Lpc2000

Use a BAT54C. Inside this component you have the 2 diodes (schotky ones 
that have only 0,3V dropout) with their cathodes connected.




Sean wrote:
> 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".
>
>
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