Lab 12: Basics of LED dot matrix display

We covered how to interface seven segment LED displays to a PIC microcontroller in two sections: Lab 6 and Lab 11. Today, we will move on to interfacing an LED dot matrix display. LED dot matrices are very popular means of displaying information as it allows both static and animated text and images. Perhaps, you have encountered them at gas stations displaying the gas prices, or in the public places and alongside highways, displaying advertisements on large dot matrix panels. In this experiment, we will discuss about the basic structure of a monochrome (single color) LED dot matrix and its interface with a microcontroller to display static characters and symbols. We will cover the animation stuff in next tutorial. I am using the PIC18F2550 microcontroller on the StartUSB for PIC board for demonstration, but this technique is applicable to any other microcontrollers that have sufficient I/O pins to drive the LED matrix.

Interfacing a LED dot matrix display with a PIC microcontroller

Theory of LED dot matrix display

In a dot matrix display, multiple LEDs are wired together in rows and columns. This is done to minimize the number of pins required to drive them. For example, a 8×8 matrix of LEDs (shown below) would need 64 I/O pins, one for each LED pixel. By wiring all the anodes together in rows (R1 through R8), and cathodes in columns (C1 through C8), the required number of I/O pins is reduced to 16. Each LED is addressed by its row and column number. In the figure below, if R4 is pulled high and C3 is pulled low, the LED in fourth row and third column will be turned on. Characters can be displayed by fast scanning of either rows or columns. This tutorial will discuss the method of column scanning.

Structure of a 8x8 LED dot matrix

The LED matrix used in this experiment is of size 5×7. We will learn how to display still characters in a standard 5×7 pixel format. The figure below shows which LEDs are to be turned on to display the English alphabet ‘A’. The 7 rows and 5 columns are controlled through the microcontroller pins. Now, lets see in detail how it works.

Suppose, we want to display the alphabet A. We will first select the column C1 (which means C1 is pulled low in this case), and deselect other columns by blocking their ground paths (one way of doing that is by pulling C2 through C5 pins to logic high). Now, the first column is active, and you need to turn on the LEDs in the rows R2 through R7 of this column, which can be done by applying forward bias voltages to these rows. Next, select the column C2 (and deselect all other columns), and apply forward bias to R1 and R5, and so on. Therefore, by scanning across the column quickly (> 100 times per second), and turning on the respective LEDs in each row of that column, the persistence of vision comes in to play, and we perceive the display image as still.

A standard 5x7 LED dot matrix display structure

The table below gives the logic levels to be applied to R1 through R7 for each of the columns in order to display the alphabet ‘A’.

Row values of each column for displaying the alphabet A

Scanning across the columns and feeding with appropriate row values

You should have noted that across each row, one pin is sourcing the current for only one LED at a time, but a column pin may have to sink the currents from more than one LED. For example, the column C1 should be able to sink the currents from 6 LEDs while displaying the alphabet ‘A’. A microcontroller’s I/O pin cannot sink this much of current, so external transistor arrays are required. I am using ULN2003A IC which has seven built-in Darlington transistor arrays (see below). The inputs of ULN2003A are active high. This means the input pins must be supplied with logic high in order to bring the corresponding output pins to ground. The schematic of the Darlington transistor array inside the ULN2003A chip is shown below.

Pin diagram and schematic of ULN2003A (Darlington transistor arrays)

Circuit Setup

The circuit setup for this experiment is quite simple. You need seven 330 ? resistors in series with rows R1 through R7 to limit the current through the LEDs. Then the rows are driven by RB0 through RB6 pins of PIC18F2550. The columns are connected to the five outputs of ULN2003A. The corresponding five input pins of ULN2003A IC are controlled by RA0 through RA4 pins of PIC18F2550. The microcontroller will, therefore, scan across the column by sending appropriate bits to PORTA. For example, setting RA0 to 1 and clearing RA1 through RA4 bits, will select the first column. The microcontroller will wait for about 1 ms before switching to the next column. At each column, the microcontroller will output the corresponding row value at PORTB to turn on the appropriate LEDs in the column that are required to display the specific character. The switching between columns is fast enough to deceive the human eyes and a steady character is displayed.

.

Circuit diagram for interfacing a 5x7 LED dot matrix with PIC18F2550

Circuit setup showing a StartUSB board with a 6x7 LED dot matrix (the sixth column is discarded here)

Software

The major part of this experiment is the software routine to scan the columns and feed the rows with appropriate values. The column-specific row values for display characters can be either defined in RAM or stored in the program memory in case the on-board RAM is not sufficient enough. In mikroC, the variables are saved in RAM and constants are stored in program memory. So, if your PIC does not have enough RAM, you can define a constant array to store the row values so that a part of the program memory is occupied by it to free up the on-board RAM. PIC18F2550 has quite a bit of RAM (2 KB), so I have used RAM to store the row values for alphabets A through Z. Here’s how I define it in mikroC,

