Re: OT: Is this question too challenging for a BSEE graduate?

On 10/26/2010 12:05 PM John Fields spake thus:

No; what article? (I take it this is the real John Fields, right?)

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

In fact, _it most certainly does_. Without external connection - the feedback - it will fail to make the inputs equal. But it will do all it can do until those voltages are equal.

You seem to have missed the fact that my entire post was discussing feedback. Sigh.

Ed

On 10/26/2010 8:06 PM ehsjr spake thus:

You mean the little op amp will huff and puff and turn blue? Not sure what you're getting at here. Without feedback, the inputs will simply be whatever they are.

See the remainder of my reply that you posted below but didn't reply to.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

--
Yeah. :-)

Here's the message  ID:

68oac6du5ec8alsq27p63a3hehpdcdvn8q@4ax.com

If you can't get it let me know and I'll email you a copy.

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In my first look at it the 120 ohms eq. of the FET was obvious, but I neglected to include the parallel path of the bias in my calculation of the total resistance. In interview questions like this, I tend to talk/question my way through it to show the interviewer how much (or how little) I know. Questions I have encountered have ranged from drawing an end-to-end block diagram of an FFT signal analyzer, and whether an interrupt routine returns a value.

Block #1: Microcontroller with built-in ADC. Maybe show a low-pass filter in front of the input, or just mention it's assumed the input signal is band-limited or it's OK if aliasing occurs. Hook up a few LEDs and switches for the output and control... mention the microcontroller has a USB or serial port if better data output is rqeuired...

Do I get the job?

I suppose the point of that question is to see if you even know what an "interrupt routine" is. It's kind of like, "Can a wheeled vehicle propel itself through the ocean?"

---Joel

Do you understand that the maximum output voltage that op amps can produce differs between devices? Some go to the rail(s) (or very close) and others go toward the rail(s) but can end up well more than a volt away.

Assuming unequal input voltages: If there is no feedback, the op amp responds to the unequal input voltages and drives the output to whatever the maximum voltage is for that specific op amp, in whichever direction the voltages on the inputs dictate. It continues responding and driving forever, or until the voltages are made equal by some external means.

If there is feedback, the output will go to whatever voltage causes the input voltages to be equal, or will go to maximum and keep on responding and driving the output to maximum if the circuit feedback is insufficient to equalize the voltages.

In all cases, the op amp has done all that it can. In the first case what it can do is limited by its maximum output voltage. In the second case, where the feedback allows the voltages to become equal, the op amp has done all that it can, because the equal input voltages limit the Vout to the specific voltage that creates equal inputs. In the third case, the op amp is once again limited by whatever the maximum (or minimum) Vout it can produce.

"An op amp does all that it can" does not mean or imply that it somehow internally adjusts the input voltages. It can't do that. It does what it _can_. It does everything that the inputs allow and that the op amps own internal design allows.

I hope that clears it up for you. You're likely to encounter the phrase again and again. It would seem that the smart course is to try to understand it, rather than engage in what you termed a semantic quibble.

Ed

On 10/27/2010 2:45 AM John Fields spake thus:

Thunderbird knows not what to do with that address, just opens up a new "compose" window.

Thanks, but as you can see, my email address is (intentionally) munged. Post it to web-space and give us a link?

Anyhow, thanks for the effort.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

That depends on the size and type of wheel. After all, a 'Paddlewheel boat does it whith a wheel. ;-)

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

Like John said, it is a message ID, not a URL. It will open the related message on most newsreaders, if you have access to the group it was posted to.

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

SMOP.

Dunno about "ocean", but...

Joel Koltner:

I was about to lose this pearl because of the "quoting style" of this NG.

Thanks, Joel.

Exactly.

Hmm, well, yes (the actual propulsion is via propeller, though -- I don't think the regular wheels rotate much while it's in water)...

I suppose I wouldn't be surprised to find that someone once designed a processor that had a special "return value from interrupt routine" instruction that the main program could check either... back in the early days of CPU design it seemed there were all sorts of weird/bizarre features someone implemented because of some highly specific problem they figured the CPU might be used to solve...

---Joel

On 10/27/2010 4:52 PM Michael A. Terrell spake thus:

I understand that. I clicked on it only half-expecting it to do anything useful.

Well, I certainly have access, as I'm reading s.e.r. using Thunderbird. But I don't know how to search by message ID. Do you, using Tbird? I assume it has this capability, but the online help is, as usual, useless.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

Well, you said, "Can a wheeled vehicle propel itself through the ocean?"

It certainly is a wheeled vehicle and it does propel itself through water. ;-)

I don't know why there would be such a thing since the ISR can do whatever is needed. It's not like the program that was interrupted would know what to do anything with the "return value" at any random time. Nested interrupts would get messy, too.

...and don't forget that not all processors have a stack so the ISR has to do work just to get back.

I still use Netscape 4.78 to access newsgroups so I'm no help with Thunderbird. I tried it last year and didn't like it.

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

Joel Koltner:

Problems like "Given this achitecture, I'm not able to read ROM, so let's RETLW?".

PICs are old, but not for that contemptible. Transistors were expensive, only after optimizations and market share Patterson & Hennessy were able to RISC(K)... well, them, actually nothing. And PICs (as far as I know, which is less than even Wikipedia) did precede (or inspire) P&H.

But I digress. Is there anybody that can avoid to put a pound of iron on a failsafe VGA splitter? ;-)

Oh, I agree completely... I've just seen such odd things in (generally old) CPUs that I wouldn't put "return value from ISR" past some designers. E.g., perhaps some machine where a "feature" could be that executing this RVFI instruction would keep interrupts disabled until the main routine specifically read the special register that RVFI wrote to or somesuch... I can't say what particular problem that might cleanly solve, just that I've seen much stranger things done.

(As F. Bertolazzi mentioned, these days everyone -- including me -- got Patterson & Hennessy in college, so modern processors all seem to look somewhat more similar to one another than in ages past... P&H has various anecdotes at the end of each chapter that they largely use to evangelize RISC anyway; there's one example of how some old IBM intelligent hard drive -- I mean, DASD :-) -- had a "search" command whereby the drive's CPU would run off and try to find a string for you, but that over time the main computer's CPU became rather faster than the hard drive's and hence it was actually slower to let the hard drive search than the main CPU. A similar thing happened with the Amiga line of computers, back in the late '80s: The original machine had a

7.14MHz 68k CPU hooked up to some fancy graphics co-processors that included a blitter that was used for scrolling text on the display. As time progressed, you ended up with the Amiga 3000 with a 25MHz 68030 but still coupling to the old 7.14MHz blitter... and which point the 68030 could so the text scrolling markedly faster than the blitter could!)

The older PICs had all of a two-level stack... definitely made for somewhat challenging programming! Some newer ones (or was it some older AVRs?) switched to an 8-level stack, which I always found "comfortable" for assembly-language programming.

---Joel

On 10/27/2010 6:28 PM Michael A. Terrell spake thus:

Well, I just "subscribed" to the Mozilla Tbird newsgroup, news://news.mozilla.org:119/mozilla.support.thunderbird (only took about a goddamn half-hour to download all 107,000 headers); no help there. Apparently Thunderbird, for all its geeky greatness, doesn't have the capability of searching my message ID. They say there's an "extension" you can download that does this; but other people say the extension can't be installed.

So much for the vaunted superiority of open-source software projects ...

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)