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

The circuit on a breadboard.

This tutorial will demonstrate an LED sequencer. The LEDs will turn on one after another from right to left. With this tutorial you will become familiar with the bit rotate function.

One remark needs to be done for the above circuit. There is only one limiting resistor for all 8 LEDs. Normally this would be wrong. A limiting resistor should be added to each LED separately to ensure the same brightness for all the LEDs no matter how many are turned on. But because only one LED is turned on each time, there is no need for separated resistors.

Let's take a look at the code

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

				movlw b'00000000'		        ;
				movwf TRISB				;Set the port pin types of the RB
				bank0					;Go to bank 0
MainLoop
				bcf Carry
				call Wait500mSec
				clrf PORTB
				bsf PORTB,0				;Set RB0 output
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				call Wait500mSec
				rlf PORTB,f
				goto MainLoop

This is a rather simple code. First, we set all the RB pins as outputs. Then the MainLoop is continuously looped. 9 steps are within this loop:

• Step 1: The PORTB is cleared and all LEDs are turned off
• Step 2: PORTB bit 0 is set. The first LED (RED) is turned on
• Step 3-9: The PORTB register is rotated left (rlf) and the LEDs are turned on in sequense

When all steps are done, the loop will run again from the beginning.

Notice the 'call Wait500mSec' instruction between each step. This instruction will create a 500 mSec delay. The subroutine 'Wait500mSec' is located inside the header file 'mSecDelays4MHz.inc' that is included in the project. As always, all files needed for this project can be found at the bottom of the page.

Following are the files for this project:

PIC Tutorials - An LED Sequencer

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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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