piclist 2002\10\24\084639a >
Thread: Winding inductor for 2 trans. LED blinker
face BY : Russell McMahon email (remove spam text)

2 related messages mixed together here:

>>   "I have no formal test equipment to test this "

> Dual-channel/trace-scope and a sine wave gen good to ?? 5 or 10 MHz?

I assumed you were talking about specific equipment. As I noted, > "I could
devise something if it it seemed liable to help". Scopes, sig gen etc
readily available. The question is the chance that this would produce useful
results. (I have since done just this as every straw is worth clutching at,
but, if anything, the amplifier in the "bad" processors is slightly better
than that in a good one.)

>>  "Microprocessor oscillators are meant to oscillate"

> Spare us the verbage and perform the test on the osc
> stage as "there is no substitute for 'the facts'".

If you think that's verbiage (aka verbage) then you just may have missed my
point, or perhaps not read through everything I wrote?
The point was that one hopes not to have to redesign one's microprocessor
after one buys it, or have it change characteristics in a major and
arbitrary manner without notification of any sort (as happened here), or
find that it does not respond to ANY of the reasonable & standard efforts
one makes to make it start oscillating properly in a circuit which falls
within the spec sheet spec. Having to place an inductor across the crystal
is not an expedient one might reasonably expect to have to take. As I noted,
there is strong evidence that the ultimate problem is some form of latchup
rather than just a badly behaved oscillator in the normal sense.

> (This is not debate class - this is "practical engineering
> 101"!)

Practical engineering 101 I can handle. Alas this is more akin to arcane
engineering 490 (with extra credit for not meeting spec sheet criteria and
requiring non standard work arounds.)

> This may or may *not* lead to a solution - but your alternative
> is 'a few well-placed shots in the dark' (you look to have
>found a work-aound in the mean-time, this, of course, is good).

Practical engineering 101 (also 220 & 370) is about gaining experience
required to be able to have one's "well placed shots" land somewhere near
the target (as I know you know). Certainly "having adequate test equipment
360" and "stopping and thinking what may actually be wrong 420" (post grad)
have their place.

> *Real* problem solution calls for UNDERSTANDING each component
> in the system - and the osc element is an important part of
> this ... the next stage to all this GIVEN component values
> and parameters would be to slap together a simple model in
> SPICE and see how that flies. This, of course, would not take
> into consideration ANY 'funnies' the 'front end' (THOSE
> who fabricated those devices on a wafer prior to their being
> cut up and mounted as 'die' in a package) fabrication process
> may have 'instilled' (been there - seen that) ...

Agree with all that more or less BUT it is my considered opinion that the
problem is the worst case one you mention - somebody has done something
arcane and isn't fessing up. MOST customers will not be affected MOST of the
time so they will probably get away with it. It is very very clear that
something changed radically along with a date code. When the manufacturer
denies that anything changed at all rather than explaining what has changed
and why so that you can try and work with it, then the situation is

I have the easy long term solution though! It's almost certain that I will
never design this manufacturer's products into anything again. We'll wait
and see how it works out - they may yet redeem themselves somewhat.

       Russell McMahon

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