# Tiva Lab 03: Seven Segment Display

## Objective

• Learn how to use a seven-segment display
• Learn how to use an array to store the predefined values

## Background Information

In this lab, you are going to interface a seven-segment display to Tiva LaunchPad. The display counts from 0 ~ F and resets itself to zero. Before going further, let us first discuss seven-segment displays.

#### Seven Segment Display

If your embedded system only needs to display numbers, you can consider using a seven-segment display. The seven-segment display is a simple device. It has 8 LEDs (the decimal point is the 8th) and can be lit in different combinations to represent the Arabic numerals and some English characters.

The segments of a seven-segment display are referred to by the letters A to G, and an optional decimal point (an "eighth segment", referred to as DP) for the display of non--integer numbers.

There are mainly two types of seven-segment displays available. In a simple LED package as shown in the figure above, typically all of the cathodes (negative terminals) or all of the anodes (positive terminals) of the segment LEDs are connected and brought out to a common pin; this is referred to as a "common cathode" or a "common anode" device. Hence, a 7 segment plus decimal point package will only require nine pins, though commercial products typically contain more pins, and/or places where pins would go, in order to match standard IC sockets.

In general, common anode displays are more popular than common cathode displays, since logic circuits can sink more current than they can source.

#### How to display a number on the seven segment display?

Depending upon which character will be displayed, the particular set of LEDs is forward biased.

Display a "0"

To display the numerical digit "0" on the display as shown in the figure above, the LED segments corresponding to a, b, c, d, e, and f must be lit up.
For common cathode display, these pins must be connected to power, and for common anode display, these pins must be connected to the ground.

Display a "1"

To display the numerical digit "1" on the display as shown in the figure above, the LED segments corresponding to b, c must be lit up.
For common cathode display, these pins must be connected to power, and for common anode display, these pins must be connected to the ground.

#### Displaying Letters

A single byte can encode the full state of a 7-segment-display. The most popular bit encodings are gfedcba and abcdefg, where each letter represents a particular segment in the display. In the gfedcba representation, a byte value of 0x06 would (in a common anode circuit) lit up segments 'c' and 'b', which would display a '1'.

Hexadecimal digit can be displayed on a seven-segment display. A combination of uppercase and lowercase letters is used for letters 'A' to 'F'; this is done in order to obtain a unique, unambiguous shape for each hexadecimal digit (otherwise, a capital 'D' would look identical to a '0' and a capital 'B' would look identical to an '8'). Also the digit '6' must be displayed with the top bar lit to avoid ambiguity with the lowercase letter 'b'.

The following table applies a common cathode seven segment display device. If you are using a common anode, the values in the table must be inverted.

Table 1: Seven Segment Display Truth Table for Hexadecimal Encodings

 Digit Display Individual Segments Illuminated Port [7:0] Port [7:0] dp g f e d c b a dp.g.f.e.d.c.b.a dp.a.b.c.d.e.f.g 0 0 0 1 1 1 1 1 1 0x3F 0x7E 1 0 0 0 0 0 1 1 0 0x06 0x30 2 3 4 5 6 7 8 9 A B C D E F

When you connect seven segment displays in a circuit, a current limiting resistor must be wired in series with each display segment. Any resistor value between 100Ω and 1KΩ will work for the LED resistor. The lower resistance will result in a brighter segment. It is best to use resistors of the same value so all the segments light up evenly.

## Required Components List

 220 Ω Resistor x 8 7-Segment Display x 1 Breadboard x 1

## Circuit / Schematic Diagram

The circuit diagram for connecting a common cathode seven segment display to the TI Tiva microcontroller port is shown below. The microcontroller I/O pins source the current required for the segment LEDs. The current limiting resistors connected in series between the microcontroller pina and segment LEDs are 220ohm each. Segments a ~ g and dp are driven through GPIO output ports.

## Procedure

Wire a circuit for the 7-segment display on the breadboard with eight current limiting resistors, then connect wires to the TI Tiva board. Make sure the power is disconnected when wiring up your breadboard.

The predefined hexadecimal encoding values need to be shifted to the corresponding bit in order to light up the correct segment LED on different Ports.

## Lab Experiments

Comment out the for-loop code in the main function, inside the while-loop.

    while (1) {
// Place your application code here
//        for (i = 0; i < sizeof(sevenSeg); i++) {
//            SegDisplay(ch & 0x0F);
//            ch++;
//            DelayMs(1000);
//        }

}