5-bit Asynchronous Counter using JK Flip-Flops, counts from 01001 to 11011

Discussion in 'Homework Help' started by cheesemaster7, Nov 17, 2008.

  1. cheesemaster7

    cheesemaster7 Thread Starter New Member

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    I need some help with a problem I'm facing. I'm to design a 5-bit asynchronous counter in multisim using 74LS76 (JKs). Here's the catch. It must count from 9 (01001) to 27 (11011) continuously. When the circuit is started up, it has to start counting from "0". It only passes through "0" once upon initialization.

    Now I've figured out how to count from 0 to 27, you just connect all the Q outputs to the next sequential JK flip-flop's clock input. Then connect the Q0,Q1,Q3 and Q4 outputs to a 4-input AND gate which is connected to each JK flip-flop's reset. When the count reachs 11011, the flip-flops get reset back to zero. I would like them to reset to nine AFTER the initial count starts at zero

    Does anyone know how I can start the circuit at zero, then count from 9 to 27? I can't really get it to start counting on anything but zero. I know there's some logic gates involved here, but I'm not sure where their outputs go. I know this is complicated and my explanation may not be the best, but any help would be much appreciated, thanks.
  2. RiJoRI

    RiJoRI Well-Known Member

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    That's why I use microcontrollers! :D

    Hint: Take a look at the 74xx76's PRESET inputs, and their effect on the outputs.

    --Rich
  3. cheesemaster7

    cheesemaster7 Thread Starter New Member

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    Thanks for the tip Rojori. I've set the PRESET inputs so that the counter counts from 9 to 27, but I'm having trouble initialize the count at zero. Essentially, this counter should count like this (in HEX): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 9, 10, 11...1B, 9...
    Currently, my counter starts at 9, counts to 27 (1B), then starts over at 9 again. I want it to start at 0, count to 27, start again back at 9, then continuously repeat.
    I built a detection circuit so that when the counter hits 27, all of the flip-flops are cleared and, simultaneously, the presets on the first and fourth bit are set low, thus enabling the count to start at 9.
    The hardest part is making this count correctly from 0 to 27 the first time, then count from 9 to 27 everytime other than the first. Any more help would be appreciated, thanks.
  4. RiJoRI

    RiJoRI Well-Known Member

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    I want it to start at 0, count to 27, start again back at 9, then continuously repeat. I built a detection circuit so that when the counter hits 27, all of the flip-flops are cleared and, simultaneously, the presets on the first and fourth bit are set low, thus enabling the count to start at 9.

    Hmmmm, sounds like we're getting to the "show me the schematic" stage. But before that, I would look at what the detector is doing at power-up; that is, why is it detecting a 1B instead of a 00? Disconnect the clock circuit, tie the input to the counter LOW, power up the circuit and snoop around, checking voltage levels.

    --Rich
  5. Ahmedthabet

    Ahmedthabet New Member

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    Can Any of you teach me how to design a 5 bit binary counter that counts from 0 to 27 then resets?
  6. Ahmedthabet

    Ahmedthabet New Member

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    0 to 25 sorry!
  7. MrCarlos

    MrCarlos Member

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    Hello Ahmedthabet

    This binary counter, will be developed with Flip-Flip-s?
    What would identification number?
    What to do when the counter reaches 25?
    Reset to zero and stop??
    Reset to zero and continue counting from 0 to 25?

    regards
    at your service
  8. crutschow

    crutschow AAC Fanatic!

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    Don't hijack someone else's thread. Please start your own new thread. ;)
  9. crutschow

    crutschow AAC Fanatic!

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    Seems simple enough. What determines the "first time"? Is it a switch closure or power-on? Either way you use that to reset all the counters to zero. Then when it detects 27 it presets the counters to 9.
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