AT89S8252 & ALE processor signal

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

Reply to
Fabrizio D'Agnolo
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Bad PCB layout or long wires.

Meindert

Reply to
Meindert Sprang

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

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

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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

Reply to
Jim Granville

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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

Reply to
fabrizio

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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.

Reply to
Peter Bushell

That is very long. How big is your board?

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

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

Reply to
Stef

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,

Reply to
fabrizio

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