> -----Original Message-----
> From: Pic Dude [SMTP:picdude@PILOTTOOLS.COM]
> Sent: Wednesday, July 10, 2002 7:53 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC]: Small battery backup?
>
> Damn dude!  Awesome answer.  Thanks!
>
> I had asked since I run my 16F872's at 4Mhz and found
> that the F's were $2.83 (actually 20Mhz versions), and
> the LF's were $2.98.  Each in 25-piece qty's from Digi-
> key.  $0.16 seemed like a no brainer case for the LF's,
> but now it all makes sense.
>
> Cheers,
> -Neil.
>
Be aware of one of he most annoying parts of using PIC's at low voltages.
The brown out detector in the 16F series has a fixed threshold of around 4
volts, which means you have to disable it to use the PIC at lower voltages.
That's not too bad a problem, you can buy cheap 3 pin supervisor chips in
virtualy any reasonable voltage range.  However, if you want to use high
voltage programming at the same time (e.g. yoiu are fresh out of port pins
and RB4 is too valuable to give up), then you have to arange something to
stop the supervisor being fed with 13 volts.  It all works out to be a
little messy because AFAICT there are no 3 pin open collector supervisor
IC's that can tolerate 13volts.  Even MChips own devices won't.  I guess
most people wouldn't think twice about adding a current limiting resistor
and a schotky diode to clamp the voltage, but when you are really pushed for
board space even a single 0402 component can be a problem.

The 18 series have a programmable brownout voltage, hopefully I'll be
migrating to them soon!

Regards

Mike

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