two-bit asynchronous counter

Discussion in 'General Electronics Chat' started by Xufyan, Apr 28, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
    114
    0
    Why my 2-bit asynchronous counter is not working ? :confused: both the LEDs are 'On' :(

    we have designed a similir 4 bit binary counter in University digital LAB n proto board it was also not working :S
    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    V2 should be 5v, not 12v. Standard TTL operates with a Vcc in a range from 4.5v to 5.5v.
    The CLR and PR inputs should all be at a logic high (5v) level. Right now they are "floating", probably low, which is an invalid condition; the outputs will most likely be oscillating at high frequency.

    Also, try slowing your V1 clock speed down to 1Hz to 10Hz; the simulation may simply be running too fast for you to observe the X1 and X3 indicators changing.
     
  3. designnut

    Member

    Apr 21, 2011
    33
    1
    As I recall the Jk connections must be connected tto the proper output to make it toggle. Fixed input doesn't work.
     
  4. Xufyan

    Thread Starter Member

    Aug 3, 2010
    114
    0
    hey thanks alot its working now, but can you please tell me why we have set both clr and present to high ??? since they are capable to completely set or reset the output
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Note the inverters on the PR(eset) and CLR inputs.
    Take a look at a datasheet for a Texas Instruments SN7476:
    http://focus.ti.com/lit/ds/symlink/sn7476.pdf
    Note the "'76 function table" on the right side of the 1st page.
    See the case where PRE and CLR are both L? To the right there are "H"'es with up-arrows next to them. In the note below, there is a statement that "This configuration is nonstable; that is it will not persist when either preset returns to its' inactive (high) level."

    Thus, if the PRE or CLR are low, they are actively trying to PREset or CLR the flip-flop.

    You must read the datasheet for the device, and understand the function/truth tables in order to be able to get the desired output from them.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    See the '76 function table in the reference datasheet; holding both J and K inputs high results in the outputs toggling when a clock is received on the clock input.
     
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