VOGONS


DS12887 drop-in replacement

Topic actions

First post, by Maeslin

User metadata
Rank Member
Rank
Member

While waiting for parts to arrive for another project, I've begun designing a drop-in replacement for the infamous integrated-battery DS12887 / M48T86 RTC/NVRAM package.

It's pretty much a small-package DS12885 mounted on a small adapter board with the clock crystal and user-replaceable batteries (probably a pair of LR44/SR44 due to space constraints) that would fit inside the footprint of an existing DS12887 package and drop right in the socket, no alteration required.

If there's interest, I might make a small production batch.

Last edited by Maeslin on 2014-07-12, 15:04. Edited 1 time in total.

Reply 1 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++

http://www.ebay.com.au/itm/130810136997?ssPag … 984.m1497.l2649

But use a socket so it can be easily replaced again.

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 2 of 173, by Maeslin

User metadata
Rank Member
Rank
Member
Mau1wurf1977 wrote:

http://www.ebay.com.au/itm/130810136997?ssPag … 984.m1497.l2649

But use a socket so it can be easily replaced again.

I know the DS12887 is still around, I just find it annoying to either have to replace the whole package or crack it open and frankenstein a new battery in there. :p Plus, when you buy a DS12887, you never know quite how old it actually is. The battery could already be near-dead.

What I intend to make will be a drop-in replacement for it, but with easily accessible and replaceable batteries while still fitting in the same volume. I'm just wondering if anyone else would be interested by that. 😀

Reply 3 of 173, by Old Thrashbarg

User metadata
Rank Oldbie
Rank
Oldbie

I really don't understand the resistance to the idea of alternatives, but every time the subject of Dallas chips comes up, there's always a few who insist that you should just buy a new one, as if there are no disadvantages to doing so.

I personally think a drop-in unit with easily-replaceable batteries would be fantastic.

As for the design of the thing, rather than two LR44 cells, what about a CR1220/CR1225? Those are about 1MM larger diameter than an LR44, but they're thinner, plus they're 3V so you'd only need one of 'em. A little bit of toying around with the layout, and you may even be able to make the whole unit a bit thinner than a Dallas chip... which really wouldn't be a bad thing, considering the awkward locations they sometimes inhabit.

Reply 4 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++

Because such an alternative is not an alternative for many. Most people don't solder and aren't into modding, don't have the tools...

Now if such a product would be available on eBay, sure people would happily buy it.

If you don't understand the resistance then you don't have an understanding of potential customers.

Maeslin:

Does your solution still require de-soldering the old chip?
How much would you charge for a unit considering all the parts and time to build?

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 5 of 173, by DonutKing

User metadata
Rank Oldbie
Rank
Oldbie

Plus, you buy a DS12887, you never know quite how old it actually is. The battery could already be near-dead[

They have a date code printed on them. I bought some from ebay that were only about a year old.

If you are squeamish, don't prod the beach rubble.

Reply 6 of 173, by Maeslin

User metadata
Rank Member
Rank
Member
Old Thrashbarg wrote:
I really don't understand the resistance to the idea of alternatives, but every time the subject of Dallas chips comes up, there […]
Show full quote

I really don't understand the resistance to the idea of alternatives, but every time the subject of Dallas chips comes up, there's always a few who insist that you should just buy a new one, as if there are no disadvantages to doing so.

I personally think a drop-in unit with easily-replaceable batteries would be fantastic.

As for the design of the thing, rather than two LR44 cells, what about a CR1220/CR1225? Those are about 1MM larger diameter than an LR44, but they're thinner, plus they're 3V so you'd only need one of 'em. A little bit of toying around with the layout, and you may even be able to make the whole unit a bit thinner than a Dallas chip... which really wouldn't be a bad thing, considering the awkward locations they sometimes inhabit.

If the old DS12887 soldered on the board, then unsoldering it would unfortunately be necessary. I've only ever seen them socketed to be honest. Soldering a fixed-lifespan component is just a dick move.

I've considered the CR1220/1225. They might fit but they have pretty much nothing for capacity. ~30-50mAh instead of ~200 for the CR2032s. That's where the LR44/SR44 would shine since a pair of them should give roughly the same lifespan as a CR2032. It's also a question of how easy it is to find each kind of cell. Going for a very common one makes things simpler.

It's only a vague cost estimate but $10-$12 plus shipping might be doable (sans batteries). Perhaps a tad more for the very first batch. I've already got most of the PCB designed and parts selected, just need to pick a battery format / matching holder and make everything as small as I can.

Reply 7 of 173, by Old Thrashbarg

User metadata
Rank Oldbie
Rank
Oldbie

I've considered the CR1220/1225. They might fit but they have pretty much nothing for capacity. ~30-50mAh instead of ~200 for the CR2032s.

Fair enough. But also remember that you don't need much capacity for this application... the battery current draw on the DS12885 is rated at 0.5μA. Even a 30mAh battery would be good for several years.

Availability I'm not so sure about, though. A lot of watches use the CR1220/1225, so the batteries themselves are pretty easy to get around here, but I don't know if that applies everywhere in the world. And I've never tried to buy a holder for one, so I don't know how easy it is to get those, nor how much it would cost.

Anyhow, it's not like I'm trying to push the idea. I just figured it was a thought worth mentioning, in case you hadn't considered the option.

Incidentally, it's got me wondering now... any idea what sort of battery they use in the DS12887? I know it's some sort of button cell, but I've never chipped one apart far enough to see exactly what it is, and I can't find it mentioned anywhere in the datasheets.

