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Difference between latch and buffer

Discussion in 'Electronic Basics' started by aman, Mar 21, 2005.

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  1. aman

    aman Guest

    Can any of you guys please comment on the following definitions which I
    think are right.

    Latch is something once enabled holds the value for infinite time as
    long has it has Vcc and ground connections.

    Buffer is something which holds the value for finite time during which
    you need to read it. It can essentially be a part of sample and hold
    circuit.

    Please correct me if I am wrong.
     
  2. Bob Eldred

    Bob Eldred Guest

    Your right about a latch, it is a digital circuit that hold a state as long
    as power is applied. They are usually made from "D" flops and must be
    clocked. This type changes state with the clock edge depending on the input.
    They do not change state with the input state only. In other words, if the
    output is high and the input goes low, the output will only change to low
    when the next clock edge comes. Some latches use "R-S" flip-flops that chang
    state with a set or reset signal and do not require a clock.

    A buffer may or may not have a latch in it. It's purpose is to drive a line
    or to drive a number of other circuits, to apply more current to a digital
    signal. In addition many buffers can operate tri-state where they have a
    high impedance mode that doesn't load other connected driving circuits. Many
    times they are bi-directional and can transmit or receive. When there is no
    internal latch, the output follows the input without a clock, some invert
    the signal.

    Analog buffers are high input impedance amplifiers and are used for sample
    and hold as well as to drive lines or other circuits. The usually have unity
    gain. Their purpose is to reduce loading and provide isolation in analog
    devices.
    Bob
     
  3. Rich Grise

    Rich Grise Guest

    Yes, you are wrong. :) A latch does hold the value indefinitely - they
    come in 'transparent' and 'D-type' or 'edge-triggered'. The transparent
    kind transfers its input to the output all of the time that the clock
    is active, and holds when the clock goes inactive; the d-type or edge-
    triggered holds what's there at the active clock edge.

    A buffer is just a straight-through inverting or noninverting amplifier,
    that has greater drive capability than the circuit it's buffering.

    HTH!
    Rich
     
  4. Jamie

    Jamie Guest

    i'll give it a shot.
    Latch
    something that holds data that was latched in
    at some time interval from a source that may or has
    already changed its value of the source since the
    latch state took place.
    in other words, once a value is latched from a
    source, it does not care what the value of the source
    changes to, the latched value will be what you see until
    you relatch again.

    ---
    Buffer.
    I can think of 2 cases here.
    one, a unity gain amplifier to represent a source value
    with out effecting the characteristics of the source.
    what this means is like Load impedance or capacitance effects
    that could change the behavior of the source.
    --
    now in the digital sense.
    A buffer is like a cache, info is backed up in memory/registers to be
    read later. this way, you don't lose any data while what ever is used to
    read it at the most convenient time is performed.
    normally in these types of buffers, when data is read out of them
    they usually do so in either FIFO(First in First out) or LIFO (Last in
    First out);
    normally the FIFO is use in most apps., but you do have the LIFO in
    cases of Stack related type functions.

    in this type of system, a Latch is not needed because the buffer is
    capturing all data for you to read when able.
    of course, there are limits to buffers! a good design should not
    allow this to exceed the limit.
    Latches can hold data when the latch state is done but, data will be
    ignored until you relatch again.

    hope that helped.
     
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