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....