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 will 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 composed of 8 LEDs (the decimal point being the 8th). These LEDs can be lit in different combinations to represent Arabic numerals and some English characters.

Figure 1: The 128 States of a Seven-Segment Display

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

Types of Seven-Segment Displays

There are two main 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 to match standard IC sockets.

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

Displaying Numbers on a Seven-Segment Display

To display a number, you need to forward-bias the specific set of LEDs corresponding to the desired character.

Display a "0"

To display the numerical digit "0," you must light up the LED segments corresponding to a, b, c, d, e, and f.
• Common Cathode: Connect these pins to power.
• Common Anode: Connect these pins to ground.
Display a "1"

To display the numerical digit "1," you must light up the LED segments corresponding to b and c.
• Common Cathode: Connect these pins to power.
• Common Anode: Connect these pins to ground.

Hexadecimal digits can also be displayed on a seven-segment display. A combination of uppercase and lowercase letters is used for letters 'A' to 'F' to ensure unique shapes for each hexadecimal digit. For example, a capital 'D' would look identical to a '0', and a capital 'B' would look identical to an '8' if not for this distinction. Additionally, the digit '6' must be displayed with the top bar lit to avoid ambiguity with the lowercase letter 'b'.

Encoding Methods for Seven-Segment Displays

A single byte can encode the full state of a seven-segment display. The most popular bit encodings is dpgfedcba and dpabcdefg, where each letter represents a particular segment in the display.

• 'dp.g.f.e.d.c.b.a' encoding: Encode segments a to g to bit 0 to bit 6, with the decimal point (dp) on bit 7.
• 'dp.a.b.c.d.e.f.g' encoding: Encode segments a to g to bit 6 to bit 7, with the decimal point (dp) on bit 7.

For example, in the dp.g.f.e.d.c.b.a encoding, a byte value of 0x06 would light up segments 'c' and 'b', displaying the number '1'.

Creating a Lookup Table for Hexadecimal Digits

A seven-segment display comprises 8 segments; we will label them a, b, c, d, e, f,  g, and dp. This is the standard way to label each segment LED. Now, we need to make a lookup table to encode hexadecimal digits.

Here, we consider the decimal point (dp) pins are not used, so the dp values are always set to 0. We only consider segments a to f. Now we have to decide for every hex number (0 to F) and figure out what segments must be lit.

Example:

• Display '0':
• We want a, b, c, d, e, f on and g off.
• In the table, put '1' on segments a, b, c, d, e, f, and '0' on segment g.
• So, the output for the "dp.g.f.e.d.c.b.a" encoding is 0, 0, 1, 1, 1, 1, 1, 1. The binary value is 0b0011 1111 (or hex value 0x3F).
• Display '1':
• We want b and c on, and the rest of the segments are off.
• In the table, put '1' on segments b and c, and '0' on the rest of the segments.
• So, the output for the "dp.g.f.e.d.c.b.a" encoding is 0, 0, 0, 0, 0, 1, 1, 0. The binary value is 0b0000 0110 (or hex value 0x06)

Below is a lookup table for hexadecimal encoding using a common cathode seven-segment display. If using a common anode display, the values must be inverted. (Hover over each character with your mouse to see which segments need to be illuminated to display it.)

Table 1: Seven-Segment Display Lookup 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
Using Current Limiting Resistors

When connecting 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. A 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.

 dp g f e d c b a Pins on 7-Seg Display 7 6 5 4 3 2 1 0 Data bit PD7 PC7 PC6 PC5 PC4 PA4 PA3 PA2 GPIO Port/Pin

Pin configurations:

DevicePort.Pin Signal TypePCTLDirectionDrive Mode

Procedures

1. Create a new folder under the EE3450 folder and name it Lab03_7SegDisplay.
2. Launch the Keil μVisio and create a new project. Save the project to the folder you created in the previous step and set the project name to Lab03_7SegDisplay.
3. Add MyDefines.h to the Source Group, then add the Common folder to the include paths which is under the "Options for Target" setting.

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 must be shifted to the corresponding bit 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);
//        }

}

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