No resistors on inputs?

[Dan East]

This should be a fundamental question for you PIC experts.

I would like to use PIC inputs with a switch without requiring a resistor. If I make the inputs high when the switch is pressed then I need an external pull down resistor to keep the inputs from floating. So I thought I could instead pull the inputs low when the switch is pressed and use the internal pull up. However I'm afraid I've already damaged a couple PICs in this way.

Can an input be taken low safely without a resistor? It seems that could be problematic, because when the PIC first starts up if the output is high then it will exceed 25 mA.

Thanks!

Dan East

 

[Nigel Goodwin]

Can an input be taken low safely without a resistor? It seems that could be problematic, because when the PIC first starts up if the output is high then it will exceed 25 mA.

Yes, you can safely take a PIC pin low without a resistor, obviously you need to use PortB with the internal pullups enabled (or there's nothing to pull it high when the switch isn't pressed). You can't short the pin out unless your program sets the pin to be an output, and sets it high - the powerup default settings are all pins as inputs, and in any case, your first action should be to set the TRIS and PORT registers accordingly.

If you check my tutorials, the switch ones work in this way, although I use external pullups so the board can be used on any port.

BTW, PIC's are extremely hardy devices, they take a lot of abuse without suffering damage!.

 

[Matt(Pic progger)]

:idea:

As a precaution, I always use a 1K resistor between the pic and the switch, this is to prevent damage from ESD, this can (and does) happen.

If you are using this as a "hobby" or "development" device, then these are not needed, although it does offer protection from setting the ports as outputs (by accident) in your code.....

Hope this helps!

 

[Dan East]

Okay, I found my problem that led me to believe something was wrong with the PIC. I switched my inputs on an 12F629 from GP3 and GP4 to GP0 and GP1. However I wasn't disabling the comparator, so GP0 and GP1 were always indicating a low input.

I made the change from GP3/4 to GP0/1 at the same time I switched from external pull downs to the internal pull ups, so I was a bit suspicious I damaged something.

Dan East