Inputs shorted to ground - current drain

[pmolsen]

If I short unused inputs to ground on an 18x chip, will the circuit draw more current than if I tie them to ground with say a 10k resister?

 

[jglenn]

No. It is just establishing a logic level, I think. Never leave open cmos inputs.

They can be triggered by stray EMF fields, and maybe ufo's. The resistor does not matter in this case.

 

[moxhamj]

The resistor is suggested in some circuits in case you wrote some code that turned an input input an output (particularly on the 08/08M where many pins can be inputs or outputs). But you are not going to write such code, right?
So just tie it to ground and save on the resistor.

 

[212]

I was just about to ask something similar, so I hope you don't mind pmolsen. Can I just ground the serial in pin???

 

[lbenson]

>Can I just ground the serial in pin???

Yes. Some recommend using a resistor, but I don't recall any reports of problems with just linking to 0V, and have certainly done that myself.

 

[212]

Thanks!...and I won't tell anyone else

 

[hippy]

I've linked direct to 0V and haven't had any problems but the potential for problems is there. It's not just badly written code which could make an input into an output but an adverse external effect, from lightning strike nearby to EMI from other machinary.

If a resistor is used you are protected from the inputs becoming ouputs, shorted to 0V then it is possible those pins and thus the chip can be damaged. This should not happen and rarely does, but can.

A convenient, and small footprint solution is to use SIL resistor packs for pull-downs alongside the DIL socket, cut the resistor pins off for PICAXE pins which do not need to be pulled sown.

Another potential disadvantage to tying Serial In to 0V is that it can never be programmed in-circuit. With a pull-down resistor it can be, though it is more complicated than attaching the 22K/10K resistor serial interface.

 

[Dippy]

Good point hippy re Serin and IC Programming.
Love the irony too.