Neil Davey wrote:
Hi Chris,
You could be running into a couple of problems... As Joerg pointed out,
breadboards/proto boards and high speed devices generally are an unhappy
mix if your not careful..
They do generally have higher capacitancies than a pcb board, so you may
have to lower, or even remove the 22pf caps on the crystal, as the extra
capacitance in the board could be loading the crystal to much... if you
Hmmm... let's try a quick calculation:
Take the common breadboard configuration of rows of 5 holes connected.
Let's calculate the capacitance between 2 parallel rows and assume that
figure at least indicates the scale of parasitic capacitances we're
dealing with.
Yanked a row of contacts out of a breadboard: it's essentially 2 flat,
parallel leaves (===== viewed from above), so between 2 rows we just
have a parallel plate cap. Separation ~ 0.5 mm. Length of base = 12
mm. Now, the leaves within a row start out vertical at the base, but
above 2 mm, deflect towards each other higher up, so here the separation
between rows is greater than 0.5 mm (up to about 2.0 mm apart). So, we
could take the height as 2 mm and assume that accounts for most of the
capacitance, but let's just double that to 4 mm just to be sure. (The
total height of the leaves is 7 mm.) So, area ~ 12 * 4 ~ 50 mm^2.
I'm not sure exactly what types of plastic breadboard blocks may be made
of, but a glance at a table of relative permittivities eps{r} shows a
few plastics in the range 2 .. 3, so take eps{r} ~ 3.
Thus, C = eps{r} eps{0} A / d ~ 3 * 9e-12 * 50e-6 / 0.5e-3 ~ 3 pF.
(Anyone have an LC meter?)
Assuming I did that right, that looks like enough to certainly affect
things a bit, but not necessarily fatally so...
(I have breadboarded a few micro designs, but of course ever since we've
had internal RC clocks we haven't had to bother with the crystal until
it was on a board. But I _must've_ tried this at least once or twice
(e.g. by also cutting the XTAL leads to about 10 mm), and don't remember
it _not_ working...)
-Terry Karlson
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