Chapter 15 74HC595 & 7-Segment Display

In this chapter, we will introduce the 7-Segment Display.

Project 15.1 7-Segment Display.

We will use 74HC595 to control 7-segment display and make it display hexadecimal character “0-F”.

Component List

ESP32-WROVER x1		GPIO Extension Board x1
Breadboard x1
74HC595	Resistor 220Ω x8	Jumper M/M	7-segment display x1

Component knowledge

7-segment display

A 7-segment display is a digital electronic display device. There is a figure “8” and a decimal point represented, which consists of 8 LEDs. The LEDs have a common anode and individual cathodes. Its internal structure and pin designation diagram is shown below:

As we can see in the above circuit diagram, we can control the state of each LED separately. Also, by combining LEDs with different states of ON and OFF, we can display different characters (Numbers and Letters). For example, to display a “0”: we need to turn ON LED segments A, B, C, D, E and F, and turn OFF LED segments G and DP.

In this project, we will use a 7-Segment Display with a common anode. Therefore, when there is an input low level to a LED segment the LED will turn ON. Defining segment “A” as the lowest level and segment “DP” as the highest level, from high to low would look like this: “DP”, “G”, “F”, “E”, “D”, “C”, “B”, “A”. Character “0” corresponds to the code: 1100 0000b=0xc0.

For detailed code values, please refer to the following table (common anode).

CHAR	DP	G	F	E	D	C	B	A	Hex	ASCII
0	1	1	0	0	0	0	0	0	0xc0	1100 0000
1	1	1	1	1	1	0	0	1	0xf9	1111 1001
2	1	0	1	0	0	1	0	0	0xa4	1010 0100
3	1	0	1	1	0	0	0	0	0xb0	1011 0000
4	1	0	0	1	1	0	0	1	0x99	1001 1001
5	1	0	0	1	0	0	1	0	0x92	1001 0010
6	1	0	0	0	0	0	1	0	0x82	1000 0010
7	1	1	1	1	1	0	0	0	0xf8	1111 1000
8	1	0	0	0	0	0	0	0	0x80	1000 0000
9	1	0	0	1	0	0	0	0	0x90	1001 0000
A	1	0	0	0	1	0	0	0	0x88	1000 1000
B	1	0	0	0	0	0	1	1	0x83	1000 0011
C	1	1	0	0	0	1	1	0	0xc6	1100 0110
D	1	0	1	0	0	0	0	1	0xa1	1010 0001
E	1	0	0	0	0	1	1	0	0x86	1000 0110
F	1	0	0	0	1	1	1	0	0x8e	1000 1110

Circuit

Schematic diagram

Hardware connection

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Sketch

In this section, the 74HC595 is used in the same way as in the previous section, but with different values transferred. We can learn how to master the digital display by sending the coded value of “0” - “F”.

Sketch_15.1_7_Segment_Display

Verify and upload the code, and you’ll see a 1-bit, 7-segment display displaying 0-f in a loop.

The following is the program code:

/**********************************************************************
  Filename    : 1 Digital 7 Segment Display
  Description : Use 74HC595 to drive the digital display
  Auther      : www.freenove.com
  Modification: 2024/06/19
**********************************************************************/
int dataPin = 15;          // Pin connected to DS of 74HC595（Pin14）
int latchPin = 2;          // Pin connected to ST_CP of 74HC595（Pin12）
int clockPin = 4;          // Pin connected to SH_CP of 74HC595（Pin11）

// Define the encoding of characters 0-F for the common-anode 7-Segment Display
byte num[] = {
  0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8,
  0x80, 0x90, 0x88, 0x83, 0xc6, 0xa1, 0x86, 0x8e
};

void setup() {
  // set pins to output
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
}

void loop() {
  // display 0-F on digital tube
  for (int i = 0; i < 16; i++) {
    writeData(num[i]);// Send data to 74HC595
    delay(1000);      // delay 1 second
    writeData(0xff);  // Clear the display content
  }
}

void writeData(int value) {
  // Make latchPin output low level
  digitalWrite(latchPin, LOW);
  // Send serial data to 74HC595
  shiftOut(dataPin, clockPin, LSBFIRST, value);
  // Make latchPin output high level, then 74HC595 will update the data to parallel output
  digitalWrite(latchPin, HIGH);
}

First, put encoding of “0”- “F” into the array.

// Define the encoding of characters 0-F for the common-anode 7-Segment Display
byte num[] = {
  0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8,
  0x80, 0x90, 0x88, 0x83, 0xc6, 0xa1, 0x86, 0x8e
};

Then, in the loop, we transfer the member of the “num” to 74HC595 by calling the writeData function, so that the digital tube displays what we want. After each display, “0xff” is used to eliminate the previous effect and prepare for the next display.

void loop() {
  // display 0-F on digital tube
  for (int i = 0; i < 16; i++) {
    writeData(num[i]);// Send data to 74HC595
    delay(1000);      // delay 1 second
    writeData(0xff);  // Clear the display content
  }
}

In the shiftOut() function, whether to use LSBFIRST or MSBFIRST as the parameter depends on the physical situation.

void writeData(int value) {
  // Make latchPin output low level
  digitalWrite(latchPin, LOW);
  // Send serial data to 74HC595
  shiftOut(dataPin, clockPin, LSBFIRST, value);
  // Make latchPin output high level, then 74HC595 will update the data to parallel output
  digitalWrite(latchPin, HIGH);
}

If you want to display the decimal point, make the highest bit of each array become 0, which can be implemented easily by num[i]&0x7f.

shiftOut(dataPin, clockPin, LSBFIRST, value);