Summary of Using Multiplexed 7 Segment Displays – PIC Microcontroller Tutorial
This article explains how to drive a four-digit seven-segment display using a PIC18 microcontroller via multiplexing. By activating only one digit at a time rapidly, the system reduces MCU I/O pin requirements while maintaining the illusion of all digits being lit simultaneously. The implementation utilizes TIMER0 interrupts in an ISR to cycle through digits and update segment data stored in an array.
Parts used in Multiplexed Seven Segment Display:
- Four digit seven segment display
- PIC18 series Microcontroller Unit (MCU)
- Transistors for common connection control
- TIMER0 module for automatic refreshing
- SEL lines for digit selection
- Segment data output lines
A four digit seven segment display. |
The image above shows a four digit seven segment display. These type of displays are widely used to present numerical data to users. Example includes clocks, panel meters, microwave ovens, refrigerators etc. As you can see in a four digit displays there are a total of 4 x 7 = 28 segments (made of leds) so you may think that they will require lots of i/o pins of MCU. But in reality a small trick can greatly reduce the number of i/o pins required.
The trick is to activate only one digit at a time. All the segments of each four digit are connected in parallel and common of each four digit is tried to MCUs i/o port via transistors. That means MCU can select any digit and put the segment data which drives the segments. Only the selected digit will light up. Then next digit is selected and segment data is changed according to the digit that must be shown in that place. Similarly each digit is selected and shown. This is done fast enough that the human I cannot see the movement. We can see all four digit lit at the same time as shown in the figure above.
Multiplexed Seven Segment wiring |
To display any four digit number, say 1234 on the display. The following steps are taken.
First the display 1 is select by making SEL1 line high (keeping all other SELx line low), now any segment data sent from MCU will be visible only on DISP1. Now we send the segment data for ’4′, thus it is shown on the DIGIT1.
After some time we select digit 2 by setting SEL2 line high (keeping all other SELx line low) and sending segment data that shows ’3′ on display. ’3′ is shown on DIGIT2
After some time we select digit 3 by setting SEL3 line high (keeping all other SELx line low) and sending segment data that shows ’2′ on display. ’2′ is shown on DIGIT3
After some time we select digit 4 by setting SEL4 line high (keeping all other SELx line low) and sending segment data that shows ’1′ on display. ’1′ is shown on DIGIT2
we repeat the above steps. And we do so fast enough that human eye cannot catch the trick and see all four digits lit at the same time.
Implementing it using PIC MCU
We will use PIC18 series MCU to implement the above technique. To make the refreshing of display automatic we will use TIMER0 module. We set it to interrupt on overflow. If you are new to TIMERs and PIC Interrupt handling please see the following tutorials.
Timer Interrupt Service Routine(ISR) is automatically called by the hardware at specified interval. This ISR switch to next display and changes the segment data according to the digit to be show on that display. If it is on last display it switch back to display number 1. The data that is shown in the display is store in an array named digits[4]. You can modify the digits[] to change the display. For example if you want to display “1234” you need to do the following :-
For more detail: Using Multiplexed 7 Segment Displays – PIC Microcontroller Tutorial
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How many segments are in a four digit display?
A four digit display contains a total of 28 segments calculated as 4 multiplied by 7. -
Does multiplexing reduce the number of required I/O pins?
Yes, activating only one digit at a time greatly reduces the number of I/O pins required from the MCU. -
What is the best way to light up all digits simultaneously?
Select each digit quickly in sequence so fast that the human eye cannot see the movement. -
How do you select a specific digit on the display?
You select a digit by making its corresponding SEL line high while keeping all other SELx lines low. -
Can you use a PIC18 series MCU for this project?
Yes, the article describes implementing this technique using a PIC18 series MCU. -
Which module handles the automatic refreshing of the display?
The TIMER0 module is used to automatically refresh the display by setting it to interrupt on overflow. -
Where is the data for the display stored in the code?
The data shown in the display is stored in an array named digits[4]. -
When does the Timer Interrupt Service Routine switch back to the first display?
The ISR switches back to display number 1 if it is currently on the last display.
