5.2 Number Display

7 Segment Display can be seen everywhere in life. For example, on an air conditioner, it can be used to display temperature; on a traffic indicator, it can be used to display a timer.

The 7 Segment Display is essentially a device packaged by 8 LEDs, of which 7 strip-shaped LEDs form an “8” shape, and there is a slightly smaller dotted LED as a decimal point. These LEDs are marked as a, b, c, d, e, f, g, and dp. They have their own anode pins and share cathodes. Their pin locations are shown in the figure below.

img_7seg_cathode

This means that it needs to be controlled by 8 digital signals at the same time to fully work and the 74HC595 can do this.

Schematic

sch_74hc_7seg

Here the wiring principle is basically the same as 5.1 Microchip - 74HC595, 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

wiring_74hc_7seg

Code

Note

  • Open the 5.2_number_display.py file under the path of kepler-kit-main/micropython or copy this code into Thonny, then click “Run Current Script” or simply press F5 to run it.

  • Don’t forget to click on the “MicroPython (Raspberry Pi Pico)” interpreter in the bottom right corner.

  • For detailed tutorials, please refer to Open and Run Code Directly.

import machine
import time

SEGCODE = [0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f]

sdi = machine.Pin(0,machine.Pin.OUT)
rclk = machine.Pin(1,machine.Pin.OUT)
srclk = machine.Pin(2,machine.Pin.OUT)

def hc595_shift(dat):
    rclk.low()
    time.sleep_ms(5)
    for bit in range(7, -1, -1):
        srclk.low()
        time.sleep_ms(5)
        value = 1 & (dat >> bit)
        sdi.value(value)
        time.sleep_ms(5)
        srclk.high()
        time.sleep_ms(5)
    time.sleep_ms(5)
    rclk.high()
    time.sleep_ms(5)

while True:
    for num in range(10):
        hc595_shift(SEGCODE[num])
        time.sleep_ms(500)

When the program is running, you will be able to see the LED Segment Display display 0~9 in sequence.

How it works?

hc595_shift() will make 74HC595 output 8 digital signals. It outputs the last bit of the binary number to Q0, and the output of the first bit to Q7. In other words, writing the binary number “00000001” will make Q0 output high level and Q1~Q7 output low level.

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.

img_1_segment

That is, the binary number “00000110” needs to be written. For readability, we will use hexadecimal notation as “0x06”.

Similarly, we can also make the LED Segment Display display other numbers in the same way. The following table shows the codes corresponding to these numbers.

Glyph Code

Numbers

Binary Code

Hex Code

0

00111111

0x3f

1

00000110

0x06

2

01011011

0x5b

3

01001111

0x4f

4

01100110

0x66

5

01101101

0x6d

6

01111101

0x7d

7

00000111

0x07

8

01111111

0x7f

9

01101111

0x6f

Write these codes into hc595_shift() to make the LED Segment Display display the corresponding numbers.