-- That's true except for multiple-emitter TTL, in which case the resistor should be added if the input is pulled high. It's all spelled out here: http://focus.ti.com/lit/an/sdya009c/sdya009c.pdf There is, however, the curious note on the bottom of page 7...
^ input
-- John Fields
Sure John, but there are plenty of us here who are lucky to see 1000 units of what we design go into production, much less a million. I think a better approach is to spend the extra pennies on the first 1000 units to get the 'time to market' advantage as well as having effectively bought 'insurance' that the design will work, and then -- if time permits -- go back and start cost minimizing.
I also find there are usually much bigger fish to fry than saving the price of an extra resistor or capacitor here of there... things like someone using a very high end DSP to perform a function that a dedicated FPGA and a microcontroller could do for 1/4 the price, using a much faster processor than needed because they don't have a good algorithm for what they want to do (which can also quickly lead to, e.g., bigger batteries!), etc.
I've worked places where we'd spend something like $10,000 on chrome-plated stainless steel 'skins' for the machines we were building; it was really hard to get that excited about saving $10 on some $300 PCBs that went into it... :-) (On the other hand, that machine also had a $40,000 air-bearing stage, and we DID sit around spending time trying to cost reduce _it_!)
I disagree. Resistors not needed for a specific reason represent money wasted, (a penny wasted on each of a million units is $10,000 of lost revenue!) and any designer worth his salt needs to learn how to work without that expensive a net.
-- John Fields
Duh?
-- John Fields Professional Circuit Designer
That's the first good example I've heard mentioned for including a series resistor.
Be aware that *some* pins are called N/C by the manufacturer. Best to leave these alone.
Graham
Sometimes getting rid of "extra" resistors makes the board layout much simpler. That could easily help time to market much more than than resistors that you never use. Judgement. What they pay us for.
I also like test points on "unused" pins. Something big enough to get a scope on when you are chasing a bug.
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Not a bad idea if you are a digital geek. Much simpler to get good timings from quartz and logic than Rs and Cs and silicon that all have relatively sloppy tolerances.
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The one about outputs?
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Presumably they don't think a lot of unloaded outputs ringing away is a good idea.
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-- I think it's just an error, where: "Unused outputs of a device should not be left unconnected (open)." should read: "Unused outputs of a device should be left unconnected (open)." in order to agree with the figure.
-- If I've designed a piece of equipment for a client, and it works like it's supposed to, then I've done my job and everyone is happy. If, then, someone comes along behing me and starts trying to make changes without knowing what they're doing, it certainly doesn't reflect badly on me, it reflects badly on whoever was supervising the idiot programmer. --- >Consider the costs then??
The other thing I like to do with programmable devices is take an unused output or two and tie them to LEDs. The first "program" I do is a counter to divide the clock enough to blink the LED. I leave this in the "code" until it's fully debugged (perhaps a compiler/assembler switch to enable it). I've wasted significant time chasing a fault that prevents the device from programming.
-- Keith
-- Well, there are liars and cheats everywhere, and if the damage was great enough to require me to defend my position it would get sorted out quickly enough.
I read in sci.electronics.design that John Fields wrote (in ) about 'Question About IC Chips', on Wed, 30 Mar 2005:
I find I could occasionally do with a reminder that says: 'Used outputs of a device should not be left unconnected.' All the voltages are correct, but there's no trace on the scope!
[Newsgroups reduced to three.]-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
-- That makes absolutely _no_ sense. If I was wrong about something, how would I be covering my ass by broadcasting it to an ever-increasing audience?
I can recount dozens of similar stories. My favourite is this one:
A little drive (
I read in alt.binaries.schematics.electronic that John Fields wrote (in ) about 'Question About IC Chips', on Wed, 30 Mar 2005:
But you've moved on, whereas he has a clear field to explain to the CEO that 'The designer didn't take standard precautions against unforeseen, but reasonably foreseeable, events. This was a disaster waiting to happen.'
This thread is spread across six newsgroups. Is that really necessary?
-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Sure it is, because Fields is desperate to cover his idiot arse!
*Good* designers think about what might go wrong, not just what the circuit will do if everything is perfect. It's analogous to software error-trapping.As you rightly say, 'standard precautions against reasonably foreseeable events'.
Stewart Pinkerton - professional hardware design engineer
No, I wouldn't really use a pretentious sig like that, because I actually am one.......................... :-)
-- Stewart Pinkerton | Music is Art - Audio is Engineering
Why not just connect the unused inputs to some output, so the input is terminated, and you don't need the resistor?
-Bill
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