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The 7-segment display is the earliest type of an electronic display that uses 7 LEDs bars arranged in a way that can be used show the numbers 0 – 9. (actually 8 segments if you count the decimal point, but the generic name adopted is 7-segment display.) These devices are commonly used in digital clocks, electronic meters, counters, signalling, and other equipment for displaying numeric only data. 
It is not different from an LED in terms of interfacing, by turning the appropriate segments ON and OFF we can display easily the numbers 0 to 9 and optionally the decimal point (DP). 
                                                  

  Figure 1: 7-Segment displaying 8 with decimal point         Figure 2: 7-Segment displaying 3

The segments of the displays are normally referred to by letters ‘a’ to ‘g’.
Figures 1 and 2 show how a 7-segment display can display digits. 
In figure 1, all the segments (LEDs) are switched on to display the digit “8” with the decimal point. On the other hand, in figure 2, segments a, b, c, d and g are switched on to display the digit “3”.  any combination can be used to display any desired digit.
The segments can also be used to display some letters, but this is limited. For example, the letter “b” can be displayed by switching on the segments  c, d, e and f and the letter “F” by switching on segments a, e, f and g.

7-segment displays are available in two configurations: common anode and common cathode. As shown in Figures 3 and 4 below.
       

    Figure 3: Common Anode 7-Segment              Figure 4: Common Cathode 7-Segment
In figure 3, the anode pins of all the segments are connected together and this pin is usually connected to the power supply. Individual segments are turned ON by grounding the required segment pin through the microcontroller by sending a “0” to the pin output. 
In figure 4 as well as in figure 1, all the cathodes of all the segments are connected together and this pin is usually connected to ground. Individual segments are turned ON by applying voltage to the required segment pin through the microcontroller by sending a “1” to the pin output. 
A PIC can source or sink 25mA of current per Input/Output pin. When designing an LED circuit, we have to know the typical voltage drop as we have learnt from the Connecting LEDs to a PIC microcontroller article. 
As with standard LEDs, it is required to use current limiting resistors in each segment of the display to limit the current as shown in figure 1. 
The easiest way to display a number on the 7-segment is to find a way to determine or look up the pattern corresponding to the digit to be displayed . This can be something like a table showing the numbers and the corresponding segment that should be turned ON or OFF to display something and the required number (this can be in decimal, hexadecimal or in binary format) to be sent to the port where the display is connected to in order to display a specific number.

Table 1 is an example to show the hexadecimal values of a common cathode 7-Segment display connected to a port of a microcontroller from bit 0 to bit 7. 

Table 1: 7-segment lookup table

Example

Figure 1: 7-Segment display connected to PIC microcontroller

The circuit in figure 1 shows a digit counter counting from 0 to 9 with 1 second delay in between when switch  SW1 is closed. when this switch is open, the counter stops and resume counting when it closed again.

Figure 8: 7-segment example source code

You can download the full project files (MPLAB XC8 source code and Proteus Schematic design) below here. 
All the files are zipped, you will need to unzip them (Download a free version of the Winzip utility to unzip files).  

Download MPLAB XC8 Project: 7-Segment.X

Download Proteus Schematic: 7-Segment Proteus