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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	LINK
Pico 2 W Starter Kit	450+	Pico 2 W Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Getting to Know Pico 2 W	1
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	1(220Ω)	BUY
6	7-segment Display	1	BUY
7	74HC595	1	BUY

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.

Wiring
74HC595	LED Segment Display
Q0	a
Q1	b
Q2	c
Q3	d
Q4	e
Q5	f
Q6	g
Q7	dp

Wiring

Writing the Code

Let’s write a MicroPython program to display digits from 0 to 9 on the 7-segment display.

Note

Open the 5.2_number_display.py from pico-2w-kit-main/micropython or copy the code into Thonny, then click “Run” or press F5.
Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Define the binary codes for each digit (0-9)
SEGMENT_CODES = [
    0x3F,  # 0
    0x06,  # 1
    0x5B,  # 2
    0x4F,  # 3
    0x66,  # 4
    0x6D,  # 5
    0x7D,  # 6
    0x07,  # 7
    0x7F,  # 8
    0x6F   # 9
]

# Initialize the control pins for 74HC595
SDI = machine.Pin(0, machine.Pin.OUT)   # Serial Data Input (DS)
RCLK = machine.Pin(1, machine.Pin.OUT)  # Register Clock (STCP)
SRCLK = machine.Pin(2, machine.Pin.OUT) # Shift Register Clock (SHCP)

# Function to send data to 74HC595
def shift_out(data):
    RCLK.low()
    for bit in range(7, -1, -1):
        SRCLK.low()
        bit_val = (data >> bit) & 0x01
        SDI.value(bit_val)
        SRCLK.high()
    RCLK.high()

# Main loop to display numbers 0-9
while True:
    for num in range(10):
        shift_out(SEGMENT_CODES[num])
        utime.sleep(0.5)

When you run this code, the 7-segment display will sequentially display the digits 0 to 9, changing every 0.5 seconds. This creates a looping counting effect where the numbers increment one by one, and after reaching 9, the display returns to 0 and repeats the cycle continuously.

Explanation of the Code

Import Modules:
- machine: Provides access to GPIO pins and hardware functions.
- utime: Contains time-related functions for delays.
Define Segment Codes:

Each entry corresponds to the segments that need to be lit to display a digit. The values are in hexadecimal format for readability.
```
SEGMENT_CODES = [
    0x3F,  # 0
    0x06,  # 1
    0x5B,  # 2
    0x4F,  # 3
    0x66,  # 4
    0x6D,  # 5
    0x7D,  # 6
    0x07,  # 7
    0x7F,  # 8
    0x6F   # 9
]
```
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”.

Initialize Control Pins:

Assigns the Pico’s GPIO pins to control the 74HC595.

SDI = machine.Pin(0, machine.Pin.OUT)
RCLK = machine.Pin(1, machine.Pin.OUT)
SRCLK = machine.Pin(2, machine.Pin.OUT)

Define the shift_out Function:

Sends 8 bits of data to the 74HC595.
Shifts out the data starting from the most significant bit (MSB).
Pulses the shift and register clocks appropriately.

def shift_out(data):
    RCLK.low()
    for bit in range(7, -1, -1):
        SRCLK.low()
        bit_val = (data >> bit) & 0x01
        SDI.value(bit_val)
        SRCLK.high()
    RCLK.high()

Main Loop to Display Numbers:
- Iterates through the numbers 0 to 9.
- Calls shift_out with the corresponding segment code.
- Adds a delay of 0.5 seconds between each number.
```
while True:
    for num in range(10):
        shift_out(SEGMENT_CODES[num])
        utime.sleep(0.5)
```

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)

Experimenting Further

Display Hexadecimal Characters:

Extend the SEGMENT_CODES list to include letters A-F for hexadecimal representation. For example, to display ‘A’, the segment code is 0x77.
Create a Counter:

Modify the code to create an up-counter or down-counter. Use button inputs to increment or decrement the displayed number.
Control Multiple Displays:

Use additional 74HC595 shift registers to control multiple 7-segment displays. Implement multiplexing to manage multiple displays with minimal GPIO usage.

Conclusion

In this lesson, you’ve learned how to use a 7-segment display with a 74HC595 shift register to display numbers using the Raspberry Pi Pico 2 W. By understanding how to control each segment through binary codes and utilizing the shift register, you can efficiently manage multiple outputs with limited GPIO pins.