to invert the signal for just a short period of time. I could add a switch, but that'd be a wrong approach, because it should be automatic.
    I feel like I am missing something very simple here... :|
     
  4. SPQR

    Member

    Nov 4, 2011
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  5. MrChips

    Moderator

    Oct 2, 2009
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    Get a Power ON Reset chip.
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    A simple way is to connect a capacitor from the shift/load input to ground and a resistor from the shift/load to the V+ power. When power is applied the shift/load voltage will initially start at ground (logic low) due to the capacitor and charge to V+ (logic high) by the resistor with a time-constant of RC. You want the RC time-constant to be longer than the turn-on time of the power.
     
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  7. rafaltrus

    Thread Starter New Member

    Nov 16, 2011
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    Thanks, guys!

    power-----resistor-----capacitor-----shift/load

    Something like this? Sorry, I forgot how RC works..
     
    Last edited: Nov 28, 2012
  8. SPQR

    Member

    Nov 4, 2011
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    Oh....very nice.
    I've learned about the existence of a new chip! Power On Reset.
    http://www.futurlec.com/Maxim/MAX699CPAa.shtml

    It looks like for a change in power, you can have a fixed delay or a variable delay in "reset".

    And yes! A simple RC circuit that you can vary the delay by varying R and C. And it is local to the chip, not affecting the other circuitry.

    What a great forum!
     
  9. SPQR

    Member

    Nov 4, 2011
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    So perhaps something like this?
    This is similar to adjusting the frequency and duty cycle of the 555.
    Thanks!
    [​IMG]
     
    Last edited: Nov 28, 2012
  10. crutschow

    Expert

    Mar 14, 2008
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    Check the "Time-domain considerations" paragraph here.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Yes, that's correct.
     
  12. rafaltrus

    Thread Starter New Member

    Nov 16, 2011
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    Thank you so much everyone!
     
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