Lpc2000

Tom:

Thanks for the code!

Is it *necessary* to have the switching in place (as opposed to use leaving 
the lithium to power the RTC all the time)?  Just curious.

Instead of using the batt->res->diode what if I used a FET instead to 
provide switching between Vcc and Vbat?  This would also extend the battery 
life (since it's also powering external SRAM as well).  From Digikey a FET 
is the same price as a single diode, so that would actually be cheaper (?)

-- Sean

At 22:24 1/30/2006, you wrote:
>Sean wrote:
>
> >Hello everyone,
> >
> >I've got a bit of a strange problem with the RTC in my setup.  I have a
> >dedicated lithium battery going to the Vbat pin to power the RTC, and it
> >usually works fine, however periodically the clock changes to an invalid
> >value and stops running.  Usually this value is something like "Year:129
> >Month:00 Day:16 Hour:02 Min:09 Sec:08", and unless I reset the RTC to a
> >valid value it stops running.  This will usually happen after several hours
> >of Vcc absent (i.e. device powered off) or after a day or two running
> >constantly.  Nothing else is disrupted.  I have a 3.6V lithium running
> >through a diode which drops the voltage at the pin down to 3.165V when Vcc
> >present and to 3.232V when Vcc absent (implying that more current is drawn
> >when the micro is powered on??)
>
>Diode switching is needed to maintain the Vbat when Vdd (3.3v main) is
>absent and to switch over to the battery when Vdd disappears.  It sounds
>like you have that working?  It will take two diodes (1N4148A) and a
>resistor (560ohm) to do this properly.  Cathodes of both diodes go to
>Vbat line, anode of one diode goes to Vdd, anode of other diode is
>series with 560ohm to positive terminal of Lithium cell.
>
>This is half the solution, the hardware half.  I found that my clock
>would also "explode" on occasion and seemingly at random.  The solution
>I took was a bit more aggressive in the software.  Study my clock
>routines, especially awakenClock() and sleepClock().  Essentially, when
>the clock registers are not needed, they are "disconnected".