Reply 8 of 173, by Maeslin

User metadata
Rank Member
Rank
Member
Old Thrashbarg wrote:

Incidentally, it's got me wondering now... any idea what sort of battery they use in the DS12887? I know it's some sort of button cell, but I've never chipped one apart far enough to see exactly what it is, and I can't find it mentioned anywhere in the datasheets.

Point, the CR122x would last for a LONG while.

When I tore a M48T86 in half, the cell inside was either a CR12xx or a CR16xx. Going for either might just be enough after all and their holders are much lower profile than what I'd have to use for 2x LR44s. I'll look into it and possibly post a rendering in a few days.

Reply 9 of 173, by feipoa

User metadata
Rank l33t++
Rank
l33t++

If my opinion has any value, I personally think such a replacement RTC is a pretty decent idea. I like the clean look of this proposal; I have never really cared for the the look of the semi-popular RTC mod with the battery atop the RTC. The battery in a Dallas 12887 typically lasts 15-20 years. I don't particularly care what style battery you select for your mod provided that it can last at least half the time of the old Dallas 12887's.

Ultimate 486 Benchmark | Ultimate 686 Benchmark | Cyrix 5x86 Enhancements | 486 Overkill Graphics | Worlds Fastest 486

Reply 10 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++
Maeslin wrote:

It's only a vague cost estimate but $10-$12 plus shipping might be doable (sans batteries). Perhaps a tad more for the very first batch. I've already got most of the PCB designed and parts selected, just need to pick a battery format / matching holder and make everything as small as I can.

When ever you do costing, take the number you think it will cost and double it 😀

The big hurdle for most will be de-soldering the old one and installing the socket.

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 11 of 173, by Maeslin

User metadata
Rank Member
Rank
Member
Mau1wurf1977 wrote:

The big hurdle for most will be de-soldering the old one and installing the socket.

You probably wouldn't need a socket for the new one since the battery will be replaceable. 😜

It's a shame the pins aren't as prominent on the 12887 as they are on a DIP package. If it was the case it would have been possible to just snip the pins right at the package, drop the (pin-less) replacement between the stubs and solder them in place.

edit: Just realized, there's a slightly newer alternative to the 12885 that both includes a trickle charger (for rechargeable battery or supercap) and is about half the price of the original DS12885. Might end up going with that and recommending the use of a ML1220 (rechargeable 1220 variant) if there are no objections. The ML1220 would still be in a holder, not soldered or on tabs.

edit2: Here's an early render. Everything's there, the only thing missing are the pins. So far it's only ~0.180" high so there's plenty of room left. The regular DS12887 is between 0.300" and 0.330" high.

2uy.gif

Reply 12 of 173, by feipoa

User metadata
Rank l33t++
Rank
l33t++

Just the term "rechargeable" might remind too many people of those leaky 386-era barrel batteries. The term alone might be enough of a deterrent for sales, leaky or not. Are you sure you want to use a rechargeable battery?

Ultimate 486 Benchmark | Ultimate 686 Benchmark | Cyrix 5x86 Enhancements | 486 Overkill Graphics | Worlds Fastest 486

Reply 13 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++

Got a few Dallas replacements today. Can the battery voltage level be measured from the pins? Or is opening them up the only way.

Unfortunately I haven't got the sockets yet, so I will wait for these first.

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 14 of 173, by Maeslin

User metadata
Rank Member
Rank
Member
feipoa wrote:

Just the term "rechargeable" might remind too many people of those leaky 386-era barrel batteries. The term alone might be enough of a deterrent for sales, leaky or not. Are you sure you want to use a rechargeable battery?

Wouldn't be much of a choice if I go with the newer DS12R885. The inbuilt charging circuit would cause problems with primary cells. I'll see if there's enough room to fit a protection diode but that would slightly lessen the lifespan of the battery due to the voltage drop.

If it's any consolation, the rechargeable cell used in this project is a tiny lithium one instead of the messy NiCads that were used on 286s and 386s.

On the other hand, the design and circuit is identical for both the 12885 and 12R885. The only thing that changes is the trickle charge present on the battery terminals, so making either variant is just a matter of picking a chip and associated battery.

Reply 15 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++

CR2032 gets my vote. Every supermarket sells it 😀

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 16 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++

Got my de-soldering station today. I practices a little on a new board and didn't have much luck. But it worked really good on the 486.

Here is the nasty ODIN chip removed 😀

Yk7fsh4h.jpg

vHVYW7bh.jpg

0EmRBLhh.jpg

Original: http://i.imgur.com/rEMNXx1.jpg

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 17 of 173, by Maeslin

User metadata
Rank Member
Rank
Member

If you have calipers, could you measure the height of the chip you just removed? I might end up replacing the battery socket with a top-loaded model due to space constraints (jumpers, other chips & co around the DS12887s)

Reply 18 of 173, by Mau1wurf1977

User metadata
Rank l33t++
Rank
l33t++
Maeslin wrote:

If you have calipers, could you measure the height of the chip you just removed? I might end up replacing the battery socket with a top-loaded model due to space constraints (jumpers, other chips & co around the DS12887s)

Using a ruler I measured 9mm!

I think a top loaded option would be a good idea.

My website with reviews, demos, drivers, tutorials and more...
My YouTube channel

Reply 19 of 173, by feipoa

User metadata
Rank l33t++
Rank
l33t++

You can get all this information from the datasheet. See attachment.

I hope you find a way to keep the battery internal to the unit.

Attachments

  • Filename
    Dallas_Pins.png
    File size
    72.29 KiB
    Downloads
    No downloads
    File license
    Fair use/fair dealing exception

Ultimate 486 Benchmark | Ultimate 686 Benchmark | Cyrix 5x86 Enhancements | 486 Overkill Graphics | Worlds Fastest 486