DS12887 pcb substitute with battery

[Maciej W. Rozycki]

Hi Chuck,

 It's taken me a little while to respond to this one, however certain 
matters do need time to develop.

On Sun, 22 Jan 2017, Chuck Guzis wrote:

> On 01/22/2017 01:49 PM, Maciej W. Rozycki wrote:
> 
> > A problem with reworking is there is sometimes very little clearance
> >  available, so any modification has to be made in a clever way or you
> > risk a short-circuit.  For example in DECstation 5000 systems their
> > DS1287 chip is located in the TURBOchannel option slot area with the
> > top of the chip almost reaching the solder side of the PCB of any
> > option plugged in there.
> 
> Although there are several illustrations of the DS1287 rework where a
> coin-cell holder was placed atop the chip, there's absolutely no reason
> why that has to be so.  A couple of wire leads away to an off-board
> battery holder, holding, say, an ER14250 half-AA.will work just fine and
> keep the battery away from the PCB--or a couple of alkaline AA cells,
> which should last a few years.  No reason to stick to lithium.

 Yeah, putting the aesthetic of the solution aside, in the DECstation 
there's the practical issue of making sure the wires do not obstruct 
shuffling TURBOchannel option cards and also finding a place where the 
substitute battery does not disturb the airflow such as to cause a 
component somewhere to overheat -- the whole side of the enclosure is a 
grille.

> You know--"use your wetware".

 That always helps, as long as it's got enough input to process, in 
particular as to what resources are available.

 Mine has got that now, thanks to this thread of discussion, where I 
learnt of the existence of this small coin cell holder, and also the other 
thread mentioning wire-wrap wire and its excellent properties for small 
circuit repair/modification works.  Naturally I picked up the green 
insulation color to match what DEC used to use, at least in their 
TURBOchannel equipment. :)

 So here: <ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/ds1287/> 
are a couple photos of the result, meeting both the clearance requirements 
and my subjective aesthetic criteria, and avoiding the issues I identified 
above.

 Taking into account the rated DS1285's standby power consumption of 0.5µA 
and the 35mAh 2.5V discharge level capacity of the CR1220 cell it will 
guarantee keeping the chip powered for 8 years.  Which I think is enough 
given the ease to replace the cell now.

 For comparison the 48mAh capacity of the BR1225 cell originally embedded 
in the chip allowed for 11 years of operation and the CR2032 cell people 
seem to commonly choose allows for 50 years at its 220mAh capacity (which 
I find a bit of an overkill).

 Note that I have calculated approximate cell capacities based on 
discharge characteristic curves published by cell manufacturers, as the 
rated capacities published are calculated for the 2.0V discharge level 
whereas DS1285 is specified for 2.5V minimum battery voltage required for 
correct operation.

 For the record, I have measured the clearance available and it's 0.1875" 
(3/16") or 4.76mm from the top of the RTC chip to the bottom of the PCB of 
a TURBOchannel option right above it.  The TURBOchannel mechanical spec 
allows for 0.075" (3/40") or 1.91mm underneath component or lead height 
(e.g. 24-plane HX+ options have memory chips there).  Which leaves 0.1125" 
(9/80") or 2.86mm of space only.

 I may yet stick a piece of insulating tape on top of the RTC chip just to 
make sure no contact happens by chance, though I think it might be an 
overkill after all.  It's not like the cell is going to pop out by itself.

  Maciej