Possible to Output_high & check input in place of a pull

iso9001 · Joined: 02 Dec 2003 Posts: 262

I have a switch that when pressed connects to ground.

Normally I would just add a pullup to 5V and check the state of the pin... high, the switch is not pressed, low and the switch is pressed. Easy.

But imagine if I had to do this w/o the pullup.... Can I output_high on that pin, and then check the inputstate... ? My guess is that maybe it would work the same way no ?

I know that Port B pins have pullups built in that can be enabled but you have to enable all of them right ? ie: not JUST the pullup on B1 ?

Thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

iso9001 · Joined: 02 Dec 2003 Posts: 262

Ah that makes sense about shorting the pin out.

I'm using a 16F872 and 76A series (they are very compatible for what my board is doing)

Its a sort of one off.

I'm bound to using the B0 pin b/c of its int_isr. So I'm not sure if charging the pin would work for me, but I do like that idea as a quick fix... gotta remember that.

I looked in the help for the port B pulls ups but the help doesnt show how to specify just one pin (am i wrong?). I'm not sure if the 16F87x series could either, I'm not sure where that would be in the datasheet.

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

You can't enable pull-ups only on one pin on the PIC16F87x series. It is all or none.

Ttelmah · Guest

There is a method that will do what you want, without any excessive loading. Have a resistor running from the PIC pin to a small capacitor. Have another smaller resistor from this junction to the switch. Pull the pin high. If the switch is open, the capacitor will charge via the resistor. Then switch the pin to input, and read the value. If the signal reads as high, the switch is open, if it reads as low, the switch is closed (assuming the switch pulls to ground).
If you are running really fast, and switch/read quickly, you can do this with just one resistor, and rely on the capacitance in the keyboard itself. If you are happy that your keyboard contacts wll not be harmed by directly connecting the charged capacitor across them, then you don't need the second capacitor (this just serves to lmit the current flow through the contacts in this case).

Best Wishes

edi · Joined: 22 Dec 2003 Posts: 82

Thanks a lot.
PU will be added...

temtronic · Posted: Wed Mar 05, 2014 6:55 am

There is a way to read if the a is high,low, or hi-Z but I can't quickly find the Microchip app for that....I like loking at real databooks not online.
I did find Mark Sullivan's 'software switch pull-ups' in the quickcode section of the 'designing for dollars' book circa 1998. That you might be able to use for your project.While code is assembler, it is dead simple to translate.
I have to agree though real pullups are 'simpler'.

hth
jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

It's tip 3 in the "compiled tips 'n tricks" guide.

Requires a resistor _and_ capacitor.....

Best Wishes