Hacking into a Dallas NVRAM

A project log for Earth Lander 84

Like the classic Lunar Lander video game but on Earth and with no graphics, only some 7-segment displays

matsengmatseng 02/03/2016 at 07:094 Comments

Since my EPROM burner is so old it connect via a parallel port I needed to find another solution for that. I found an old but never used Dallas DS1235 32Kx8 NVRAM module that I got in a goodie bag at some electronics seminar I attended way back in the past.

It's dated 1988 and looking at the data sheet the internal battery should last at least 10 years. Considering it's almost 20 years past the expiry date I wanted to make sure that the batteries wasn't dead.

The module consists of a SRAM with a DS1210 Nonvolatile Controller Chip and two lithium batteries potted in epoxy.

Luckily the top of the DS1210 was exposed at the bottom so it was rather easy to just use a hot soldering iron to scrape off the epoxy layer by layer to get down to the legs of the DS1210 so I could measure the voltages of VBAT1 and VBAT2.

It was dead... VBAT1 had just a few millivolts and VBAT2 had fared batter and was at 250mV.

I will dig out some more of the epoxy so I can cut one of the battery leads and attach an external battery to keep the module alive without VCC.

I'm not sure if I can use a single 3 volt coin cell battery and expect the SRAM to be within data retention limits since the DS1210 have a voltage drop of up to 0.3 volts. But the maximum voltage of VBAT1/2 on the DS1210 is 4.0 volts so I can't just series two coin cells.

So I have three choices. Either run it on a single standard 3v cell, get a 3.6 volt Li-ion coin cell or series two 3 volt cells and then hook it up with some diodes to drop off the extra voltage. But at nano amps the Vf of a diode is not much so I'd need a lot of them. Maybe I can use a LED instead....


quadzillatech wrote 12/24/2021 at 16:25 point

did you ever finish this?,ive 2 of these in a tek scope with dead batterys.

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Yann Guidon / YGDES wrote 02/03/2016 at 15:23 point

"Low Data Retention Voltage: 1.5V(Min)"

That's a 2002-era chip and older chips might have a slightly higher retention voltage but if your CR2032 is fresh, and you're only using during this development period, you should be good.

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Yann Guidon / YGDES wrote 02/03/2016 at 14:07 point

I believe you're safe with a primary cell at 3V, because that's what it's designed for. The SRAM's data retention voltage should be around 2V or 1.5V at worst, according to some of my datasheets. Since you're just building a prototype, it's ok to keep it simple and working :-)

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matseng wrote 02/03/2016 at 15:02 point

Yes, that sounds very likely. I'll just tack on a cr1220 on top of the chip and call it a day :)

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