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Battery-backed RAM

Hi Everyone

I'm designing a system which uses low power SRAM powered by battery as a backup device. The data sheet states that all I/Os mus be kept within

0.2V of either Vcc or GND for the standby current to be guaranteed. This is no surprise, one CMOS device must be held firmly in the off state.

The I/Os will be connected to a powered down CMOS buffer while in standby mode, and my question is: will the powered down buffer have enough leakage current hold the I/Os at ground potential, or should I provide weak pull-down resistors to prevent these pins from floating above 0.2V?

TIA Cheers Geoff

Reply to
Geoffrey
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Sorry for the double post - can only use Google from workplace.

Reply to
Geoffrey

I would use the pulldowns. I wouldn't even make them all that weak -- during normal operation the bus will either be driven fast which takes current anyway, or it'll be allowed to float. Good strong pulldowns will keep the lines that start high from spending much time in the transition region as they float down.

--

Tim Wescott
Wescott Design Services
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Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
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Reply to
Tim Wescott

Hello Geoffrey,

I would not count on that. Think about it, CMOS outputs at a VCC of zero have both devices in the "fully off" region. The respective node can float about within the bounds of the substrate diodes and that is far more than 0.2V. More like 0.6V.

So I'll second Tim's opinion, I'd also use pull downs. But I don't believe they have to be very strong because a chip that has its power turned off with quickly drag its port pins down because of the substrate diodes.

Regards, Joerg

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Reply to
Joerg

I was thinking more about the case when it's power on and the bus has just gone high impedance -- anything that's sitting at VCC is now going to slowly fall to ground, with a nice long interval in the middle.

I wouldn't swear to any one value being right, though -- I'd test.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Very nearly 30 years ago I designed a power failure sequencer for NVRAM (non-volatile RAM), for use by NCR in cash registers. The NVRAM needed no battery. Is that sort of stuff still made?

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
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Reply to
Jim Thompson

Hello Jim, Hello Tim,

I'd have a few words with the designers of that part if they left the bus floating.

Yep:

However, even the old HP analyzer I just repaired uses regular CMOS RAM and a backup battery. I guess nowadays they'd have used a Li coin cell but even that old Yuasa NiCd in there lasted a whopping 20+ years. Amazing. The bad news is now I have to do another trip to the recycling station.

Regards, Joerg

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Reply to
Joerg

Check out the ZMD Capstore NVRAM: We switched from the ST battery backed RAM when we ran into problem of them failing in the field, and because we had some radios going into space where NASA wouldn't allow any Lithium batteries.

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Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

Have you considered using FRAM devices from Ramtron? You'll still need to figure out a way to keep the control lines inactive during power-up and power-down, but you can ommit the battery.

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Reply to
Nico Coesel

We're using quite a large device, 1M x 16 (the software guys want a big event log space) so I think we're stuck with SRAM. We've used Simtek NVRAM before, but with limited success - in fact the device we used in the last project had many problems, and is now to be withdrawn from the market.

I also think we will have to fit the resistors (82 in total, for two SRAMS). Thanks for the help, everyone, and apologies for multi-posting. Anyone know how to cross post from Google? Our organization doesn't allow an NNTP feed ...

Cheers Geoff

Reply to
Geoffrey

On Mon, 15 May 2006 04:35:43 -0700, Geoffrey wrote: ...

Yes. Instead of clicking the "Reply" link at the bottom of the post you're replying to, look up in the header, and click the "show options" link. That will give you another "Reply" link. Use that one. Then, google will present you with an entry form that has a textbox for the newsgroups - just type in newsgroup names, (appropriate NGs, of course) up to a total of five, comma-separated.

This is also the preferred form of reply (the other reply link), as it quotes context and allows bottom-posting.

Cheers! Rich

Reply to
Rich Grise

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