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'[PIC:] Differences between 62256pc and 76c256c sra'
2003\11\14@080537 by Arno Cammans

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face
Hai,

I've created a logic analyzer with help of a few 76c256c 32kx8 memory
ic's. Strange thing is if I replace them by 62256Pc memory cells the
thing doesn't work anymore.
The memory is connected with a few counters to generate addresses. If I
strobe the WE low and OE high, and then only change the adresses , the
76c256 writes perfectly. Putting the contents of the cell into a graph
makes a perfect line. When I use the 62256 ram my graphs get distorted.
Lots of random bits in it. So my question is ... are there any
differences between these 2 memory ic's. I have the datasheets, but
can't find differences.

I enclosed graphs to illustrate my problem.

Thanks for the help.

Arno

Sorry .. Forgot the [pic:] the first time

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2003\11\14@094849 by SM Ling

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Decoupling cap ??  Supply?

Cheers, Ling SM
>The memory is connected with a few counters to generate addresses. If I
strobe the WE low and OE high, and then only change the adresses , the
76c256 writes perfectly. Putting the contents of the cell into a graph
makes a perfect line. When I use the 62256 ram my graphs get distorted.
Lots of random bits in it. So my question is ... are there any
differences between these 2 memory ic's. I have the datasheets, but
can't find differences.

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2003\11\14@100057 by Alan B. Pearce

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>The memory is connected with a few counters to generate
>addresses. If I strobe the WE low and OE high, and then
>only change the adresses

I don't think you can do this and expect the data to store properly. If you
look at the timing diagrams I think you have to strobe one of either of
these two signals false while you change the address.

I am surprised that one of the chips stores the data apparently uncorrupted,
and think you are just lucky about this, but I do not know that particular
chip, so it may have a feature where it can be driven this way.

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2003\11\14@113829 by William Chops Westfield

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On Friday, Nov 14, 2003, at 05:07 US/Pacific, Arno Cammans wrote:

>  If I
> strobe the WE low and OE high, and then only change the adresses , the
> 76c256 writes perfectly. Putting the contents of the cell into a graph
> makes a perfect line. When I use the 62256 ram my graphs get distorted.
>
The 62256 data sheets specifically says that you can't change the
address lines while WR/ and CE/ are low.  What you're seeing is about
what I'd expect as the
chip writes to several memory cells as the address signals propagate at
slightly different speeds internal to the chip.

I didn't see a similar caution for the 76c256, but I'd be surprised if
you
don't have similar problems, perhaps with a different pattern....

BillW

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