Developers' Corner, NIT Silchar, India: SEVEN SEGMENT DISPLAY (SSD) INTERFACE ON 8051

SEVEN SEGMENT DISPLAY (SSD)

It is an output device and this is how it looks like so don't get confused when you face it as you're about to get familiar with it in the most basic way. Don't worry, I will explain the output pins soon enough.

Seven Segment Display (SSD)

As you can see, it's called Seven Segment Display because it has actually got Seven Segments (that's too obvious I guess). These seven segments can be used to display different shapes but it is designed such that it's meaningful to display only numbers on it. And this is how they will actually look like

SSD Output (Numbers)

You can think of the segments as individual LEDs which can be turned ON/OFF. There you are, I just told you the working secret behind it. So when it comes down to LEDs, you already know that it looks like this

LED Symbol

And I hope you also know that it will glow ONLY when current flows from ANODEtowards CATHODE. Before moving forward, you can read my article about LEDs and interfacing them to 8051 if you want to know more about them. However, let's stay focused on SSD here. SSDs are also composed of individual LEDs and work on the very same principle. So basically they come in two forms

Common Anode
Common Cathode

Common Cathode

Common Anode

Pin Description of SSD

FOR COMMON CATHODE

Connect INPUT VOLTAGE to A
Connect GROUND to COMMON (either upper or lower as both are shorted)

FOR COMMON ANODE

Connect INPUT VOLTAGE to COMMON
Connect GROUND to A

8051 SSD INTERFACE

Till now, you might have noticed that displaying something on the SSD is all about giving it the right type of sequence. The SSD itself is just a combination of LEDs. It's your job to know what you're doing. For instance, you have to turn ON segment 'b' and 'c' for displaying '1' (verify from above diagram). You require seven control lines to completely utilize an SSD for display i-e a, b, c, d, e, f, and g (dot is optional). As for the common pin, connect it to

GROUND for Common Cathode
3-5V for Common Anode

For interfacing it with 8051,you can use 74LS47 IC. It is basically BCD to 7-segment Decoder/Driver which is specially designed for driving an SSD. It's function is such that it gives out the corresponding sequence for SSD when the microcontrollers sends a number 0-9. You can find more details about the IC in it's Datasheet Here is the truth table for common cathode configuration

BCD to 7-SEGMENT Decoder Truth Table

As you can see, providing the number (0-9) as INPUT will produce the correspondingSEQUENCE at its output which is used to display that number on the SSD. However, I'm not going to use this IC for interfacing. Why complicate things by introducing some extra hardware when you can manage it through software? I am going to present an interfacing scheme for SSD which doesn't include any other component. I won't keep you waiting, here it is

8051 SSD Interface

And trust me when I say it, it's as simple as it looks. Nothing more, nothing less. I have directly connected the SSD to 8051 because the MCU don't have the 'guts' to damage it and the program code can take care of the sequences which is even more simpler! You can have a look by yourself