Problem with 74LS373 latching!

[UroBoros]

74ls373

hai

I am using 74LS373 chip to latch the PORTD output of PIC to illuminate some LEDS. I have no previous experience with this Latch.

As there is no need to enable or disable the output of this latch I have tied the Output Enable pin of this chip to ground so that the output is always latched.


So in software I put the required data into the concerned port and toggle the Latch enable pin of this 74LS373 and the output gets latched into this chip.

So far it is ok.

but after some time the output of the latch turns off or change data. I chechked the software and this change is not happening at any particular portion of code or in a prefixed time.

Is there any experience or suggession you can share?


picstudent

 

[IanP]

74ls373 pin configuration

Double check microcontroller's pin that controls LE (Latch Enable) input of the 74LS373 ..
Most likely it generates very short pulse(s) ..
There is now way the outputs of this LATCH would change their stages by themeslves, unless there is a transition(s) (L-->H) at the LE pin ..

And one more thing, make sure that the power at the Vcc pin of the 74LS373 is "clean" (100nF cap wouldn't hurt) ; this "stuff" can pass through power ..

Regards,
IanP

 

[UroBoros]

74hc373 or 74ls373

Hai
Thanks a lot IanP for the suggession.Will double check it.

btw is there any possibility to set a breakpoint in ICD to break if a pin goes low or high, So that I can monitor the LE pin.(I think no such fesility in ICD.Any clue on how to catch this bug?)

Actually by normal inspection I cannot locate where in code it gets triggered.But as you pointed out ,it is happening somewhere in my code,otherwise "fairchild" would have been out of this business years before!

Thanks a lot.

Will be back
PIcstudent

 

[XNOX_Rambo]

74hc373 es el mismo 74ls373

How much current are you sinking (sourcing?) through the output pins of the
74LS373?
I'm thinking that you might be exceeding the allowed package power dissipation,
which could lead to erratic behaviour.

 

[UroBoros]

74ls373 driving led

Hai

Initially I was cnnecting leds with 330R resistor to GND.(Directly driving )


In that case I suspected this problem and now I am driving 8 transistors (BC548) with 10k to base and collector load as LED with 330R series.


But still same behavier.

Thanks
picstudent

 

[XNOX_Rambo]

pic and 74ls373

OK, then the next question is if you are running your circuit on 5V, as this is
the nominal voltage for a 74LS373.

You could also try the 74LS374 instead, the pin-compatible edge triggered
version. There could be some timing flaw in your code that will be obvious when
using a 374, as it demands a more stringent timing than the transparent 373.

As a sidenote, I prefer the 574 as it has a flow-through design, giving a simpler
layout.

 

[UroBoros]

what is 74ls373

Hai

Thanks for the reply.

My supply voltage is 5 Volts itself.
I have to enquire about the availability of 374 and 574 here to test that.

But nowhere else in my code the Latch Enable pin is being "Touched".

As this is the prototype the wiring is very shabby and probabily ground may not be perfect.Is there any chance that while operating like xoring or ioring other pins of the same port of PIC ,the Latch Enable Pin "vibrates" so that latch gets triggered?

Any experience like that?(Microchip datasheet says something about reading and writing back to ports while xoring..)

picstudent

 

[IanP]

74ls373 con 8 leds

Personally, I wouldn't bother with other ICs .. The 373 is as good as any other latch from 74-family ..
Try to concentrate on finding the problem ..
Maybe you can try another port pin that doesn't have alternative finction, or add a pull-up, ..

Regards,
IanP

 

[kar2kn]

74ls373 led

hi, check out the fan out of the ic.
and also no of lines that is driven.
or try using 74HC373

 

[UroBoros]

pic + led + 74ls373

..... or add a pull-up, ..

Regards,
IanP

Hai

Thank you very much IanP for the suggession, and happy to say that I have solved the issue.

As you said I have connected a pull up resistor to the LE pin and a .001uF capacitor to GND. It solved the issue.Actually the noise picked up through the wiring of the LE pin was the problem.

So again you helped me to get out of a deadlock.

Thanks a lot.
I am donating 30 Points for you for the help.
this is not being done to create any waves in your already flooded point account.

But what else I could do for the most helpfull memeber of EDAboard?

With thanks and regards
picstudent

 

[IanP]

74ls373 test

You are always welcome ..
It is really good to hear that you solved the problem not by changing ICs ..

Good luck and regards,
IanP

 

[gautamsakthi]

74ls373 pin description

wat is the voltage level of inputs(d0 to d7) to be given to latch