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Old 5th Oct 2015, 11:30 pm   #1
glowinganode
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Default Dallas DS1216 Smartwatch repair

I'm working on a piece of test equipment that has a data logging feature, and contains a Real Time Clock to provide time stamps in the event history.
It was found necessary to reset the time and date every time I powered it up, so suspected a failed back-up battery.
The RTC function is provided by a Dallas DS1216 "Smartwatch" module coupled to an 84256 RAM. This contains an embedded lithium battery and the clock and control electronics.
Googling around suggested these were still available, but not particularly cheap and of a similar date code, over 20 years old.
Inspired by some of Kat Manton's exploits I thought I'd explore further and attempt a repair or find a workable solution.
I prodded around and found a few hundred mV's floating around some of the pins, but wasn't sure whether this was coming from elsewhere on the board.
I went to great pains removing the 28 pin device, only to find the underside potted with epoxy.
My biggest breakthrough was chancing upon the DS1215 which I took a gamble upon being the smaller IC set inside the 28 pin socket. The datasheet indicated a 3 V lithium battery externally connected to pin 4 and gnd.
I measured 0.3 V here, and nothing at the output (pin15) which confirmed a depleted battery.
In preparedness, I had already been to Maplins and bought a CR2032 battery and holder. Fearing nothing to lose, I connected it to the battery connections on the IC, soldering to the shoulders of the pins.
Probed about again, now the voltages seemed to be correct.
I re-soldered the module to the pcb, refitted the RAM and went for it.
Brilliant, all working as it should, and remembering the correct time.
Connecting a new battery directly in parallel with the depleted embedded one did play on my mind, however I don't really have much choice unless I can find a way of disconnecting the original battery.
The DS1215 has an input for a second battery (pin 14) which if not used should be tied to gnd. I discovered it was actually connected to the other battery (pin 4). I did consider applying a low voltage between the two pins to fuse the trace, but thought best to give up while I was winning.
I originally had the new battery connected between pin 4 and pin 8, but re-worked it to connect to pin 14 and pin 8 as it seemed a tidier job.
Interestingly enough with the new battery temporarily disconnected, the original battery was reading just over 3 volts steady, so was obviously still capable of holding a charge. This made me feel a bit happier. Also proved that the job could be done in-situ.
The only real problem is mounting the new battery, currently it is taped up and secured with a self adhesive cable tie base.
Thanks for following, hopefully this may help someone out, we are bound to see more of this kind of technology in our radios and test gear as time goes on.
Cheers, Rob.
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Old 5th Oct 2015, 11:59 pm   #2
paulsherwin
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Default Re: Dallas DS1216 Smartwatch repair

I think your work looks very neat.

There are stories on the net of people fixing similar RTC modules by chopping the top off with a Dremel, which exposes the battery.
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Old 6th Oct 2015, 12:01 am   #3
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Default Re: Dallas DS1216 Smartwatch repair

Most of the Dallas clock chip repairs I have seen done on line have involved digging into the package to cut the internal wire to the duff battery.
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Old 6th Oct 2015, 11:08 am   #4
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Default Re: Dallas DS1216 Smartwatch repair

Excellent- it's always heartening to see "impossible" and "BER" overcome with ingenuity and dexterity.
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Old 6th Oct 2015, 9:10 pm   #5
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Default Re: Dallas DS1216 Smartwatch repair

It may make sense to connect the new battery via a schottky diode. This will lower the voltage applied to the internal dead cell and make it less likely to fail disastrously or indeed drain the new battery. It will also prevent any fault voltage coming out from the chip from potentially driving reverse current through the new battery.

It's beginning to sound like there may be a market for carefully recelled parts like this!
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Old 6th Oct 2015, 10:38 pm   #6
glowinganode
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Default Re: Dallas DS1216 Smartwatch repair

Chris, thanks for your input.
Adding a series diode did cross my mind, it would also protect against the effects of inadvertently inserting the battery with incorrect polarity.
I chickened out though, I'd already twice soldered directly to the pin shoulders and didn't want to push my luck any further.
A pin compatible replacement for this device is available, the DS1244Y-70, although not that cheap at 15-20 a go. Offers a 10 year service life.
As a cheap fix to get you going, that can be done in situ, it works (so far).
I liken it to "repairing" a failed section of a multi-section electrolytic, by soldering an axial 'lytic directly across the pins of the failed section.
Yes there's a chance it could go bang, that's the risk we run.

Cheers, Rob.
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