Question About IC Chips

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

^ input

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.

Oh, go on, intend away! No-one was ever punished for a really good pun.

You know what is paved with good intentions? (;-)

Duh?

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

The one about outputs?

Presumably they don't think a lot of unloaded outputs ringing away is a good idea.

I read in alt.binaries.schematics.electronic that John Fields wrote (in ) about 'Question About IC Chips', on Tue, 29 Mar 2005:

Keep it up, guys. Where do atheists worship?

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!

I can recount dozens of similar stories. My favourite is this one:

A little drive (

Why not just connect the unused inputs to some output, so the input is terminated, and you don't need the resistor?

-Bill

Why not just connect the unused inputs to some output, so the input is terminated, and you don't need the resistor?

-Bill

I sure do. BUT, I didn't post a "line of text". I posted a hyperlink to a site giving pointers as to the do's and dont's of connecting unused pins on IC's. If your newsreader doesn't show a simple and fairly short hyperlink correctly, that's not my fault.