AT89S8252 & ALE processor signal

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Hei to everyone,

I have a strange problem with the ALE signal.
Address Latch Enable signal is used to latch the low byte of the address during
access of external memory. In my case the ALE signal is directly connected to
the Latch Enable input of SN74HCT573. My problem that the Address latch is
only working well when an oscilloscoop is connected on the ALE pin.
What's the reason for this behaviour?
A small condensator, 82pf, over ALE solved the problem. Is this a good
solution.
Thank you in advance, for help

Fabrizio

Re: AT89S8252 & ALE processor signal
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Bad PCB layout or long wires.

Meindert



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Probably lousy PCB layout, lack of decoupling or long wires without
termination.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Re: AT89S8252 & ALE processor signal
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Better layout is the best solution, but you can try a 74HC573
as that has more tolerance of trace ringing.
-jg

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Vladimir,

My circuit is Build on Experimetal printboard, Cabling is done with wires.

The distance between AT89S8252 ALE pin and the input of SN74HCT573 is more
or less 30cm !

When i replace this single wirewrap wire with a thicker wire the problem is
solved.

So the distance between the Processor signal and the Buffer ic is to Big !!!

Thanks for help,
Fabrizio

Re: AT89S8252 & ALE processor signal

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is
!!!
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I hope you're "designing" whatever this is as a hobby, and not to put in an
aicraft, or something!

Peter.



Re: AT89S8252 & ALE processor signal
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That is very long. How big is your board?

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The thickness of the wire should not be important (within limits),
length and routing are much more likely to be a problem.

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Yes. On most boards this is only a few centimeters.

Do you use proper supply decoupling on each ic on the board? This is
always important, but even more so on a wire board with long wires.

Stef



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There are in fact two exeprimetal boards involved.
(One board is not big enough for al ic's)
One board contains the AT89S8252 with The RS232 connections for programming
and communications. The other board has the IO and address decoding IC's.
Every IC has an 100nf decoupling condensator connected to +VCC and GNC has close
to the IC has possible.

The problem is that both exp boards are connected via flatkable, 40 wire.

The lenght +-25Cm of this cable seems to be a problem for the ALE signal.

This problem is solved, for me, and i learned that the lenght of cabling is not
so innocent has it looks

fabrizio,

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