Before reading this tutorial, make sure you have read the How to use our PIC Tutorials page!

This is a circuit for understanding the ports. The LED connected to the RB0 port of the PIC will simply turn on and off when the user presses or releases the pushbutton connected on the RA0 port of the PIC accordingly.

The circuit is very simple. When the pushbutton (NO) is released, the input RA0 is taken LOW through the 10K resistor. When the pushbutton is pressed, the RA0 is taken HIGH through the pushbutton. The rest are taken care from the program of the PIC. Following is the assembly listing. Bellow is the PIC code of this tutorial. You will only find a part of the tutorial that is interesting for our discussion. The rest can be found attached at the end of this page.

; Main Program ------------------------------------------------------------
Start
			bank1				;Go to bank 1
			movlw b'11111111'		;Set the port pin types of the RA
			movwf TRISA			;All RA ports are inputs

			movlw b'11111110'		;Set the port pin types of the RB
			movwf TRISB			;All RB ports are input except RB0 that is output
			bank0				;Go to bank 0
MainLoop
			btfss PORTA,0			;Read RA0 pin
			goto Button_Is_Off		;If RA0 is '0', then goto Button_Is_Off
Button_Is_On
			bsf PORTB,0			;Set RB0 output
			goto MainLoop			;And go back to the Main Loop
Button_Is_Off
			bcf PORTB,0			;Clear RB0 output
			goto MainLoop			;And go back to the Main Loop

At the end of this tutorial, you will find all the files including the above asm file in one zip to download and test it.

It may look a little bit complicated but it is not. Everything up to the comment line 'Main Program' are bits and bytes to initialize and set the PIC into operation. There is also this 'Init_normal.inc' header file included along with the standard 'P16F88.inc'. If you are ne to programming, just do not give them any attention and include them as is. I will explain the code from the 'Main Program' and under.

The first thing to do is to set the RA and RB pin port types. I start with the RA port setupr. To access the TRISA register, i need to change to bank 1. Then i load the byte '11111111' in the TRISA register. With this way, i make all the RA pins inputs. Remember that i need one input for the pushbutton. If you look in the schematic, this input shall be the RA0 pin for me.

Then i declare the RB port pin types. For this, i load the byte '00000000' to the TRISB register. This way, i declare all RB port pins as outputs. I could have load the '11111110' as well, because i need only the RB0 pin to be an output.

When bith port pin types are declared, i switch back to bank 0, the place where the most commonly used registers are located.

Next, it is time for the conditional loop. I check the RA0 state. The btfss PORTA,0 does this work. If the button is NOT pressed, then the RA0 will be zero. If so, then the command right after the btfss PORTA,0 shall be executed. This command is a goto command and shall send the program flow to the Button_Is_Off label. Under this label, there is a bcf PORTB,0 command that will simply clear the RB0 output and the LED shall be turned off. The next command (goto MainLoop) will send the program flow back to the MainLoop label and the program will re-check the RA0 again

If the button is pressed, then the RA0 input shall be '1'. When the program flow reaches the btfss PORTA,0 under the MainLoop label, then the very next command shall be skipped. This means that the program flow shall NOT jump to the label 'Button_Is_Off'. Instead, the command 'bsf PORTB,0' shall be executed. This command will set the RB0 output and the LED will light. The next command (goto MainLoop) will send the program flow back to the MainLoop label and the program will re-check the RA0 and so on and so on...

The above described routine shall loop infinitely.

Following are the files for this project:

PIC tutorial - Lighting an LED with a pushbutton

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Basics       What will you need       Choosing the right PICThe MPLAB       Getting familiar with the MPLAB environment       Creating a new project       Open and close projects       Creating new files and including them in the project       Your very first assembly program       Compile a program and transfer to the PICSection 1: Beginner's theoryMemory Organization       The Data Memory Organization       The Program Memory OrganizationThe instructions       General knowledge about instructions       Value Loading Instructions       Program Flow Instructions       Mathematic Instructions       Logic Function Instructions       Bit Orientated Instructions       Byte Orientated Instructions       Miscellaneous InstructionsThe Basic Special Function Registers       The Status Register       The Option_Reg Register       The TRIS and PORT registersBeginner's PIC Tutorials       How to use our PIC Tutorials       A Pushbutton turning an LED on and off       A Simple LED Flasher       Interfacing Multiple Switches - The internal Pull-Up resistors       An LED Sequencer       Interface a Single 7seg Digit       Interface Multiple 7seg Digits       A 3-digits Decimal Counter       A Clever ButtonSection 2: Intermediate theory       Instruction Cycle Duration and Calculated Delays       The Timer Modules - Timer0       The Timer Modules - Timer1       The Timer Modules-Timer2

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