PIC Interface Design & Development
Experiment 2 LED 5x7 DISPLAY
Objective:
1. To use I/O of PIC Microcontroller to design a LED display
2. To develop a C code for Running the LED display
We will design a 5x7 LED Display and PIC microcontroller will control each LED as shown below.
Materials:
x FlashPIC-Dev’ evaluation board, with PIC 16F877 or PIC 18F458 Microcontroller.
x LEDs display (http://www.jameco.com)
x 4017 chip (you can conduct the function of this 4017 using the “software” approach)
x Other necessary equipment
We use CD4017 counter chip to control only one column of LEDs turns on at a time. B0 is clock for 4017
and B1 is the reset. 4017 chip Output "0" goes HIGH on the rise of the first clock cycle. On the rise of
the second clock cycle, 4017 chip output "0" goes LOW and output "1" goes HIGH. After each five cycle
of clock, we can reset the counter to start over again.
You may conduct the function of 4017 by the software. In which case, the 4017 chip can be omitted.
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Seven of the lines of port D drive the LEDs via 470Ohms resistors. The output lines of the chip
correspond to Port D and by turning off these lines, clocking the 4017 then turning on the outputs again,
the second column of LEDs will be illuminated. This is repeated for the 3rd, 4th and 5th columns. When
this is repeated at a rate above 50 times per second, the whole screen of LEDs appears to be ON at the
same time. This is how a picture or effect is produced on the screen - it's called scanning.
Test:
x
x
Make sure all the LEDs are working.
Test each LED display
x
Display each character of “EGR 592” 30 seconds such as
x
Running Sign
The 5x7 Display can display running messages. Data can be loaded into 5 Ghost locations and
this data is displayed on the screen. One byte is transferred at a time from the table and loaded
into the Ghost section. A routine then outputs the five locations to the screen.
The data in the Ghost section is then shifted one place to the left and a new byte loaded into the
5th location. This gives the effect of a message scrolling across the screen, from right to left. To
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work out the value of each column of LEDs, the individual HEX values are added together.
The program looks for "FF" and repeats the message.
Reference:
http://www4.tpg.com.au/users/talking/5x7%20Display%20Index.html
An example of report and the corresponding code is listed below.
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Experiment #2 LED 5x7 Display Sign
Abstract
Robert Quesada II
California State University Pomona
Pomona, CA 91768
In this experiment students will demonstrate
displaying letters and numbers on a 5x7
LED matrix display. The program to drive
the 5x7 LED matrix display will be written
in C code. The 5x7 display will be
connected to the Port D and Port B of the
PIC microcontroller.
1. Introduction
There are different steps to this experiment;
the first part is to be able to light a single or
a row of LED’s on the 5x7 display as shown
below:
2. Background
The 5x7 display is connected to Port D, but
Port D only has 8 outputs total. In order to
display all the output, the individual rows of
LED’s are placed together. So when one say
Pin D6 is set to turn on the whole row.
Therefore in order to capture just one LED
to turn on the columns are connected
together.
But this is not all that is need to display,
what is also needed are transistors connected
to the columns to hold or switch on the
column, however the transistor can not hold
the voltage for long, therefore the program
must cycle the output to both Port D and
Port B (controls the transistor indirectly).
What controls the transistors is a 4017 chip,
which is a simple divider; Port B controls
the reset and the clock. The 4017 works like
a shifter in the circuit.
Figure 1 Test single column
Using the knowledge gained from this first
step the next step is to letters like the “E”
“G” “R” “5” “9” “2” and display each letter
for 30 seconds.
3. Equipment/Materials
(1) PC with USB
(1) PIC16F877 development board
(Various) Resistors
(35) LED’s
(5) Transistors
(1) 4017 chip
(Various) Clip leads / wires
4. Circuit Description
The circuit is shown below
Figure 2 Displaying the letter “R”
The final step is to make the letters move or
what is called a “running sign”. Essentially
the letter move across the LED display
which a person is able to read.
Figure 3 5x7 Circuit
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Port D (D0-D6) is used for output to display
rows and Port B (B0-B1) is used for the
clock and reset on the 4017 chip to control
the columns on the 5x7 display. The
transistors are used as ighting-up control of
the LEDs columns (to provide enough
power).
~0x40,~0x40,~0x7F,~0x40,~0x40,~0x00,//T
~0x7F,~0x49,~0x49,~0x41,~0x41,~0x00,//E
~0x3F,~0x40,~0x40,~0x3F,~0x40,~0x40,~0x3F,~0x00,
//M
~0x3E,~0x41,~0x41,~0x41,~0x3E,~0x00, //O
~0x70,~0x0E,~0x01,~0x0E,~0x70,~0x00, //V
~0x7F,~0x00,//I
st
The 1 step to display anything on the 5x7
display is to reset the 4017 chip. Next is to
st
send the desired output to Port D, the 1
column should be lit with the output of Port
D because reset send high to pin 0 of the
4017.. Then to light the next column, clock
the 4017 and turn on the desired output to
nd
Port D this will light up the 2 column. Do
this for all 5 columns, after 5 the 4017 needs
st
to be reset to start over at 1 column. In
order to see the image this needs to be
cycled many times so it will appear the
whole screen is on, this is called scanning.
st
To program this the 1 step is the header
file:
#include <16F877A.h>
#fuses HS,NOWDT
This allows us to use directives from the
16F877A.h header file, the NOWDT
disables the watch dog timer which would
cause the PIC to automatically reset.