unsigned short Alphabets[130]={ 0x7e, 0×09, 0×09, 0×09, 0x7e, // A
0x7f, 0×49, 0×49, 0×49, 0×36,  // B
0x3e, 0×41, 0×41, 0×41, 0×22,
0x7f, 0×41, 0×41,0×22, 0x1c,
0x7f, 0×49, 0×49, 0×49, 0×63,
0x7f, 0×09, 0×09, 0×09, 0×01,
0x3e, 0×41, 0×41, 0×49, 0x7a,
0x7f, 0×08, 0×08, 0×08, 0x7f,
0×00, 0×41, 0x7f, 0×41, 0×00,  // I
0×20, 0×40, 0×41, 0x3f, 0×01,
0x7f, 0×08, 0×14, 0×22, 0×41,
0x7f, 0×40, 0×40, 0×40, 0×60,
0x7f, 0×02, 0×04, 0×02, 0x7f,
0x7f, 0×04, 0×08, 0×10, 0x7f,
0x3e, 0×41, 0×41, 0×41, 0x3e,
0x7f, 0×09, 0×09, 0×09, 0×06,
0x3e, 0×41, 0×51, 0×21, 0x5e,
0x7f, 0×09, 0×19, 0×29, 0×46,
0×46, 0×49, 0×49, 0×49, 0×31,  // S
0×01, 0×01, 0x7f, 0×01, 0×01,
0x3f, 0×40, 0×40, 0×40, 0x3f,
0x1f, 0×20, 0×40, 0×20, 0x1f,
0x3f, 0×40, 0×30, 0×40, 0x3f,
0×63, 0×14, 0×08, 0×14, 0×63,
0×07, 0×08, 0×70, 0×08, 0×07,
0×61, 0×51, 0×49, 0×45, 0×43 // Z
};

And this is how mikroC allows you to store arrays in the program memory.

const unsigned short characters[30]={

0×24, 0x2A, 0x7f, 0x2A, 0×12, // \$

0×08, 0×14, 0×22, 0×41, 0×00, // <

0×41, 0×22, 0×14, 0×08, 0×00, // >

0×14, 0×14, 0×14, 0×14, 0×14, // =

0×36, 0×49, 0×55, 0×22, 0×50, // &

0×44, 0x3c, 0×04, 0x7c, 0×44, // PI

};

I have written a simple program in mikroC to display the alphabets A through Z sequentially, and some special characters too. You can watch the video below to see how they look like on the dot matrix display.

Displaying symbol PI

• senthil.b

i have need more c program and assembly language program

• hemaraj

• sir, plz help me.

• Sir, i would like to know more about dot matrix display . How can send data to the display ..?

• In the Theory of LED dot matrix display says “For example, a 8×8 matrix of LEDs (shown below) would need 64 I/O pins, one for each LED pixel”. Is not the number of I/O that should be required is 65 (because it takes 1 extra I/O as the common) ? Please the explanation, thank you ..

• can you help do a project liker the one you done that, but my project want more word as such as: welcome to souphanouvonguniversity ………& each other). i will play you money

• AMPOLU HAREESH

u are always provide code for rows but can u please provide the code for columns . i mean can we connect columns to controller or any other device or there is any software for that……………………..
and also kindly please explain about how to connect and write the program of two or more than 2 led dot matrix

thanking you sir,

• Sir,
I have a PIC32MX320F128H, a led matrix 7×5 and ICs 74HC595, but i dont know how to make connections in the breadboad. Could you give me the schematic?
Thank you

• abdullah rafiuddin

ULN2003 and ULN2803
What is the diffrence betweenCD4082 and CD4081 ?

• What is the diffrence between ULN2003 and ULN2803 ?

thank you!
marC:)

• can you please explain in few words how you converted the above digits corresponding to different rows,because where you said:
unsigned short Alphabets[130]={ 0x7e, 0×09, 0×09, 0×09, 0x7e, // I am not getting how you get those values ;on my behalf when I convert I am getting: {0x7F,0X88,….}

Thanks

• Nabil Hossain

these are individual BCD codes converted to hexadecimal codes.

ex: 0x7e means,
0b01111110
______________
7= 0111
3=1110, whereas 0x / 0b are syntax for writing hex ob bin code.

• satheesh

thank u so much for urs info..

• Jigme Norbu

Its very interesting but how can how to I add more LED dot matrix and how does C code changes generally. I am a beginner in Microcontroller.

• thx for this project
please can you help me with the isis shematic of lab 12 and the christmas messsage because they dont work with me and i dont know the problem

• Sir.Please me ,full details of Dot-matrix,construction.

• Welmer Dampayla

Could you share me how a real time message to be display on the dot matrix. please share some techniques on how to do it. Please help me how to decode a web message and put it into the dot matrix display. thanks ahead.

• sir please sir help me with circuit diagram,codes and component needed for the construction of led dot matrix display with atleast 150 LED, which will be displaying DEPARTMENT OF ELECTRICAL/ELECTRONIC ENGINEERING IMT ENUGU CAMPUS BY GROUP 2.

• Hello sir,am given project on moving message display using bicolor LED.pls help me with circuit diagram and how to connect them.as sson as possible.

• Sir,
Our project is to make a clock using 4 dot matix,89C51 & decoder .how can I do this by using assembly language.I need the solution(code & circuit diagram) before April 25th.

• Sir,
Our project is to make a clock using 4 dot matix,89C51 & decoder .how can I do this by using assembly language.I need the solution(code & circuit diagram) as soon as possible.

• if there are more columns in dot matrix led display how i can interface these columns with micro controller ports,as pins of micro controller are limited in number

• Dear Sir,
I want 10 such led displays to be mounted in one line. what changes will i have to make in the software and the hardware. Please help.

Thanking you,
Regards,
Naren

• Superb!

• Is there a reason why you swap the Rows and Column headings in your chart ‘Row values for displaying the alphabet A’?

It took a second to figure out why it didn’t match up with the scanning columns which followed. Sure its not that difficult to translate once you figure out what’s going on, but why make it more difficult for the reader.

I think it would make for an easier read if you chart orientation was the same as the rest of the posting. That is, with Rows going from top to bottom and Columns from left to right.

Otherwise, a nice article.

Thanks,
– Bill

• Hey Bill,
I realized what you said, and I have modified the table accordingly. Thanks.