[SI-LIST] Engineering Effort vs. Time
- From: "Peter Arnold" <parnold@xxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 25 Apr 2006 13:08:54 -0700
Dear Community,
Entrenched layout practices often come up for discussion on this list
and sometimes these are exposed as lacking an engineering foundation. It
is interesting to step back and see why there such practices abound. I
think it boils down to efficiency.
In a perfect world we would have as much time as we liked to nail down
everything we do on boards with justification from verifiable research.
In the real cost-driven world we cannot do that, and we have to put our
trust in various streams of knowledge. These include, in approximate
order of reliability:
1) Solid research performed in your own lab
2) Advice from colleagues and component vendors
3) Rules of thumb
4) Advice from experts-at-large, email lists etc.
5) Tradition (old wives' tales, folk wisdom, the 20H rule etc.)
6) Superstition
An engineer faced with a real-world project in real time must select
amongst these sources, trading off time vs. potential inaccuracy, and
will inevitably find himself doing things that cannot immediately be
justified. This is not necessarily bad engineering practice. For
example, traditions or advice from others could be good things to bet on
- they are most likely based on some good work someone did once.
Every engineer has to develop some intuition or meta-rules-of-thumb to
determine which level of advice is good enough to apply to a given
decision. Trouble is, things change over time (and are a function of
frequency :)) What was once safely treated by rule-of-thumb may now
require simulation. The hard part is knowing when to discard those
successful habits of a lifetime.
The points I am slowly getting around to proposing are these:
* In real time, it's hard to justify every design decision from scratch
* Rules of thumb are highly time- and cost-effective if intelligently
selected
* It is essential to periodically audit your rule of thumb collection
to determine if they are still good (or were ever good!)
Thanks to the members of SI-List for publicly thrashing these things out
so I can keep my thumbs up-to-date!
Regards,
peter arnold
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