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Edge-Triggered S-R Flip-Flop

Discussion in 'CAD' started by Jim Thompson, Sep 11, 2003.

  1. Jim Thompson

    Jim Thompson Guest

    ISTR that there was recently a discussion of how to make an
    Edge-Triggered S-R Flip-Flop... a rising edge on one input sets the
    flop, a rising edge on the other input resets.

    I should keep notes... I can't remember how it was done.

    Anyone still have a copy?

    Thanks!

    ...Jim Thompson
     
  2. James Meyer

    James Meyer Guest

    I thought they all worked like that.

    Unless you mean that the DC level on either input is arbitrary. In that
    case maybe you can program a PIC to do the job. :cool:

    Jim "the other one"
     
  3. Jim Thompson

    Jim Thompson Guest

    What's a PIC? Ah, yes, what Picadores use ;-)

    ...Jim Thompson
     
  4. mike

    mike Guest

    I'd use a gate-delay "differientiator" on each input of an S-R flop.
    You can also do some interesting things with a two-bit Johnson Counter
    with separate edge clocked inputs for S and R. Decode the output
    states with an xor gate.
    mike

    --
    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    4in/400Wout ham linear amp.
    Honda CB-125S
    400cc Dirt Bike 2003 miles $550
    Police Scanner, Color LCD overhead projector
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  5. Jim Thompson

    Jim Thompson Guest

    I just invented a method that avoids the "delay-game"... I'll post it
    tomorrow... it's time now for Cab/Merlot blend and a filet ;-)

    ...Jim Thompson
     
  6. Does it get rid of metastability too? ;-)
     
  7. Jim Thompson

    Jim Thompson Guest

    Does anything ?:)

    ...Jim Thompson
     
  8. With 2 flip flops, or with 8 NAND gates. Replace them with NORs and
    swap the outputs for negative edge trigger.

    OUT
    |
    |
    +-------|--------------------------+
    | | |
    | | +-------------+ |
    | | | | |
    ^ | | | ^ | |
    | *********** | | | *********** |
    | * R * | | | * R * |
    +---*D Q*--+ | +---*D Q*--+
    * * | * *
    * _* | * _*
    +---*CLK Q*O------+ +---*CLK Q*O
    | * * | * *
    | *********** | ***********
    | |
    | |
    | |
    SET RESET


    I traced the circuit below with 4 possible initial states.

    below: Q(initially) = 0 RESET = 0


    /1
    SET -----+------------------------|
    | |NAND>--+ \2
    +-------| \6 +--| | /7x
    |NAND>--+-----+ |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| /5 | |
    |NAND>-----+ |
    \4 +-------| +--| Q
    | |NAND>--+------- /3
    +--------------------------------+--| |
    | |
    +----------|--+
    | |
    +----------+ |
    | | _
    +--------------------------------+--| | Q
    | |NAND>-----+---- \4
    /3 +-------| +--|
    |NAND>--+ \4x |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =1 | |
    |NAND>-----+--+ |
    +-------| +--| |
    =0 | |NAND>--+ =1
    RESET -----+------------------------|


    below: Q(initially) = 0 RESET = 1


    /1
    SET -----+------------------------|
    | |NAND>--+ \2
    +-------| \6 +--| | /7x
    |NAND>--+-----+ |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| /5 | |
    |NAND>-----+ |
    \4 +-------| +--| Q
    | |NAND>--+------- /3
    +--------------------------------+--| |
    | |
    +----------|--+
    | |
    +----------+ |
    | | _
    +--------------------------------+--| | Q
    | |NAND>-----+---- \4
    /3 +-------| +--|
    |NAND>--+ =1 |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =0 | |
    |NAND>-----+--+ |
    +-------| +--| |
    =1 | |NAND>--+ =1
    RESET -----+------------------------|


    below: Q(initially) = 1 RESET = 0


    /1
    SET -----+------------------------|
    | |NAND>--+ \2
    +-------| \2 +--| | /3x
    |NAND>--+-----+ |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =1 | |
    |NAND>-----+ |
    =0 +-------| +--| Q
    | |NAND>--+------- =1
    +--------------------------------+--| |
    | |
    +----------|--+
    | |
    +----------+ |
    | | _
    +--------------------------------+--| | Q
    | |NAND>-----+---- =0
    =1 +-------| +--|
    |NAND>--+ =0 |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =1 | |
    |NAND>-----+--+ |
    +-------| +--| |
    =0 | |NAND>--+ =1
    RESET -----+------------------------|


