Question About IC Chips

ground?

It depends entireley on the device in use.

Chris

I suggest leaving _used_ pins alone...;)

I read in alt.binaries.schematics.electronic that snipped-for-privacy@columbus.rr.com wrote (in ) about 'Question About IC Chips', on Mon, 28 Mar 2005:

You mean 'unused pins'. It depends on the device; you need to read the data sheet. Some devices require a resistor to ground or to supply. Some MUST NOT be connected to anything at all.

Newsgroups reduced to FOUR.

I read in alt.binaries.schematics.electronic that Vidar Løkken wrote (in ) about 'Question About IC Chips', on Mon, 28 Mar 2005:

Once they have been used, they should be cut off and discarded.

Newsgroups reduced to four.

Sell them on Ebay?

Try reading the data sheet? On some I/C's spare pins are just floating objects... on others they may be used as test points during manufacture and are designated NC (no connection).

...Jim Thompson

Generally, unused output pins should be left unconnected.

What you do with unused input pins will depend on the chip - for CMOS logic parts, all input pins must be connected to something, and unused input pins must be connected in a way that will allow the chip to work as required. For example, and active low "reset" pin must be connected to the positive supply, otherwise the chip will be permanently held in its reset state.

That depends on the chip used. Generally, "inputs" can be grounded while outputs are left unconnected. Check the chip's datasheet for current consumption with inputs at Vss and Vcc, it might consume less power if the pins are pulled up to Vcc instead.

All the jokes aside, an unused input to a logic circuit should never be left floating. You never know what state it will take on powerup. Worse, you never know when it's going to change.

Tie each unused input to whatever value won't change the output. For instance, if you use a two-input NAND gate as an inverter, you can tie one input high or to the other input. Both will work, although paralleling inputs increases the fan-in, which may be significant. With an OR or NOR gate, unused inputs need to be held low. Some logic families may require a resistor instead of a direct connection to either supply rail; check your data sheets.

Unused outputs can float.

And everyone who replied meant to say: "It's entirely dependent on the chip used, and the circuit it's used in." In general, unused outputs can be left alone, unused inputs should probably be tied high or low, sometimes through a resistor, but you can't tell without a detailed schematic and a datasheet and/or application note for the specific IC used. I can probably think of a dozen examples of where my "general" rules above are wrong, for instance...

Sorry for my typing skills. I ment to say:

What do you do with unused pins?

ground?

Along those same lines, what about unused microcontroller ports? Say a PIC

If they're truly unused, it shouldn't matter. If they're connected internally, then logic should prevail.

That would certainly seem like the logical thing to do. No pun intended.

The recommendation for logic gates is to connect unused AND gate inputs to the positive supply through a 1K resistor.

OR gate inputs are to be connected to ground.

Why this is recommended is obvious from the logic function the gates perform.

If the unused inputs are not logic gate inputs it gets more complicated, as Jim and others have mentioned.

Robert

It was only "necessary" for TTL, where the base-emitter junction following a strapped-high input could be stressed if forced into breakdown by excessive VCC. This is not a concern with CMOS, where, by the time an input gate is in jeapardy, so are many of the internal gates.

Another reason I've seen for strapping thru a resistor is so, during board test development, canned routines can be applied to the part, testing used and unused gates alike. It takes a tiny extra effort to not test an unused gate or ignore one that fails because its input cannot be wiggled enough. (That practise always stuck in my craw, since unit cost was among my responsiblities, but I heard it from enough different test folks that I guess it could be part of their lore of "good" practise.)

I know digital designers who do this as standard practice... it allows easily forcing a test signal when trying to track a fault.

...Jim Thompson

Some of the microchip datasheets say you should connect the MCLR input to VCC through a 10k resistor. I'm not sure, but I believe they are are afraid of SCR latchup, which can occur with CMOS inputs if there is more than some small amount of current into the pin above Vcc. Perhaps they have seen this happen during startup with a big cap near the power pin, or with glitches on the power rail.

I bet JT will know if this is really possible, or just an application engineer's fantasy. I think the newer pics don't have this in their datasheet anymore.

these

One very good designer I knew (he's now deceased) not only adamantly required resistors on unused inputs, but edicted, "No system shall contain a one-shot." ;-)

...Jim Thompson