Latch circuits

Discussion in 'General Electronics Chat' started by othello, Aug 29, 2008.

  1. othello

    Thread Starter New Member

    Aug 29, 2008
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    i am a new learner in this world & also this forum.
    can u help me to get this answer?
    Why we use LATCH CIRCUITS in digital ?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,536
    A common term in digital is latch and hold. The idea is to capture a moving number so we slow humans can see it. It is also used a lot in displays to keep it stable between updates.

    Latches are basically flip flops of various types.
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    With latch circuits we can create memory and store data, an example is the RAM we use in our PC's.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    A latch circuit is not a flip-flop. It is a memory circuit made from combinatorial devices, but it is fundamentally different. The basic definition has two inputs, called Data (D) and Latch Enable (LE). It has one output(Q). The usual implementation calls for Q to be equal to D when LE is high and for Q to hold its value when LE is low.
    Code ( (Unknown Language)):
    1.  
    2. Q = (D & LE) | (Q & LE*)
    3.  
    If you draw a Karnaugh map of this equation you can see the possibility of a hazard or race condition. To avoid this hazard we add a completely redundant covering term and the hazard goes away. The new latch equation becomes
    Code ( (Unknown Language)):
    1.  
    2. Q = (D & LE) | (Q & LE*) | (D & Q)
    3.  
    The redundant covering term (D & Q) removes the hazard. Cool beans! eh?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, a latch CAN be a D-type flip flop, if a data item is placed on the D input and the clock is toggled, the data will be latched into the flip flop.

    A latching shift register with parallel inputs and serial output is very useful for determining which of numerous data lines may have been responsible for triggering an event. A CMOS CD4021B IC is an example of this type of latching shift register.

    It is comprised of numerous D-type flip flops.
     
  6. veritas

    Active Member

    Feb 7, 2008
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    The term "latch" usually describes a memory device that is NOT clocked, unlike a flip-flop, which is edge-sensitive.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    There are some interesting information on latch circuit types and their various application contained in the AAC ebook here.

    hgmjr
     
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