Interfacing LEDs to PIC microcontrolers

I'm trying to use one I/O for both input and output (RA1 on a 16C84). Software - Every [so often] the pin is changed to an output and set high to drive an LED for about [a while]....then turned off and changed back to an input. Between the LED blinks I poll the input to check whether it's HI. To debounce, the input must be HI once and again 20msec later. I've got the weak pull-ups disabled.

Hardware - The pin has a 470 ohm resistor and (Smurf-) blue LED to ground. Also a pushbutton switch and 10k resistor to V+.

What should happen:

If pushbutton not pressed - LED should just blink. If pushbutton is pressed - start other routine.

What does happen:

Other routine starts by itself. Even if 10k resistor is pulled out of the circuit and circuit left open (so pushbutton is not malfunctioning). Also, never noticed the problem occurring during the night - always during the day. I can trigger it with a flashlight!

Why:

The [input pin] is floating - worse, the LED [is] generating a (very very tiny) current from light. Try putting a pull down resistor (100K) from the pin to ground, or across the LED.

I use the inverse (LED to vcc, push button low) of your circuit (with the extra resistor) on my graphic LCD checker:- http://ourworld.compuserve.com/homepages/steve_lawther/t6963tst.gif with no problems.

PIC countdown timer

PIC 16F84A elapsed time counter on 4, 7 segment LED's

Rotate some bits without affecting others

Fashing LED program for 16f877

Driving multiple bicolor LEDs in a matrix from and embedded controller

Using lookup tables to convert numbers to seven segment LED display patterns

Driving 8 or 9 7 segment displays with 8 or 9 io pins (and no extra circuitry)

General LED interfaceing

From the CHEAPIC tutorial:

• bincnt.asm - Count in binary on 4 LEDs
• swcnt.asm - Count switch closures.
• eggtimer.asm - Countdown timer, displays minutes and 10's of seconds on LEDs
• binclock.asm - A clock which displays hours and minutes in BCD on LEDs at keypress

See also:

• http://www.rentron.com
• http://www.mindspring.com/~shadedemon/electronics/electronics.htm very bright PIC controlled LED array sign.
• http://www.bobblick.com/bob/projects/sign2/index.html 7x10 LED sign using an EPROM and some counters
• http://ken.webster.org/clock/ Large LED (7-Segment) Clock PIC16C73B

PICList post "How to matrix bicolor LEDs?"

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