"Olin Lathrop" wrote
>
> You probably don't need a crystal that allows hitting the baud rate 
> exactly.
> If you need the cycles, just use a 10MHz crystal with the 4x PLL.  You'll 
> be
> able to hit 115.2Kbaud close enough.  I've done this many times.  Also 
> note
> that you'd be better off with the 7.3728MHz crystal and 4x PLL than the
> 14.7456MHz crystal if cycles are important.
>

I knew that for lower baud rates, since I get 9600 with 4MHz very easily, 
but I assumed that for higher speeds, I needed a very small error or no 
error at all. Well, according to my estimates, 7.3MHz is enough to do the 
kind of processing I'm gonna do, but I wanted to step up just to be sure. 
Besides the additional power consumption, what other disadvantages there are 
in using the 14MHz crystal? (cost is irrelevant as this will never make 
production level quantities)

<snip>
>
> So I usually stick to the short answer and put 22pF on both OSCO and OSCI
> and be done with it.  I'm sure your crystal rated for 18pF load will work
> fine with 22pF to ground on each side.  Those two capacitors alone account
> for 11 of the 18 required pF.  It seems about right that the non-zero
> impedence of the OSCO driver and the various pin and stray capacitances 
> can
> make up the other 7pF.  Unfortunately "seems about right" is as close as
> you're going to get.  Fortunately, the system is fairly forgiving of 
> errors
> on this value.
>

According to the datasheets and app notes I've been reading, there is room 
to be really anal about the exact and precise value for the caps. One of the 
app notes even suggests that I have several blank boards populated only with 
the mcu, xtal circuit and power source to test it under several vdd/temp/cap 
situations, but I prefer the short answer. Someone suggested 5pF for stray 
capacitance. Assuming it is pretty close, then my caps should be 26pF, but 
based on what I read so far, given it is within a certain range the precise 
value is not critical.


>> 2-What's the recommended type of capacitor in this case (ceramic,
>> tantalun, electrolytic, etc).
>
> At 22pF you're not going to find tantalum or electrolytic.  Ceramic has
> better characteristics anyway.

Wuppps... my newbie birthmarks showing again

>
>> Is SMD fine?
>
> It's actually better since it has less stray inductance, assuming the
> circuit is laid out well.

Given that I am designing the circuit, that's a big assumption 8^D

Cheers

Padu 

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