One simple quesiton

Discussion in 'VHDL' started by parag_paul@hotmail.com, Nov 8, 2007.

  1. Guest

    This might be very simple , sorry for wasting your time.

    I have seen terms like level sensitive and edge sensitive..

    Now hoe can anythign be level sensitive, the value has to change at
    the edge no matter what,
    I want to understand , that latches are said to be level senstitive,
    now, the x output of SR latch ( for example) Will become s or r from
    0,0 input state only when one of them changes to 1 right, How can we
    call it a level sensitive device.

    Thanks in advance again
     
    , Nov 8, 2007
    #1
    1. Advertising

  2. Guest

    Concerning latches, "edge-sensitive" indicates that the latch contents
    may change only at a clock transition; "level-sensitive" suggests
    that the contents may change while the "enable" signal is in its
    "active" state (transparent latch).

    The terms "edge-triggered" and "level-triggered" are also used.

    Don't seek any deep meaning or accuracy in these terms. They are used
    (like most words) without much thought. You may see them in data
    sheets, but you should read the detailed description in order properly
    to understand the function.


    >I have seen terms like level sensitive and edge sensitive..
    >
    >Now hoe can anythign be level sensitive, the value has to change at
    >the edge no matter what,
    >I want to understand , that latches are said to be level senstitive,
    >now, the x output of SR latch ( for example) Will become s or r from
    >0,0 input state only when one of them changes to 1 right, How can we
    >call it a level sensitive device.
     
    , Nov 8, 2007
    #2
    1. Advertising

  3. Ole Nielsby Guest

    <> wrote:

    > I have seen terms like level sensitive and edge sensitive..


    Me thinks parachutes are level sensitive and airbags are edge
    sensitive. Your parachute will (hopefully) automatically unfold
    when the air pressure reaches a certain level. Your airbag will
    explode at a sudden move.

    If you want a better explanation, you should incorporate the
    terms you want explained in the subject line. Call it:

    Level/edge sensitive - could someone explain?

    or something like that. That's more likely to catch the eyes of
    the experts.
     
    Ole Nielsby, Nov 8, 2007
    #3
  4. Duane Clark Guest

    wrote:
    > This might be very simple , sorry for wasting your time.
    >
    > I have seen terms like level sensitive and edge sensitive..
    >
    > Now how can anythign be level sensitive, the value has to change at
    > the edge no matter what,
    > I want to understand , that latches are said to be level senstitive,
    > now, the x output of SR latch ( for example) Will become s or r from
    > 0,0 input state only when one of them changes to 1 right, How can we
    > call it a level sensitive device.


    level_sensitive: process (A, EN)
    begin
    if EN = '1' then
    B <= A;
    end if;
    end process level_sensitive;

    Notice that there is no clock involved. There are real situations where
    you might want that behavior, though almost never in an FPGA. That is
    why the synthesis tools warn you if you have inadvertently created a latch.

    In the (very) old days of building microprocessor memory interfaces with
    TTL logic, they were used rather commonly. These are commonly referred
    to as transparent latches, because when the enable is active, the device
    is transparent. For example, google for the 74LS75 device. Notice that
    there is no clock pin on the device.
     
    Duane Clark, Nov 9, 2007
    #4
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Yamin

    deep copy quesiton

    Yamin, Nov 30, 2004, in forum: Java
    Replies:
    9
    Views:
    614
    charely
    Dec 4, 2004
  2. Hunter
    Replies:
    16
    Views:
    863
    Michael
    Jan 10, 2004
  3. JustSomeGuy

    Newbie quesiton.

    JustSomeGuy, Dec 5, 2004, in forum: XML
    Replies:
    3
    Views:
    420
    Bjoern Hoehrmann
    Dec 5, 2004
  4. JustSomeGuy
    Replies:
    15
    Views:
    575
    JustSomeGuy
    Sep 20, 2003
  5. Andrew Bullock

    bit of a maths quesiton

    Andrew Bullock, Feb 5, 2005, in forum: C++
    Replies:
    2
    Views:
    316
    Victor Bazarov
    Feb 5, 2005
Loading...

Share This Page