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5.2 Displaying Numbers
In this lesson, we’ll learn how to use a 7-segment display to show numbers using the Raspberry Pi Pico 2 W and a 74HC595 shift register. The 7-segment display is a common electronic component used in devices like digital clocks, calculators, and appliances to display numerical information.
By combining the 74HC595 shift register with the 7-segment display, we can control all the segments using only a few GPIO pins on the Pico, saving valuable I/O resources for other components. * 7-segment Display
Required Components
In this project, we need the following components.
It’s definitely convenient to buy a whole kit, here’s the link:
Name |
ITEMS IN THIS KIT |
PURCHASE LINK |
|---|---|---|
Pico 2 W Starter Kit |
450+ |
You can also buy them separately from the links below.
SN |
COMPONENT INTRODUCTION |
QUANTITY |
PURCHASE LINK |
|---|---|---|---|
1 |
1 |
||
2 |
Micro USB Cable |
1 |
|
3 |
1 |
||
4 |
Several |
||
5 |
1(220Ω) |
||
6 |
1 |
||
7 |
1 |
Understanding the 7-Segment Display
A 7-segment display consists of 7 LEDs (segments) arranged in a figure-eight pattern to display digits from 0 to 9. There’s also an eighth LED for the decimal point. Each segment is labeled from a to g, and the decimal point is labeled dp.
Here’s the segment labeling:
In a common cathode 7-segment display, all the cathodes (negative sides) of the LEDs are connected together to a common ground.
Schematic
Here the wiring principle is basically the same as 5.1 Using the 74HC595 Shift Register, the only difference is that Q0-Q7 are connected to the a ~ g pins of the 7 Segment Display.
74HC595 |
LED Segment Display |
|---|---|
Q0 |
a |
Q1 |
b |
Q2 |
c |
Q3 |
d |
Q4 |
e |
Q5 |
f |
Q6 |
g |
Q7 |
dp |
Wiring
Writing the Code
We’ll write a program that controls the 7-segment display by sending serial data to the 74HC595 shift register. The display will cycle through the numbers 0 to 9 in sequence.
Note
You can open the file
5.2_number_display.inounder the path ofpico-2w-kit-main/arduino/5.2_number_display.Or copy this code into Arduino IDE.
Don’t forget to select the board(Raspberry Pi Pico) and the correct port before clicking the Upload button.
// Define the pins connected to the 74HC595
const int DS = 0; // GPIO 0 -> DS (Pin 14)
const int SHCP = 1; // GPIO 1 -> SHCP (Pin 11)
const int STCP = 2; // GPIO 2 -> STCP (Pin 12)
// Array of hexadecimal codes for digits 0-9 on a common cathode 7-segment display
const byte numArray[] = {
0x3F, // 0: 00111111
0x06, // 1: 00000110
0x5B, // 2: 01011011
0x4F, // 3: 01001111
0x66, // 4: 01100110
0x6D, // 5: 01101101
0x7D, // 6: 01111101
0x07, // 7: 00000111
0x7F, // 8: 01111111
0x6F // 9: 01101111
};
void setup() {
// Initialize the control pins as outputs
pinMode(DS, OUTPUT);
pinMode(SHCP, OUTPUT);
pinMode(STCP, OUTPUT);
}
void loop() {
// Iterate through each number 0-9
for (int num = 0; num < 10; num++) {
// Set STCP to LOW to prepare for data
digitalWrite(STCP, LOW);
// Shift out the data to the shift register
shiftOut(DS, SHCP, MSBFIRST, numArray[num]);
// Set STCP to HIGH to latch the data to the output pins
digitalWrite(STCP, HIGH);
delay(1000); // Wait for one second before displaying the next number
}
// Turn off all segments after displaying 0-9
digitalWrite(STCP, LOW);
shiftOut(DS, SHCP, MSBFIRST, 0x00);
digitalWrite(STCP, HIGH);
delay(1000);
}
After uploading the code, the display should cycle through the numbers 0 to 9, showing each number for one second. After reaching 9, all segments should turn off for one second before starting the sequence again.
