[SI-LIST] Re: timing analysis
- From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, "Hal Murray" <hmurray@xxxxxxxxxxxxxxx>
- Date: Sat, 28 Mar 2009 12:53:06 -0000
Hal Murray wrote:
"or young kids who don't understand the big picture?"
- I know lots of old engineers who dont understand the big picture, and they
dont even try to understand it. On the other hand, they are treated as great
professionals by the managements at several companies (just because they are
old. they call it "experience"), they are senior/lead engineers. I saw this
at several places, I think it is quiet usual in the industry,
unfortunatelly. They make ugly mistakes very often, what they could have
avoided just by having proper knowledge and understanding (timing analysis,
signal integrity, pi, return currents, logic delays, stackups,
impedance...). they never read a tech book or a publication, then they spend
a half year or one year to try to fix those mistakes by trial... At least i
am trying to understand everything related to my work, and its going well,
actually much better than for these old guys.
you saw old engineers who actually had understanding, ok but i think it is
not representative, there are old/young engineers with/without proper
understanding on the "big picture", maybe not evenly distributed, but even
then...
regards,
a "young kid"
----- Original Message -----
From: "Hal Murray" <hmurray@xxxxxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Cc: "Hal Murray" <hmurray@xxxxxxxxxxxxxxx>
Sent: Wednesday, March 25, 2009 11:46 PM
Subject: [SI-LIST] Re: timing analysis
>> As I mentioned, there is a camp in our company that says the
>> methodology I described above is too pessimistic. As evidence, they
>> point to the fact that all our boards work at 100 MHz over
>> temperature, even though worst-case timing analysis says we have
>> negative margin above 85 MHz. They are asking me to adjust my
>> methodology in order to show positive "worst-case" margin at 100 MHz.
>
> Are those arguments being made by seasoned engineers or young kids who
> don't
> understand the big picture? Or maybe managers who don't like your
> answers?
> Have they gone over the timing budget and pointed at the numbers they want
> you to shave and provided some justification? Have they commented on the
> risks?
>
> I think there are two issues here. One is engineering. The other is
> management.
>
> Management has to make trade offs between cost, time to market, and
> quality.
> A major aspect of quality is reliability. The importance of reliability
> and
> cost differ between consumer gear and medical/defense equipment, PCs and
> servers. Time-to-market may be critical for a startup. ...
>
> Engineering has to provide the information to make those decisions and
> tune
> the design in response to feedback.
>
> There is another dimension to this discussion. It's the interface between
> engineering and management. One issue is risks. Another issue is
> confidence/trust.
>
> If engineering says X for reliability, what are the chances they are
> wrong?
> One argument for having a conservative timing analysis is that it is lower
> risk. That simplifies the interface between engineering and management.
> Engineering doesn't have to spend time explaining and justifying their
> methodology to management and management doesn't have to spend time
> wondering
> if engineering's designs will run reliably. Even a solid design still has
> risks, it's just that other types of risks become much more important than
> timing errors. (Consider vendors of key parts going out of business, or
> lead
> times on caps going through the roof, or software bugs.)
>
> A design that runs at 85 MHz on paper but 100 MHz in the lab doesn't seem
> very surprising to me. The above said "over temperature", but didn't say
> voltage. It's also using typical silicon and typical PCB and ...
>
> As somebody else mentioned, is 85 MHz fast enough to get the job done? If
> so, run at 85 MHZ and go work on more important problems. Or ship it
> sooner.
> Again, part of the goal is to reduce the interaction between engineering
> and
> management.
>
> It might be interesting to investigate where the differences come from.
> Are
> there any numbers you think you can shave? Why? What are the risks?
>
> For example, do you really think you can save a ns off the tester guard
> bands? Let's
> assume the vendor isn't stupid. Why do you know more about their setup
> than
> they do? If there was a ns to save, wouldn't they have saved it already?
> For high volumes, you might get the vendor to sell you specially binned
> parts. For low/modest volumes, you might be able to test them yourself,
> but
> I think it would be a lot of work to setup the testing procedure. Do you
> need that hassle? Is it worth it? It might be if you really need to run
> at
> 100 MHz.
>
> Another example, I might be willing to cut corners on temperature for hand
> held consumer gear. But what happens if it gets left in the sun?
>
>
> --
> These are my opinions, not necessarily my employer's. I hate spam.
>
>
>
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