P
Paul
Gabriel said:
You could check with an oscilloscope, that a buffered clock signal
coming from the chip, is working. For example, there is an
MCLK signal on one side of the scaler. I'd connect my oscilloscope to
that and check for something running at hundreds of megahertz.
That proves chip is powered, PLLs are working, and there is a
clock signal on output. If the 14.318 was dead, MCLK would be too.
Probing right at the crystal terminals is not recommended,
since the slightest capacitance (a couple pF) can shift
the operating point, detune the thing, change the Q, etc.
You could just as easily stop it from oscillating as anything
else. I've never tried probing a crystal with a FET probe,
just to see what would happen. I wouldn't even try. It
wouldn't be my first point of attack. (If there was no
buffered copy of the clock signal available, then I'd have
to investigate the right kind of probe for the job. A FET
probe is the closest I know of.)
(Down to 0.5pF loading now. Impressive. One of these
would cost as much as a very old used car
)http://www.tek.com/datasheet/active-probe-4-ghz-active-probes
And as a belt and suspenders guy, I don't even like to
design things that way anyway. I like tin can oscillators
with four legs on them, for that "guaranteed to work"
design result. I've seen enough disasters related to
improperly specified two-lead crystals, to not want
any part in it. As long as the boss isn't all "those
are too expensive - get that outta there!", I'll continue
to use the four legged tin cans instead. As long as they
have a tri-state pin on them, for the guys in the factory
to run test, they're great.
I've seen processor designs, that used the two leg quartz
crystal, and the processor crashes every couple of days,
and people are scratching their heads. Turns out, the
oscillator is the weakest link, not the rest of the
hardware. That's where I learned about the value of the
four legged tin cans. No more "grrr... shit crashed again".
(Four legged friend)
http://www.smcelectronics.com/XTALOSC.JPG
One of my managers, took me aside one say, and tried
to explain how to select AT-cut crystals. Went through
the steps you'd follow and so on. I'd do that again
if I had to, but when you buy the ones with the four
legs, you're off the hook in terms of whether it
works properly or not. The ones with the four
legs are digital devices. Feed them clean power
and away you go.
Paul