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

This tutorial uses exactly the same circuit as the previous one, as well as the technique to interface three displays with using only 11 pins from the PIC. Therefore, you should read first the previous tutorial to be familiar with this technique as it will be not discussed here. This tutorial will demonstrate a binary counter. It will count from 000 to 255. The number 255 is the max decimal value for an 8-bit byte. It could be of course extended to any length, but because we are still at the beginner's section it will be kept as easy as possible.

The circuit is the same as the previous tutorial:

For detailed informations on this circuit, look at the previous tutorial

Here is the code for this tutorial:

; RAM preserved -----------------------------------------------------------
	cblock 0x20
		WaitCounter,WaitCounter2
		Counter
		Huns,Tens,Ones
		PassArg1,PassArg2,PassArg3
		DelayCounter
	endc

; Main Program ------------------------------------------------------------



Start
				bank1				;Go to bank 1
				movlw b'11111000'		;
				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

				clrf Counter
				bcf Left_Seg
				bcf Mid_Seg
				bcf Right_Seg
MainLoop

				movf Counter,w
				movwf PassArg1
				call Bin8ToDecimal

				movf Huns,w
				movwf PassArg1
				call BinaryTo7_Seg
				movwf Huns

				movf Tens,w
				movwf PassArg1
				call BinaryTo7_Seg
				movwf Tens

				movf Ones,w
				movwf PassArg1
				call BinaryTo7_Seg
				movwf Ones

				movlw d'10'
				movwf DelayCounter

ShowNumbers
				bcf Left_Seg				
				movf Ones,w
				movwf Segments
				bsf Right_Seg
				call Wait4mSec

				bcf Right_Seg				
				movf Tens,w
				movwf Segments
				bsf Mid_Seg
				call Wait4mSec

				bcf Mid_Seg				
				movf Huns,w
				movwf Segments
				bsf Left_Seg
				call Wait4mSec

				decfsz DelayCounter,f
				goto ShowNumbers

				incf Counter,f
				goto MainLoop

BinaryTo7_Seg
				movlw b'00111111'		;Number 0
				movf PassArg1,f
				btfsc zero
				return

				movlw b'00000110'		;Number 1
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01011011'		;Number 2
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01001111'		;Number 3
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01100110'		;Number 4
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01101101'		;Number 5
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01111101'		;Number 6
				decf PassArg1,f
				btfsc zero
				return

				movlw b'00000111'		;Number 7
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01111111'		;Number 8
				decf PassArg1,f
				btfsc zero
				return

				movlw b'01101111'		;Number 9
				return

The code has grown larger. The idea is still simple. A counter ('Counter') is increased on every cycle loop. The counter carries a binary number from 0 to 255. Then, the value of the Counter is loaded every time to a temporary register named 'PassArg1'. The subroutine 'Bin8ToDecimal' is then called. This subroutine is located inside the header file 'bcdToDecimal_3Digits.inc' that is included in the project. When this subroutine is called, it will convert the binary number in 'PassArg1' into three decimal digits. Those digits will be at the 'Huns', 'Tens' and 'Ones' registers.

We still need a way to convert them into the corresponding binary numbers to be visualized on the 7-segs. This is done by calling the subroutine 'BinaryTo7_Seg'. Immediately after the binary to decimal conversion, the 'Huns' register is loaded to the temporary 'PassArg1' register, and the 'BinaryTo7_Seg' subroutine is called. The first thing done in this subroutine is to load the number for the '0' digit to the W register. Then it will check if the 'PassArg1' was zero and if it was, it will return back. If not, it will load the number for the digit '1'. The 'PassArg1' will be decreased ones and if it becomes zero it will return back. Then it will load the digit '2', decrease once more the 'PassArg1' and if it then becomes zero it will return back. This is done up to the digit '9'. So, according to the number that exists in the 'PassArg1' register, the W register will have the corresponding binary number that will be visualized on the display.

When the program flow returns from the 'BinaryTo7_Seg' subroutine, the W register will have the binary number that corresponds to the number in the 'Huns' register. this binary number is immediately stored back to the 'Huns' register for later use. The same procedure is done for the 'Tens' and the 'Ones' registers. Remember that those 3 registers carries the three digits for the decimal number of the Counter. Each register carries a number from '0' to '9'.

When all digits have been converted from decimal to 7-seg digits, it's time for visualization. The procedure is detailed explained at the previous tutorial. The same procedure is done here in the 'ShowNumbers' subroutine.

You should notice that above and bellow this subroutine, the following code exists:

				movlw d'10'
				movwf DelayCounter

ShowNumbers
				.
				.
				.
				.
				decfsz DelayCounter,f
				goto ShowNumbers

This code will now create the delay between the number changing. It would be easier to call out well-known delay subroutine to make a 250mSec delay for example. But in that case, for 250mSec the displays would be all turned off. So this is not the correct way. Instead, we will use the same subroutine that displays the numbers to create the delay. We load the number '10' to the 'DelayCounter' register. Then we run the subroutine that displays the numbers. At the end, we decrease once the 'DelayCounter' register. If it becomes zero, the program flow continues normally and it will loop back to the beginning. But if it is still not zero, it will display the numbers once more. This will happen for 10 times. The delay between each number change is approximately 110mSec.

As you understand, the time calculation is a big headache when dealing with simultaneous procedures. Here for example, we want to have digit display and time counting at the same time. There will be situations that more than 3 procedures will have to run at the same time. For example, the display, the time keeping, the button watching and an alarm will need to run at the same time if you make a clock! During the Advanced lessons, the time calculation will be explained in details.

Following are the files for this project:

PIC Tutorial - A 3-digits Decimal Counter

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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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