    below: Q(initially) = 1 RESET = 1


    /1
    SET -----+------------------------|
    | |NAND>--+ \2
    +-------| \2 +--| | /3x
    |NAND>--+-----+ |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =1 | |
    |NAND>-----+ |
    =0 +-------| +--| Q
    | |NAND>--+------- =1
    +--------------------------------+--| |
    | |
    +----------|--+
    | |
    +----------+ |
    | | _
    +--------------------------------+--| | Q
    | |NAND>-----+---- =0
    =1 +-------| +--|
    |NAND>--+ =1 |
    +--| | |
    | | |
    +----------|--+ |
    | | |
    +----------+ | |
    | | |
    +--| =0 | |
    |NAND>-----+--+ |
    +-------| +--| |
    =1 | |NAND>--+ =1
    RESET -----+------------------------|
     
  9. Jim Thompson

    Jim Thompson Guest

    Thanks, Tom! It's all coming back to me now.

    ...Jim Thompson
     
  10. mike

    mike Guest


    I don't think you want to do that with flip-flops. Too many
    race issues with clocking and resetting at the same time.
    If you rewire the flops into a Johnson counter configuration,
    each flip-flop has to contend only with it's own metastability
    issues. One clock makes the outputs the same, the other
    makes 'em different. Decode the state with an XOR.

    The stuff below looks a lot like a MC4044 Phase Detector.
    mike


    --
    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    4in/400Wout ham linear amp.
    Honda CB-125S
    400cc Dirt Bike 2003 miles $550
    Police Scanner, Color LCD overhead projector
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  11. Hadn't thought of that! It's a much cleaner solution and purely
    synchronous. Something like:

    +---------------------------+ __
    | +----------------|---------|<>|
    | | | | |-----o Out
    | | +---------|__|
    | .---. | .---. |
    +---|D |--+ +------|D |--+
    | | | | |
    R o------------->Ck |o----+ +--->Ck |o
    '---' | '---'
    |
    S o--------------------------+
     
  12. James Meyer

    James Meyer Guest

    I find that the more I drink, the stabler I get. Sometimes I get so
    stable I can hardly stand up.

    Jim (Burp) Meyer
     
  13. Well, as long as you're inventing, I thought you'd go for
    something useful! ;-)
     
  14. Sometimes I'm (meta) stable on the bar stool. I could fall into
    the potential well on the right, or the left (both stable
    conditions). ...but I'm (meta) stable sitting on the local
    maximum inbetween.
     
  15. This will also work in many cases,
    It eliminates loss of state when both inputs go LOW.
    (Use NOR if required)

    Stepan

    +-------| |-------|
    |NAND>--+-----+ |NAND>--+-----
    +--| | +--| |
    | | | |
    +----------|--+ +----------|--+
    | | | |
    +----------+ | +----------+ |
    | | | |
    +--| =0 | +--| =0 |
    |NAND>-----+--+ |NAND>-----+--
    +-------| |-------|
     
  16. That's very neat.
    It looks similar, although I derived it. Robert came up with the same
    circuit in response to the same problem.
     
  17. Where is that Jim?
     
  18. Fred Bloggs

    Fred Bloggs Guest

    This is identical to a version of JK-FF phase detector specialized for
    PLL application and used in the 74HC297 ADPLL manu'd by TI still. You
    might take a look at the gate level for that. The J and K inputs are
    edge clocked and set/reset the FF.
     
  19. Jim Thompson

    Jim Thompson Guest

    RSN!

    ...Jim Thompson
     
  20. Jim Thompson

    Jim Thompson Guest

    Ah, yes! MC4044, I remember it well... I described what an ideal
    edge-matching phase detector would do and Ron Treadway conceived the
    logic... first day was 360 gates, second day was 90, and on the third
    day it was NINE! Then I designed and coined the phrase "charge-pump".

    ...Jim Thompson
     
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