Using a PORT for input and output in the same program?

flip · Joined: 15 Feb 2005 Posts: 11

Can I use PORT B as input and output in the same program? I'm using the 18f458. My program works like this:

In one subroutine, I initialize PORT B as output and output B to LEDs. A control bit makes the program jump to this subroutine and this control bit also is hooked up to 8 AND gates, one for each bit from PORT B. I pull down all of PORT with 1k resistors, then AND PORT B with the control bit.
The output of the ANDs go to 8 LEDs.

In the other subroutine, PORT B is iniatialized as input. The next control bit is hooked up to 8 more AND gates. This control bit is ANDed with 8 input switches (which are also pulled down using 1k resistors). The output of these AND gates are then hooked up to PORT B.

Will this work? I tried straight hooking up the LEDs and the switches to PORT B but the program wouldn't work.

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Guest · Posted: Tue Feb 22, 2005 6:02 pm

What would you recommend? All I need to have as input are switches and all I need for output are LEDs. Thanks

cyril · Guest

I'd think you would want to use a gate with open collector (or drain) outputs. And of course use pull up resistors

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

One of our designs has switches with LED indicators that are driven in the manner that you desire. That is a single pin is both an input and an output. For the output, the pic supplies the current to the LED. So the LED is connected to the PIC and the other end of the LED is connected to a resistor to ground (680ohms). The PIC is also connected to 2 series resistors to ground. The first a 220 ohm and the second a 47K. The junction between these two resistors is connected to a switch. The other side of the switch goes to 5V.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Mark,
I think there is an error in your schematic drawing; your text and the drawing don't match. Shouldn't the switch be connected between the 47k and 220 Ohm resitors? Nice idea!
Just something I don't understand is why you need the 47k resistor at all?

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Yep, text is right and drawing is wrong. In a hurry this morning. The 220 ohm resistor will prevent you from attaching VDD to the pin on the PIC. Could cause damage to the PIC if the switch were pressed and the PIC was set to output a 0. The 47K pulls the input to ground when the switch is not closed.

flip · Joined: 15 Feb 2005 Posts: 11

Let me get this straight. I use the same idea as in your diagram, but put the switch in between the 220ohm and 47k?

flip · Joined: 15 Feb 2005 Posts: 11

I'm also wondering will the LED light up when the switch is on? Can the output to the LED work when the switch is on?

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Guest · Posted: Thu Feb 24, 2005 10:19 am

I'm just looking for a design that will allow me to use PORTB to output to LEDs in one subroutine, and have switches as input in another subroutine, using the same PORT B line without affecting eachother. Do you think I should use AND gate logic?

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

I'd stick a 74HC245 to read the switches. The LEDs could be driven off the PIC or you could even stick a 74HC373 to drive them. Phillips also makes I2C driver chips. They can be configured to blink LEDs and such.

[email protected] · Guest

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