Should the used pins be left unconnected or should they be taken to ground?
- posted
19 years ago
Should the used pins be left unconnected or should they be taken to ground?
Sorry for my typing skills. I ment to say:
What do you do with unused pins?
ground?
Unused inputs should be tied to either VCC or ground. If they are left open, it is possible the chip will start "oscillating", if the input is moving between on and off. This will probably not affect the chip, but may cause odd problems elsewhere in the circuit.
At one time, it was the military practise to attach unused inputs through a 1K resister to VCC. Doing this is longer thought necessary, just attach it directly.
Aidan Grey
Right, and everything will be fine, until a new program is uploaded that zeros the tris bit for port a, when he/she meant port b... let's just say leaving it floating is a FAR better result...
The resistor is still necessary IMHO, if only to save a person from their own mistakes.
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
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_!)
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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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
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
The only place I've seen where unused outputs should be properly terminated is when using a differential output in single-ended mode. Proper termination is required to reduce ground bounce. Fairly rare circumstance...
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