Digital input to an IC - do I need a resistor?

If the switch is going to ground or the processor's Vcc, you don't need a series resistor, but you will need a pull-up or pull-down resistor to ensure that the input goes to the opposite state when the switch is open.

From long experience with TTL logic, I would connect the switch from the input pin to ground, and a 3 - 5K resistor from the input pin to

If im not mistaken, PICs are voltage controlled devices. Meaning that if you are inputting TTL voltages, you will not need a series resistor to limit current flow (assuming your logic voltages are being generated by other such devices). If your logic is somehow directly connected to your Vcc (ie, a switch, or a transistor switch) then you will need a series resistor because you will short your entire Vcc source to ground..and that quite frankly...is bad..

eP

PIC16F914:

No. You don't need a series-resistor to limit the current. As others noted: you may need a pull-up or pull-down resistor.

But if you realy want to connect a push button, you may need something else: a debounce circuit.

If you push the button near your frontdoor, your bell will (simply) ring. If you connect a push button to an electronic circuit it will 'see' a train of pulses, for upto 1 or 2 mSec at least. Normally, you have to cope with that, either in software or by a hardware circuit.

Thanks very much, that's really helpful.

How crucial is the resistor value, though? I ask because it seems my colleagues are planning to use a lot of 10K resistors elsewhere; would these be OK or is your 5K pretty much the limit?

Thanks,

Pete

You might also put an 0.01 uF capacitor across the switch to reduce the contact bounce problem.

Thanks. The order is going in very shortly!

Pete