Next step is to set up the port definitions
#byte PORTD = 0x08
/* output port
D definition */
#byte PORTB = 0x06
/* output port
B definition */
#bit clock = PORTB.0
/* clock*/
#bit reset = PORTB.1
/* reset*/
To store the columns a array is used
This array is for the EGR592 display
int displayEGR[35]={
~0x7F,~0x49,~0x49,~0x41,~0x41,//E
~0x3E,~0x41,~0x49,~0x49,~0x2E,//G
~0x7F,~0x48,~0x4C,~0x4A,~0x31,//R
~0x7A,~0x49,~0x49,~0x49,~0x46,//5
~0x32,~0x49,~0x49,~0x49,~0x3E,//9
~0x27,~0x49,~0x49,~0x49,~0x31,//2
}; //array to store hex numbers for display
This array is for the scrolling text
int displaySCROLL[80]={
~0x7F,~0x10,~0x0C,~0x02,~0x7F,~0x00,//N
~0x3E,~0x41,~0x45,~0x45,~0x36,~0x00, //G
~0x32,~0x49,~0x49,~0x49,~0x26,~0x00,//S
~0x7F,~0x00//I
~0x3E,~0x41,~0x45,~0x45,~0x36,~0x00,//G
~0x7F,~0x10,~0x0C,~0x02,~0x7F,~0x00,//N
~0x00,~0x00,~0x00,~0xFF}; //end of
array
Next setup the integers needed
int y=0, column,columncounter;
long int timecounter=0;
y is what is used to control the amount of
shifts of columns, to display for a letter at a
time increment y by 5 and to scroll in a new
column increment y by 1.column is used for
the “for” loop this cycles through the
columns 1 through 5. Columncounter is to
scan Port D on the desired column over and
over to eliminate the “ghosting” effect. And
timecounter is used to display the columns
for a desired amount of time.
Two functions are declared, one is to clock
the 4017 chip and the second is to reset the
4017 chip.
void clockshift(void){
clock=0;
clock = 1;
}
void resetchip(void){
reset=1;
reset=0;
}
//reset 4017
Next is main part of the program
void main(void){
set_tris_b(0x00); // port B is all output
set_tris_d(0x00); // port D is all output
resetchip();
This setups the ports for output and resets
st
the chip for the 1 column.
~0x00,~0x00,~0x00,~0x00,~0x00,~0x00,//blank space
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//clock the 4017
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The next section is only for displaying “E”
“G” “R” “5” “9” “2”
while(1){
for(timecounter=0;timecounter<3000;timecounter
++){//timing loop, adjust!
for(column=0;column<=4;column++){
for(columncounter=0;columncounter<50;columnc
ounter++)
output_D(displayEGR[column+y]);
clockshift();
}
}
y=y+5; //shift 5 to next "letter"
if (y==30) //end of array restart
y=0;
}
}
The timecounter loop controls how long the
letters stay on the LED display. The column
count changes the columns 1 – 5, the
columncounter is what prevents ghosting by
display the column over and over many
times. To display the next letter y needs to
be shifted by 5 because each 5 columns
represents one letter in the array. When
y=30 or the end of the array reset the y
value.
5. Testing Scenarios
We will test our LEDs to see if they’re all
working at first by lighting up all the
columns of LEDs starting from column 1
and working our way to column 5. Then
after we show that all the LEDs are working
properly. We will start off by showing that
we can hold the letter E, then G, then R,
then 5, then 9 and lastly 2 for 30 seconds
each. This will show we are capable of
programming the LEDs accordingly. Then
we will demo the scrolling message on the
LED screen “TE MOVING SIGN.”
6. Conclusion
The testing scenario was demonstrated to
work correctly. The only difficulty in this
lab was the running sign and EGR592
display in one program so two programs
were needed because storing both arrays in
one was too much memory for the PIC
microcontroller to hold. This lab
demonstarated
how
important
the
correctness of coding needs to be when
connecting a PIC to some external circuit,
in this case the 5x7 LED and 4017 chip.
This next part is from the scrolling text the
program is very similar.
while(1){
for(timecounter=0;timecounter<200;timecounter+
+){//timing loop, adjust!
for(column=0;column<=4;column++){
//output to columns 1-5
for(columncounter=0;columncounter<50;columnc
ounter++) //prevent ghosting by scanning
output_D(displaySCROLL[column+y]);
//output to port D
clockshift(); //next column
}
resetchip(); //restart afete 5
}
y=y+1;
if (displaySCROLL[5+y]==~0xFF) //reached
end of usable array restart
y=0;
}
}
The only difference of the scrolling text is
that the time displayed is much shorter and y
is incremented by 1 to shift in next column.
The terminating part of the array is ~FF
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