Hi Mike,

I don't know the short-circuit current of the battery he chose, but is it
really worth taking chances by not including a cheap resistor? It's just
good insurance. Limiting the battery's short-circuit current is always a
good idea.

And what happens if the real-time clock chip happens to fail in a way that
the lithium is in a position of being charged (a no-no)? A series resistor
is a good idea in this case as well. (I presume that the RTC has a blocking
diode internally to take care of this.)

Regards,

Ken Pergola

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Michael Rigby-Jones
Sent: Monday, July 14, 2003 3:03 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC] PIC as an RTC, is it recomended?


> -----Original Message-----
> From: Ken Pergola [SMTP:no_spam@LOCALNET.COM]
> Sent: Saturday, July 12, 2003 3:14 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC] PIC as an RTC, is it recomended?
>
> Hello Mauricio,
>
> You *may* find out it will cost more not to use a specialized RTC chip,
> but
> it is worth investigating alternatives (time is money too). You may have
> to
> spend more money on a heftier lithium to compensate for the larger current
> draw of a non-RTC solution (such as a PIC -- unless the nanoWatt devices
> fit
> the bill).
>
> Also if you have not already implemented the following on your PCB, please
> do...
>
> Two side note caveats regarding your lithium battery:
>
> 1) It is always a good idea to place a current-limiting resistor (~470
> ohm)
> in series with the positive lead on the lithium battery (as close to the
> lithium battery as possible). You don't anyone accidentally shorting out
> the
> battery during PCB troubleshooting/testing and having a lot of current
> dumping through a PCB trace. Not a pretty sight.
>
The OP is refering to a lithium coin cell, they have enough internal
resistance that the chances of drawing enough current to blow a PCB trace is
negligible.

Mike


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