Seven segment displays are electronic display devices used to display decimal digits as a simpler low cost alternatives to more complex dot matrix displays. The Seven Segment Board, from Zilogic, is an 8 digit seven segment display. In this first article, in a series of articles on the Seven Segment Board, we will introduce the reader to seven segment display fundamentals, and usage of the Seven Segment Board.

Selecting a Digit

A digit of the Seven Segment Board can be selected by using three control lines DSEL0, DSEL1 and DSEL2. The following table shows the DSELx pin states, the corresponding digit selected.

TableDigit Selection
DSEL0 DSEL1 DSEL2 DIGIT Selected
0 0 0 DIGI1
0 0 1 DIGI2
0 1 0 DIGI3
0 1 1 DIGI4
1 0 0 DIGI5
1 0 1 DIGI6
1 1 0 DIGI7
1 1 1 DIGI8

Displaying a Single Digit

The algorithm for displaying a 7-segment digit is given below.

  • Drive the DISPON# high to turn off the display.

  • Turn on/off segments by driving the segment signals SEG[A:H]. The segment signals to be driven for each decimal digit is given in the following table.

  • Select the 7-segment display by driving DSEL[0:2]

  • Drive the DISPON# high to turn on the display.

Figure 1. Segment Naming
TableCharacter to SEG[A:H] Map
TableCharacter to SEG[A:H] Map
Character SEG[A:H] in Binary SEG[A:H] in Hex
0 0b1111_1100 0xFC
1 0b0110_0000 0x60
2 0b1101_1010 0xDA
3 0b1111_0010 0xF2
4 0b1010_1010 0xAA
5 0b1011_1010 0xBA
6 0b1011_1110 0xBE
7 0b1110_0000 0xE0
8 0b1111_1110 0xFE
9 0b1110_0110 0xE6

The MSB of segment signals (7th bit) is used to switch on the dot separator.

Multiplexed 7-Segment Display

Since the signals that drive the segments are shared by the 8 displays, the segments of only one display can be driven at a time. Each display is turned on successively for a small period of time (1.5ms, to avoid flicker), and by persistence of vision all of them appear to be on simultaneously.

Persistence of vision is the phenomenon of the eye by which even nanoseconds of exposure to an image result in milliseconds of sight. — Wikipedia.org

The algorithm for displaying multiple 7-segment digits simultaneously is given below.

  • Turn off the display.

  • Select the next 7-segment digit.

  • Drive the segment data pins corresponding to the value to be displayed.

  • Turn on the display.

  • Generate a 1.5ms delay using a timer.

  • Goto step a.

Credits

The Seven Segment Digit image is based on this Seven Segment image from Wikimedia Commons.

About the Author

Vijay Kumar heads the firmware and application division in Zilogic. He is a technologist and trainer specializing in Linux, Embedded Systems & Python. He has been instrumental in promoting open source software over the past 20 years, by organizing meetups and conferences.

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