Help with older computer Real Time Clock

[Salvatore]

I pulled a pluggable RTC module due to a dead battery (internal) hoping to
be able to replace same.

I was able to remove the cover but the package had the soldered on
crystal, caps and battery completely encapsulated in dense and abundant
black material.

I managed to separate most of it and can replace the battery with a more
current cell. The crystal is still embedded in potting material and
cannot be removed to view without destroying the IC.

In the process of de encapsulating, one of the oscillator components
crumbled. It is a ceramic disk (typical yellow) and there is no marking
remaining. I need to know the value of this cap to resolve the issue.

The IC is a VIA VT82885N 24 pin DIP package

the crystal connects to 2 Xtal input pins.

each pin has a capacitor wired to it and they form a junction which
connects to Batt minus. I am fairly sure of this wiring.

One cap is a 10 (line under) and the other is virtually identical in size,
but destroyed. I am not sure about the 10 value being mmf or nf, or ??
Is it common to form this arrangement using identical values.
should either be neg temp coeff caps??

The caps are about 3/16" dia and thin.

SO: Is the circuit as I see it correct?? What should the cap values be??

Thanks for any suggestions (other than tossing the MoBo)

 

[ph12]

The IC is a VIA VT82885N 24 pin DIP package

These people may be able to help

http://www.epower2go.biz/

Peter

 

[Salvatore]

These people may be able to help

http://www.epower2go.biz/

Peter

Thanks

I do not need a battery

I need to know what the capacitor values are and if one might be a
negative coefficient one.

 

[Franc Zabkar]

I pulled a pluggable RTC module due to a dead battery (internal) hoping to
be able to replace same.

I was able to remove the cover but the package had the soldered on
crystal, caps and battery completely encapsulated in dense and abundant
black material.

I managed to separate most of it and can replace the battery with a more
current cell. The crystal is still embedded in potting material and
cannot be removed to view without destroying the IC.

In the process of de encapsulating, one of the oscillator components
crumbled. It is a ceramic disk (typical yellow) and there is no marking
remaining. I need to know the value of this cap to resolve the issue.

The IC is a VIA VT82885N 24 pin DIP package

the crystal connects to 2 Xtal input pins.

each pin has a capacitor wired to it and they form a junction which
connects to Batt minus. I am fairly sure of this wiring.

One cap is a 10 (line under) and the other is virtually identical in size,
but destroyed. I am not sure about the 10 value being mmf or nf, or ??
Is it common to form this arrangement using identical values.
should either be neg temp coeff caps??

According to the following app note, the caps should be identical:
http://pdfserv.maxim-ic.com/arpdf/AppNotes/app58.pdf

Using the formula on page 3, the capacitances should be of the order
of 10pF each.

Here is a datasheet for a similar (?), but discrete, RTC:
http://pdfserv.maxim-ic.com/en/ds/DS12885-DS12885T.pdf

The caps are about 3/16" dia and thin.

SO: Is the circuit as I see it correct?? What should the cap values be??

Thanks for any suggestions (other than tossing the MoBo)

- Franc Zabkar

 

[Salvatore]

According to the following app note, the caps should be identical:
http://pdfserv.maxim-ic.com/arpdf/AppNotes/app58.pdf

Using the formula on page 3, the capacitances should be of the order
of 10pF each.

Here is a datasheet for a similar (?), but discrete, RTC:
http://pdfserv.maxim-ic.com/en/ds/DS12885-DS12885T.pdf



- Franc Zabkar

Thanks Frank.

The VIA VT82885N Datasheet included very little, and did not even identify
the ground pin on the IC where the ext caps and batt - connect.