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    why does output of 74LCX573 go high during powerup

    Hello,

    I am using 74LCX573 whose inputs are connected to MCU while the output control a device (lets consider Leds for example)

    When I powerup (ie MCU is in reset for about 1200ms and then activates), the outputs of latch are high even when the inputs are low and once the MCU is active, it all works fine.


    My requirement. The outputs of latch should be low during powerup.

    The Latch pin is connected to MCU. The output enable pin is grounded.

    What can I do so that the output of latch are 0 on powerup.

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    Re: why does output of 74LCX573 go high during powerup

    Hi,

    Show your schematic.

    ***
    The "latched state" at power up is not defined in the datsheet.
    And most probably you left the signals floating during power up. (This is not the problem of the 74LCX573, you have to take care for that, if you need predictable power up behaviour)
    Thus I recommend to
    * either choose "high impedance" on power up by using !OE,
    * or using transparent mode by pulling D-lines and LE-line to the correct state.

    Klaus
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    Re: why does output of 74LCX573 go high during powerup

    Do you have pull-down resistors connected to the D0-D7 Input pins ?



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    Re: why does output of 74LCX573 go high during powerup

    Quote Originally Posted by summitville View Post
    Do you have pull-down resistors connected to the D0-D7 Input pins ?
    No I don't have them.

    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,

    Show your schematic.

    Klaus
    Click image for larger version. 

Name:	LATCH.jpg 
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ID:	153274


    If I remove ground on !OE and assert a pullup. Its works fine

    If I connect !OE to some MCU pin with pullup. It again behaves the same say (High during powerup)

    I need to have all the outputs low on powerup.



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    Re: why does output of 74LCX573 go high during powerup

    Hi,

    Your schematic shows unconnected input pins.
    General recommendation: Don't leave (unused) input pins floating. Neither digital one nor analog ones.
    This has been discussed many times ... thus you will easily find moee informations, if you need...

    The 573 has three output states: LOW, HIGH, high-z.
    With !OE = HIGH you make the outputs high-z!
    Where "high-z" is no valid logic level, it may be recognized as HIGH, as LOW...it even may oscillate.
    If you want it to be a valid level, you have to care for it ... usuall with "pull-xx" resistors.

    Your "schematic" is just the 573 .... without any information about pull-up, pull-down, source of signal, target of signals...

    Klaus
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  6. #6
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    Re: why does output of 74LCX573 go high during powerup

    In the worst case scenario, you can add a delay to the OE pin and add pull-down resistors at the output but I agree with others, this sounds like a problem with the signals driving the 573, not the IC itself.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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