Dale wrote: I'm just curious.  Am I the only one who has never used
capacitors with
the crystal, and never had one fail to start?  Or is this one of those "i=
t
will work for more repetitions than you can test for one-offs and
prototypes but don't trust it for production" things?

Answer: If you use a crystal it is just plain luck that makes it to work =
or
maybe the capacitance in the wire that makes things rock. When I have use=
d
Weroboards and wires hanging in the air I have seen this in my own design
but I would definitely not recommend it. Generally oscillator design is n=
ot
tested as much as I describe things and usually things work by just apply=
ing
some capacitor with approximately right value but then there are those ca=
ses
when you experience strange behaviour in your design. Even if my suggesti=
ons
are basic oscillator design testing there is a lot of qualified designers
out there that never test this. If you do not do similar test a design ca=
n
be failing when the customer using it, and you do not want that. If you h=
ave
a marginal design in production it can start failing if the temperature g=
oes
up or down or if the capacitor ages and get slightly different values.
It is also good to do a oscillator design well to enable transitions to
other PICmicro or newer revisions as the internal inverter amplification =
in
the PICmicro can change. The amplification of the internal inverter is no=
t
mention in the data book. Its not there because it's a parameter that is =
not
that important as its the whole oscillator design that matters (witch
oscillator used, manufacturer of crystal, capacitor and PCB layouts). Its=
 up
to the designer to develop a secure oscillator design that takes differen=
ces
in consideration. To test at max and min Vdd makes the design to be just
perfect. If its perfect it does not matter if the internal inverter
amplifies a bit more or a bit less or if the capacitor ages and change
values slightly. You are secure anyway.

Niklas Wennerstrand


-----Ursprungligt meddelande-----
Fr=E5n: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.ED=
U]
F=F6r Dale Botkin
Skickat:        den 11 juni 2001 21:26
Till:   PICLIST@MITVMA.MIT.EDU
=C4mne: Re: [PIC]: Downsides to these oscillators?

On Mon, 11 Jun 2001, Niklas Wennerstrand wrote:

> You cant toss away the caps, they are needed. You can experiment with
their
> values to get the oscilator design to fit your microcontroller.

I'm just curious.  Am I the only one who has never used capacitors with
the crystal, and never had one fail to start?  Or is this one of those "i=
t
will work for more repetitions than you can test for one-offs and
prototypes but don't trust it for production" things?

Dale
--
A train stops at a train station.  A bus stops at a bus station.
On my desk I have a workstation...

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