Understanding the Code
Defining Control Pins:
DS (Data Serial Input): Receives serial data to be shifted into the register.SHCP (Shift Register Clock Input): Controls the shifting of data into the register.STCP (Storage Register Clock Input): Controls the latching of data from the shift register to the output pins.
const int DS = 0; // GPIO 0 -> DS (Pin 14) const int SHCP = 2; // GPIO 2 -> SHCP (Pin 11) const int STCP = 1; // GPIO 1 -> STCP (Pin 12)
Creating Data Patterns:
numArray: An array holding the hexadecimal codes for displaying numbers 0-9 on a common cathode 7-segment display.Each hexadecimal value corresponds to the segments that need to be lit to display a particular number.
const byte numArray[] = { 0x3F, // 0: 00111111 0x06, // 1: 00000110 0x5B, // 2: 01011011 0x4F, // 3: 01001111 0x66, // 4: 01100110 0x6D, // 5: 01101101 0x7D, // 6: 01111101 0x07, // 7: 00000111 0x7F, // 8: 01111111 0x6F // 9: 01101111 };
Suppose that the 7-segment Display display the number “1”, we need to write a high level for b, c, and write a low level for a, d, e, f, g, and dg.
That is, the binary number “00000110” needs to be written. For readability, we will use hexadecimal notation as “0x06”.
Setup Function:
Sets the
DS,SHCP, andSTCPpins as outputs to send data to the shift register.void setup() { // Initialize the control pins as outputs pinMode(DS, OUTPUT); pinMode(SHCP, OUTPUT); pinMode(STCP, OUTPUT); }
Loop Function: The
forloop cycles through each pattern in thenumArrayarray.Shifting Out Data:
shiftOutsends the byte of data one bit at a time.MSBFIRSTindicates that the most significant bit is sent first.
shiftOut(DS, SHCP, MSBFIRST, numArray[num]);
Latching Data:
Setting
STCPLOWprepares the shift register for new data.After shifting out the data, setting
STCPHIGHlatches the data to the output pins, updating the 7-segment display.
digitalWrite(STCP, LOW); // shiftOut(...) digitalWrite(STCP, HIGH);
delay(500);adds a half-second pause between each pattern for visibility.Turning Off All Segments: After displaying numbers 0-9, the code sends 0x00 to turn off all segments. The display remains off for one second before the loop repeats.
digitalWrite(STCP, LOW); shiftOut(DS, SHCP, MSBFIRST, 0x00); digitalWrite(STCP, HIGH); delay(1000);
Troubleshooting
No Numbers Displayed:
Check all wiring connections.
Ensure the 74HC595 is properly powered.
Verify that the GPIO pins on the Pico are correctly connected to the shift register.
Make sure the 7-segment display is connected correctly, with each segment connected through a resistor.
Incorrect Numbers Displayed:
Double-check the hexadecimal codes in numArray.
Ensure that the shift register outputs are correctly connected to the corresponding segments.
Flickering or Unstable Display:
Verify that the power connections are stable.
Ensure that the resistors are properly connected to limit the current to each segment.
Understanding the Segment Codes
Each segment code corresponds to the segments that need to be illuminated to display a specific digit. Here’s how the segments map to each digit:
0: Segments a, b, c, d, e, f (code 0x3F)
1: Segments b, c (code 0x06)
2: Segments a, b, g, e, d (code 0x5B)
3: Segments a, b, c, d, g (code 0x4F)
4: Segments b, c, f, g (code 0x66)
5: Segments a, c, d, f, g (code 0x6D)
6: Segments a, c, d, e, f, g (code 0x7D)
7: Segments a, b, c (code 0x07)
8: Segments a, b, c, d, e, f, g (code 0x7F)
9: Segments a, b, c, d, f, g (code 0x6F)
Further Exploration
Controlling Multiple 7-Segment Displays:
Chain multiple 74HC595 shift registers to control additional 7-segment displays, enabling multi-digit displays.
Implementing LED Animations:
Create dynamic animations or scrolling text by modifying the data patterns sent to the shift register.
Integrating with Sensors:
Combine the 7-segment display with sensors (e.g., temperature, light) to display real-time data.
Building a Digital Clock:
Use multiple 7-segment displays and real-time clock modules to create a functional digital clock.
Adding Decimal Points and Indicators:
Utilize the decimal point (dp) and additional indicators (e.g., colons) for more complex displays.
Conclusion
In this lesson, you’ve learned how to use the 74HC595 shift register with the Raspberry Pi Pico to control a 7-segment display. By sending serial data to the shift register, you can efficiently manage multiple outputs using just a few GPIO pins. This technique not only conserves valuable I/O resources but also opens up possibilities for expanding your projects with more LEDs, displays, or other peripherals.