Constructing a couter from D-Flip-Flops

Discussion in 'Homework Help' started by pkizzz, Oct 11, 2009.

  1. pkizzz

    Thread Starter New Member

    Oct 11, 2009
    1
    0
    I have a assignment about constructing a two bit downwards counter with the help of D-Flip-Flops and NOR-gates.

    We use Digital Fundamenmtals 10th edition, and it covers counters based on JK-Flip-Flops. So I don't understand how to do it with only D-flip flops.

    What I have done:

    __________________________________________
    Counts from: 00 --> 11 --> 10 --> 01 --> 00....

    Current state | Next state | Flip-Flop inputs

    Q1 Q0----------Q1 Q0---------D1 D0
    --0 0------------1 1------------ ? ?
    --0 1------------0 0------------ ? ?
    --1 0------------0 1------------ ? ?
    --1 1------------1 0------------ ? ?
    __________________________________________

    How can I see what the states of the D inputs should be?

    With JK inputs we use a table like this:
    _______________________________________
    |Output transitions --------- Flip-Flop Inputs |
    | QN -- QN+1 ----------------- J ------ K --|
    ---------------------------------------------
    --0 ---> 0 -------------------- 0 ------ X
    --0 ---> 1 -------------------- 1 ------ X
    --1 ---> 0 -------------------- X ------ 1
    --1 ---> 1 -------------------- X ------ 0
    ---------------------------------------------

    So, how can I know about the D Flip-Flops inputs?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A D flip flop's Q output follows the state of the D input when clocked. This is no help to your counter, but as an illustration, you can make a D flip flop toggle on each clock by tying the Q^ output to the D input.
     
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