Digital Logic Circuit - With a RS Flip Flop

Discussion in 'Homework Help' started by joelwalker71, Apr 12, 2012.

  1. joelwalker71

    Thread Starter New Member

    Mar 15, 2011
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    Hi there,
    I would really appreciate some help with my homework.
    I was tasked to create a logic circuit that worked for the truth table I have attached.
    I then converted it to NAND gates and simplified it down; I have attached a screenshot.

    The second part of my task was the include a RS Flip Flop into my circuit. So when the bulb (LED) comes on, it latches on and stays on until the circuit is reset.
    I have no idea where to start with this, so any help will be greatly appreciated!

    Thanks,
    Joel
     
  2. crutschow

    Expert

    Mar 14, 2008
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    You have not made a FF. An RS flip-flop needs memory (feedback from output to input) to maintain a state after the input control signal is removed. Your truth-table does not explicitly show that and that is perhaps why you did not build the correct circuit.
     
  3. joelwalker71

    Thread Starter New Member

    Mar 15, 2011
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    Thanks for your reply.

    Going back and re-reading the question. I'm not sure that it says I have to use a RS Flip Flop. (it just gives it as an example)
    It states, Modify your circuit (the one I attached) so once it activates under any of it's conditions that like latches on and the bulb remains on.
    Any ideas how it can be done??

    Thanks,:)
     
  4. crutschow

    Expert

    Mar 14, 2008
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    3,220
    It says it needs a latch which is basically the same as an RS Flip Flop. Here's a reference.
     
  5. joelwalker71

    Thread Starter New Member

    Mar 15, 2011
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    Where would you add this into the circuit though?
    Thanks,
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Delores: The link you gave is bad. Please try again.

    Who is trying to work the problem? Joel or Delores? Or are both in the same course? Just trying to keep the players straight.

    Based on what Joel has said earlier, the problem tells you to modify the circuit so that if any of the states that result in the LED turning in is entered that the LED stays on even if the inputs move to a state that does not correspond to the LED being on. In other words, the circuit has to remember that it was once, at some point since the last reset, in a state that corresponed to the LED being on. To remember something requires memory. The problem may not have specified the form of the memory element, but the specified behavior requires some form of memory. Since no mention has been made of a clock, I recommend against using a clocked register like a D or JK flip flop because that leads to playing ad hoc asynchonous logic games with clock and set/reset inputs and that road doesn't end well at some point. So use some type of clockless latch, such as an RS or /R/S latch made from crosscoupled NOR or NAND gates.

    As for Joel's question about where it goes, think about it. You have a logic circuit that is producing an output that has to be remembered and the signal going to the LED has to be whatever was remembered. Where does it make the most sense to put a memory element in that case?
     
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