7 Segment Display

A Seven Segment Display (SSD) or Seven Segment Indicator is an electronic display device¹ often used to display a single digit number in decimal or hexadecimal form. This post briefly talks about it.

This is what it looks like :

Decimal Display

• A decimal system uses 10 symbols to represent all numbers : 0 1 2 3 4 5 6 7 8 9

• The seven elements of the display can be lit in different combinations to represent the Arabic numerals².

0 1 2 3 4 5 6 7 8 9

• Often we arrange them in an oblique (slanted) way, which aids readability.

0 1 2 3 4 5 6 7 8 9

• By slanting digits, we’re making sure that we don’t mistake 7 for a 1 and vice-versa.

• SSDs may use LCD³s, LED⁴s for each segment, or other light-generating or controlling techniques.

Hexadecimal Display

• Hexadecimal number system uses 16 symbols to represent all numbers : 0 1 2 3 4 5 6 7 8 9 A B C D E F.

• Here A = 10, B = 11, C = 12, D = 13, E = 14, F = 15.

• To display hexadecimal numbers : 0 1 2 3 4 5 6 7 8 9 A B C D E F with a SSD, we needed to think of a way to display letters with our 7 segments without creating an ambigous or non-unique shape.

• Today, we use a combination of uppercase and lowercase letters to display A b (c C) d E F. Otherwise, a capital ‘D’ would look identical to a ‘0’ and a capital ‘B’ would look identical to an ‘8’. However, this modern scheme wasn’t followed in the past.

• Also the digit ‘6’ must be displayed with the top bar lit to avoid ambiguity with the letter ‘b’.

Display Rules

• A 7 segment display has segments which we label as : a b c d e f g for our ease.

• In the device, corresponding to each segment there’s a pin which sends ON or OFF signal to that segment.
• To display an ‘8’ all segments must be switched ON.
• To display a ‘2’, a b g e d must be on.
• Similarly, we can eye-ball rest of the conditions and make a list or a truth table.

Common Cathode & Common Anode SSDs

• When we say Common Cathode or Common Anode in relation to Segment Displays, we assume that all the segments in our SSD are LEDs. A Light Emitting Diode looks like this :

• Much like their diode cousins, LEDs are polarized. There are a handful of identifiers for finding the positive and negative pins on an LED. You can try to find the longer leg, which should indicate the positive, anode pin.

• Or, if someone’s trimmed the legs, try finding the flat edge on the LED’s outer casing. The pin nearest the flat edge will be the negative, cathode pin.

• But, what may be the meanings of Common Anode SSDs & Common Cathode SSDs ? The difference between the two displays, as their name suggests, is that the common cathode has all the cathodes of the 7-segments connected directly together and the common anode has all the anodes of the 7-segments connected together and are illuminated as follows:

• In CC SSDs, all cathode (negative) pins are connected together to logic 0, which means segments are illuminated by supplying logic 1 to respective pins.

• In CA SSDs, all anode (positive) pins are connected together to logic 1, which means segments are illuminated by supplying logic 0 or GND to respective pins.