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clh333
May 25th, 2017, 02:47 PM
A Biostar '486/33 VLB board that dates back to the 1980s developed a problem a few years ago that prevented it from booting: The OEM ODIN clock chip also contained BIOS configurations and was backed by a self-contained battery. The battery died and the machine would not complete its POST, complaining about low voltage in the CMOS battery.

Internet research led me to several proposed solutions, including the piggy-back battery modification of the chip. I was unsuccessful in all attempts, so I resolved to get a replacement clock chip. ODIN is no longer in business, but Dallas Semiconductor made a replacement that was pin-compatible. Dallas was purchased by Maxim, but I bought a replacement Dallas chip (pictured below) from JDR.

After unsoldering the ODIN chip I socketed the board to make replacements easier. My board appears to be a Rev. 5 or 6, and by the time this one was made the CMOS clear / set jumper pins had been eliminated. I added a 4-post header at the spot where J8 had been. However, the new chip did not fix the problem.

Finally after consulting with Maxim I found that JDR had sold me the wrong chip (a variant with no internal battery). I also learned that Maxim estimates their chip's battery life at 5 years, so judging from the date codes this would have been DOA if it had a battery. What I want, Maxim explained, is DS12887-A (CMOS backup and an internal battery) which exactly replaces the ODIN chip.

With the -A variant in place and a jumper between pins 2 and 3 on J8 the board now boots. The first boot complained that the BIOS settings needed to be updated; since then all is well.

-CH-

38803

Chuck(G)
May 25th, 2017, 05:11 PM
The no-internal-battery variant is what? Does it also include a crystal, just like the battery-equipped one does?

Or are you talking about a DS1285/DS12885 (just the bare DIP?)

clh333
May 26th, 2017, 03:44 AM
The no-internal-battery variant is what? Does it also include a crystal, just like the battery-equipped one does?

Or are you talking about a DS1285/DS12885 (just the bare DIP?)


The no-internal-battery variant is what? Does it also include a crystal, just like the battery-equipped one does?

Or are you talking about a DS1285/DS12885 (just the bare DIP?)

The DS12885 requires an external oscillator; the 12887 has one on-board and is factory adjusted. The 12885, 12887A and 12C887A (century counter) all have RAM Clear function, the 12887 apparently does not.
http://pdfserv.maximintegrated.com/en/ds/DS12885-DS12C887A.pdf

That may have been the critical difference. Unfortunately I can't find the datasheet I was referring to when I made this discovery - the one on DigiKey has been updated and Maxim has a new version of the chip out now. So I'm relying on memory when I assert that the 12887-A variant has an internal battery and the 12887 does not. (We all know what that's worth.) But it definitely is true that the 12887-A features a RAM clear input, which was incorporated in the BIOS (and therefore left off the motherboard) and the 12887 does not. Screen shot of data sheet below.

-CH-

38814

Edit: Found the email from Maxim TS (1/17/17) and the link to the datasheet provided: https://datasheets.maximintegrated.com/en/ds/DS12885-DS12C887A.pdf. DS12885 uses external crystal and external battery. Sheet says 12887s include lithium battery but makes the distinction about RAM clear between 12887 and 12887-A. Email from TS also notes:

"NOTE: The + symbol indicates lead free/RoHS compliant devices. Almost all parts over the past 9 years include the + symbol. Without the + the part will have lead on the leadframe. Parts with lead are generally special orders."

and adds:

"The DS12887 RTC has no freshness seal, so the battery life is 10 years from the date of manufacture marked on the package.

NOTE:
JDR is not and has not been an authorized distributor of Maxim components so we have no way of guaranteeing 1) their authenticity or 2) that they have been handled, stored, shipped, packed by the 3rd party trader or by any of their previous owners so they would not have the Maxim guarantee which is provided to all buyers with components purchased through authorized Maxim Sales channels."

From the email I concluded my purchase from JDR was more than 10 years old and probably had a dead battery as a result. I obtained the replacement from DigiKey. Hope this clarifies things a little...

-CH-

Chuck(G)
May 26th, 2017, 07:44 AM
So, essentially what I said. From the DS12885 datasheet:


The DS12885, DS12887, and DS12C887 real-time clocks (RTCs) are designed to be direct replacements for the DS1285 and DS1287. The devices provide a real-time clock/calendar, one time-of-day alarm, three maskable interrupts with a common interrupt output, a programmable square wave, and 114 bytes of battery-backed static RAM (113 bytes in the DS12C887 and DS12C887A). The DS12887 integrates a quartz crystal and lithium energy source into a 24-pin encapsulated DIP package.

Further on it states that pin 21 is omitted (clipped off) on the DS12887 encapsulated version, but is left on with the DS12887A. This is the "RAM Clear" pin and is designed to be connected to an external (manual) jumper. Since the pin has an internal pullup, you can convert a DS12887A to a DS12887 by simply clipping the pin off.

My point from donkey's years ago is that one could purchase the original DS1285 chips, add the 32.767 KHz crystal and use an external battery to make a unit that would be good virtually forever. Some third-party versions of the encapsulated chip used a removable "hat" to get to the battery for replacement. There's really no mystery to the 1287/12887; it's mostly a matter of convenience.

clh333
May 30th, 2017, 10:47 AM
So, essentially what I said.

Yes.


My point from donkey's years ago is that one could purchase the original DS1285 chips, add the 32.767 KHz crystal and use an external battery to make a unit that would be good virtually forever.

For you, yes. For me: It took me forever to figure out how to solve the problem; it would have taken me another forever to configure the crystal and battery properly. But I got a couple of Kaypros that will need similar fixes, so maybe next time I'll be smarter. My first mistake was looking for answers on the Internet...

-CH-

P.S. No Internet access since last Friday as the ISP (Spectrum) made a "clerical error" in their coding of our account and left us in the dark, so to speak.