SRAM 'OE' pin query

[mjones]

Okay, just showing my ignorance here ...... the OE signal is an input.
The OE pin (22) on a 41256SJ surface mount (J-Lead) SRAM on a board I
have is stuck high *even when the pin is lifted* (ie the problem isn't
originating elsewhere).

I'd guess that means the SRAM is at fault, correct?


Thanks

 

[Anthony Fremont]

mjones wrote:

Okay, just showing my ignorance here ...... the OE signal is an input.
The OE pin (22) on a 41256SJ surface mount (J-Lead) SRAM on a board I
have is stuck high *even when the pin is lifted* (ie the problem isn't
originating elsewhere).

I'd guess that means the SRAM is at fault, correct?

Not necessarily. The OE pin is usually "active low" meaning that an
external circuit must drive the pin to a low state to enable the chips
outputs. It may have a weak internal pullup resistor that you see as a
voltage present on the pin. Try grounding the pin and see if your
circuit starts working.

 

[mjones]

On Fri, 24 Dec 2004 20:33:50 GMT, "Anthony Fremont"
<spam@anywhere.com> wrote:

Not necessarily. The OE pin is usually "active low" meaning that an
external circuit must drive the pin to a low state to enable the chips
outputs. It may have a weak internal pullup resistor that you see as a
voltage present on the pin. Try grounding the pin and see if your
circuit starts working.

I see, thanks. Well I tried grounding the pin and that pulled it low
but made no difference.

I should also have mentioned that this particular chip gets rather hot
compared to the others that are the same as it on the PCB.



Cheers

 

[mjones]

On Sat, 25 Dec 2004 02:25:56 GMT, Rich Webb
<bbew.ar@mapson.nozirev.ten> wrote:

Ahhh... It's an AS7C256. Different critter.

Ah, good.

It's always possible that it's faulty. CMOS is sensitive to static
discharge damage. I'd guess that even in the UK it gets a *bit* dry in
the winter months... ;-)

Well, *sometimes* ............

Otherwise, CMOS typically won't dissipate energy unless it's switching,
so perhaps it's more active than it should be. Are all of the address
and control lines connected to a driver or pulled high?

From what I recall, I think they're all high.

Also, there's a
note that "CE or WE must be High during address transitions."

Hmmm, I see. Worth swapping it out though do you think?

If you don't have an o-scope or logic analyzer handy, consider one of
the gizmos from www.rockylogic.com. I've had one of their 8-channel
analyzers since they first came out and still carry it with me in my
notebook PC bag. Eight (even sixteen) channels aren't enough to see
*everything* but are enough to get a better idea of what's happening.

Thanks for the tip, will take a look.

Happy Christmas.

 

[Rich Webb]

On Fri, 24 Dec 2004 20:55:11 GMT, mjones98@hotmail.com (mjones) wrote:

On Fri, 24 Dec 2004 20:33:50 GMT, "Anthony Fremont"
spam@anywhere.com> wrote:


Not necessarily. The OE pin is usually "active low" meaning that an
external circuit must drive the pin to a low state to enable the chips
outputs. It may have a weak internal pullup resistor that you see as a
voltage present on the pin. Try grounding the pin and see if your
circuit starts working.


I see, thanks. Well I tried grounding the pin and that pulled it low
but made no difference.

I should also have mentioned that this particular chip gets rather hot
compared to the others that are the same as it on the PCB.

What does the data sheet say about this pin?

Tying it low may mean that it's trying to drive its outputs at
inappropriate times. Are there chip select and read/write strobes that
you're also controlling?

FWIW, the only 41256 parts that I could locate datasheets for were all
dynamic RAMS and none had OE pins.

--
Rich Webb Norfolk, VA

 

[mjones]

On Fri, 24 Dec 2004 22:35:03 GMT, Rich Webb
<bbew.ar@mapson.nozirev.ten> wrote:

What does the data sheet say about this pin?

If I could understand it all I'd tell you.

I got it from:

http://www.farnell.com

Part no: 3005392

Just key in that number and the search result will come back.

Then click on the Orange order code to the left, and on the next page
click on the datasheet (PDF format).

Tying it low may mean that it's trying to drive its outputs at
inappropriate times. Are there chip select and read/write strobes that
you're also controlling?

Not sure I'm afraid - I don't have any schems for this board.

I do know though that this (and another RAM) is being driven by the
output of a 7414 (Hex schmitt inverter - pin 10 (this 7414 is also
connected to an oscillator via another pin). In fact, it was looking
at this 7414 with my logic probe that highlighted something being
wrong - the probe would NOT light up high or low with the probe on pin
10 of the 7414, but on lifting pin 10 it then went low as expected.

Re-connected pin 10 of the 7414 and traced it to pin 22 of the
aforementioned RAM and found that lifting pin 22 (OE) of the RAM also
caused pin 10 of the 7414 to go low (as expected) and that pin 22 of
the RAM, when lifted, was still high (which seemed unusual, especially
when it's also noted that this particular RAM is getting noticeably
hotter than the others of the same type).

FWIW, the only 41256 parts that I could locate datasheets for were all
dynamic RAMS and none had OE pins.

Curious.

Thanks

 

[Rich Webb]

On Fri, 24 Dec 2004 22:48:59 GMT, mjones98@hotmail.com (mjones) wrote:

On Fri, 24 Dec 2004 22:35:03 GMT, Rich Webb
bbew.ar@mapson.nozirev.ten> wrote:


What does the data sheet say about this pin?

If I could understand it all I'd tell you.

I got it from:

http://www.farnell.com

Part no: 3005392

Ahhh... It's an AS7C256. Different critter.

I do know though that this (and another RAM) is being driven by the
output of a 7414 (Hex schmitt inverter - pin 10 (this 7414 is also
connected to an oscillator via another pin). In fact, it was looking
at this 7414 with my logic probe that highlighted something being
wrong - the probe would NOT light up high or low with the probe on pin
10 of the 7414, but on lifting pin 10 it then went low as expected.

Re-connected pin 10 of the 7414 and traced it to pin 22 of the
aforementioned RAM and found that lifting pin 22 (OE) of the RAM also
caused pin 10 of the 7414 to go low (as expected) and that pin 22 of
the RAM, when lifted, was still high (which seemed unusual, especially
when it's also noted that this particular RAM is getting noticeably
hotter than the others of the same type).

It's always possible that it's faulty. CMOS is sensitive to static
discharge damage. I'd guess that even in the UK it gets a *bit* dry in
the winter months... ;-)

Otherwise, CMOS typically won't dissipate energy unless it's switching,
so perhaps it's more active than it should be. Are all of the address
and control lines connected to a driver or pulled high? Also, there's a
note that "CE or WE must be High during address transitions."

If you don't have an o-scope or logic analyzer handy, consider one of
the gizmos from www.rockylogic.com. I've had one of their 8-channel
analyzers since they first came out and still carry it with me in my
notebook PC bag. Eight (even sixteen) channels aren't enough to see
*everything* but are enough to get a better idea of what's happening.

--
Rich Webb Norfolk, VA