[SI-LIST] Re: Engineering Effort vs. Time
- From: "Lynne D. Green" <lgreen22@xxxxxxxxxxxxxx>
- To: <parnold@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Apr 2006 14:47:35 -0700
Hi, Peter,
A nice inclusive list.
Where would you put simulation in this list? It is better than a rule of
thumb, but not as trustworthy as a complete prototype (at least, one tested
correctly :-)
Best regards,
Lynne
"IBIS training when you need it, where you need it."
Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@xxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Peter Arnold
Sent: Tuesday, April 25, 2006 1:09 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Engineering Effort vs. Time
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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