On 22/05/2013 05:03, Matt Bennett wrote:
> On Sun, May 19, 2013 3:42 am, David wrote:
>> I have written a simple article which might be of use to some people,
>> measuring the PIC Vdd using no external components.
>>
>> This builds on an older Microchip application note designed for outdated
>> parts, so it isn't a new idea. However it works well enough for simple
>> battery monitoring and can be added to any project on a supported PIC.
>>
>> You can find it here: http://edeca.net/wp/electronics/measurin
>> g-pic-vdd-with-no-external-components-using-the-fvr/
>=20
> Unfortunately, there is a very good reason why you don't see this very
> often- the FVR isn't very 'F' - it can vary by as much as 5% on the PIC18
> you mention, and by -8 to +6% on the 12LF1840- and these aren't errata-
> these are the published and tested specs- they aren't very good
> references- you would need to take into account the amount the reference
> can vary over temperature and voltage.  I don't believe that the chart yo=
u
> posted takes the variance into account
>=20
> It is great to get a rough estimate, ... you may be able to calibrate eac=
h
> part- if you want any kind of precision, you'll need to use an external
> FVR.

Matt,

Thanks for the comments, these are really helpful.  I must admit that I
ran out of time due to real life and almost deleted the accuracy section
entirely.

I have updated my post to include a comment about this and a reference
to VR01 in the 18F datasheets.  I would never use this for anything
other than simple battery trip point detection or rough capacity, but it
is interesting to note.

David
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.