Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

why does output of 74LCX573 go high during powerup

Status
Not open for further replies.

tiwari.sachin

Full Member level 6
Joined
Aug 1, 2009
Messages
341
Helped
3
Reputation
6
Reaction score
3
Trophy points
1,298
Location
India
Activity points
4,447
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.
 

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
 

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

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

No I don't have them.

- - - Updated - - -

Hi,

Show your schematic.

Klaus

LATCH.jpg


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.
 

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
 

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.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top