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

Another simple circuit is the LED flasher. With this tutorial you will become familiar with the delays. It is very important to understand how to generate delays with the PIC.

What this circuit does, is to simply flash an LED every half second. To maintain the simplicity of the code, there is no button or other switch conencted to it, as this would be rather off-subject. Bellow is the PIC code of this tutorial.

; RAM preserved -----------------------------------------------------------
	cblock 0x20
		WaitCounter,WaitCounter2
	endc
; Main Program ------------------------------------------------------------
Start
				bank1					;Go to bank 1
				movlw b'11111111'		;
				movwf TRISA				;Set the port pin types of the RA

				movlw b'11111110'		;
				movwf TRISB				;Set the port pin types of the RB
				bank0					;Go to bank 0
MainLoop
				call Wait250mSec
				call Wait250mSec
				bsf PORTB,0				;Set RB0 output
				call Wait250mSec
				call Wait250mSec
				bcf PORTB,0				;Clear RB0 output
				goto MainLoop

Wait250mSec                     movlw d'250'
				movwf WaitCounter2		
BackWaitLoop2                   movlw d'163'			
                                call WaitWx4Cycle
                                movlw d'163'
                                call WaitWx4Cycles
			        nop
			        nop
			        decf WaitCounter2,f
			        btfss Zero
			        goto BackWaitLoop2		
			        return				



WaitWx4Cycles                   movwf WaitCounter
BackWaitLoop                    decfsz WaitCounter,f
                                goto BackWaitLoop
                                return

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.

The first thing to be noticed is that all ports are inputs except the RB0 port. This is our only output and the LED is connected there.

When the program reaches the MainLoop, it calls for a label named 'Wait250mSec'. This label is our calculated 250mSec delay. Twice a call means 1/2 second delay. Now how this delay really work:

Go to this label to follow the code. You will notice that the decimal number '250' is loaded to the WaitCounter2 register defined at the beginning of the code. Right after, a 'BackWaitLoop2' label is defined. 8 instructions below, there is this 'goto BackWaitLoop2' instruction. This block, will generate 1mSec delay and will be called as many times as the WaitCounter2 carries, in our occasion that will be 250 times equals to 250mSec.

To generate the 1mSec delay, we need to know something about the timing of the PIC. Each instruction cycle needs 4 clock pulses to be implemented. Our PIC is clocked at 4MHz (defined in the OSCCON register within the Init_normal.inc header file). So, if you divide the 4.000.000 Hz by 4, you will get the astonishing number of 1.000.000 instructions per second! Our PIC is capable to execute 1.000.000 instructions per second, and this is our time base to calculate the delay. Follow my thinking:

The answer is hidden in plain mathematics. The formula for the above calculation is:

And that means that we need to execute 1000 instructions to generate 1mSec delay. That is the basic idea how to calculate delays. During the intermediate lessons, there will be e fully explanation on how to measure the instruction cycle and delay, because not all instructions have the same cycle. But to avoid all this mess, you can just use the code provided to this tutorial. This will work perfectly for the 4MHz oscillator. It can be used for direct delays from 1mSec to 256mSec. Just load the value in mSec that you want to the WaitCounter2 register and call the BackWaitLoop2 subroutine. If you want more than 256 mSec delay, you can call more than once the same subroutine, as done in this tutorial for the 500mSec (half second) delay.

Now, back to the MainLoop. It is most obvious what is done over there. At first, a 500mSec delay is called. Then, the RB0 output is SET, the LED turns on. Afterwards, another 500mSec is called and the RB0 output is CLEARED, the LED is turned off. That keeps going infinitely, and there is your LED flasher!

Following are the files for this project:

PIC Tutorial - LED Flasher

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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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