[time-nuts] Simulation

J. Forster jfor at quik.com
Sat Aug 14 16:01:55 UTC 2010

I think you missinterpret what I meant. Two examples:

I've seen programmers who use "instructions" that are not part of a uP
instruction set and are undocumented, just to be "clever". If a different
brand of chip, or even a different rev., the chip does something
completely different. These guys should be strung up by their tender

I've also seen transistors used as avalanche switches (basically a failure
mode). If a different production run has improved normal mode performance,
the avalanche function may vanish.




> J. Forster wrote:
>> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters
>> or
>> instructions in a production design is a fool.
>> If you are that silly, you must fully specify the selection criteria.
>> -John
> Or, has their back against the wall and can't do it any other way.
> How is this any different than using trimpots or hand select?
> For years, folks have hand selected matched pairs of devices, since the
> circuit requires tighter tolerances than the mfr guarantees.
> Many, many RF designs have "select at test" pads to set levels or tuning
> stubs depending  on what the actual gain or impedance properties of the
> active devices are, or for trimming temperature dependencies.
> Would you say that the engineer is a fool for not just specifying
> tighter tolerances.. the tighter tolerances may not be available from
> the mfr (who has to respond to many customers, most of which will be
> happy with the standard performance).  It's sort of a tradeoff.. do you
> go to the mfr and say, I need a better grade of part, or do you buy the
> run-of-the-mill part, and sort them.
> You might decide to do the latter for competitive reasons, e.g. rather
> than the mfr producing a better grade of part, and potentially selling
> it to your competitors too, you keep the "secret sauce" in house.
> (Granted you could have the mfr make/select a proprietary part for you..
> that's basically changing who does the work, but doesn't change the
> underlying design)
> Even manufacturers do this, for instance with speed grades on things
> like microprocessors.  They don't have enough process control to
> guarantee a particular speed, so they make em all, and then sort them.
> The other thing is that the selection criteria might not be knowable in
> a standalone sense.  That is, you have to put the part into the circuit
> and see if it works, rather than measuring some device parameter.  I
> would agree that to a certain extent, this implies that you don't really
> know how the circuit works, but it might also be that the most cost
> effective approach is to use empiricism, rather than analysis.

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