Lesson 8: Independent Key Control of LED Display in Binary Using the Puzhong A2 Microcontroller

Core Principle Overview

The essence of displaying binary with an independent key is to use a variable to record the number of times the key is pressed, and to display the binary value of this variable through 8 LEDs. Each LED represents a binary bit, with the LED on indicating 1 and off indicating 0.

Detailed Principle Breakdown

1. Hardware Principle

• Independent Key: Connected to P3.1, active low
• 8 LEDs: Connected to port P2 (P2.0~P2.7), common anode connection
• P2.0 → Represents the least significant bit (bit 0, 2⁰)
• P2.1 → bit 1 (2¹)
• P2.2 → bit 2 (2²)
• …
• P2.7 → Most significant bit (bit 7, 2⁷)

2. Software Principle

Core Idea:

1. Define a counter variable, initialized to 0
2. Each time the key is pressed, increment the counter by 1
3. Directly assign the value of the counter to port P2, and the LEDs will automatically display the corresponding binary number

Complete Code Implementation

#include <REGX52.H>

sbit KEY1 = P3^1;  // Independent key connected to P3.1

void Delay(unsigned int t) {
    while(t--);
}

void main() {
    unsigned char count = 0;  // Counter, range 0-255
    
    while(1) {
        // Detect key press
        if(KEY1 == 0) {
            Delay(10);        // Debounce on press
            if(KEY1 == 0) {
                count++;      // Increment counter by 1
                P2 = ~count;  // Important! Invert the count value before sending to P2
                
                // Wait for key release
                while(KEY1 == 0);
                Delay(10);    // Debounce on release
            }
        }
    }
}

Code Principle Deep Analysis

Key Statement:<span>P2 = ~count;</span>

Why is inversion necessary?

Because our LEDs are connected in a common anode configuration:

• LED on requires <span>P2.x = 0</span> (low level)
• LED off requires <span>P2.x = 1</span> (high level)

However, the logic for binary display is:

• When the binary bit is 1, the LED should be on
• When the binary bit is 0, the LED should be off

Thus, the inversion operation is necessary:

count = 5 (binary: 0000 0101)
~count = 1111 1010

P2.7 P2.6 P2.5 P2.4 P2.3 P2.2 P2.1 P2.0
  1    1    1    1    1    0    1    0   (P2 output)
 Off  Off  Off  Off  Off  On   Off  On   (LED state)

Actual Display Effects

Key Press Count	Count Value	Binary	LED Display (P2.7~P2.0)	Notes
0	0	0000 0000	Off Off Off Off Off Off Off Off	All Off
1	1	0000 0001	Off Off Off Off Off Off Off On	Only D1 On
2	2	0000 0010	Off Off Off Off Off On Off	Only D2 On
3	3	0000 0011	Off Off Off Off Off On On	D1, D2 On
4	4	0000 0100	Off Off Off Off On Off Off	Only D3 On
…	…	…	…	…
255	255	1111 1111	On On On On On On On On	All On
256	0	0000 0000	Off Off Off Off Off Off Off Off	Restart

Visual Demonstration

Let’s take a look at the display effects for the first few key presses:

Key Press Count   Binary      LED Display (D8...D1)
  0      0000 0000   Off Off Off Off Off Off Off Off
  1      0000 0001   Off Off Off Off Off Off Off On
  2      0000 0010   Off Off Off Off Off Off On Off  
  3      0000 0011   Off Off Off Off Off Off On On
  4      0000 0100   Off Off Off Off Off On Off Off
  5      0000 0101   Off Off Off Off Off On Off On
  6      0000 0110   Off Off Off Off Off On On Off
  7      0000 0111   Off Off Off Off Off On On On
  8      0000 1000   Off Off Off Off On Off Off Off

Advanced Function: Adding Decrement Functionality

If you need key K1 to increment and key K2 to decrement:

#include <REGX52.H>

sbit KEY1 = P3^1;  // Increment key
sbit KEY2 = P3^0;  // Decrement key

void Delay(unsigned int t) {
    while(t--);
}

void main() {
    unsigned char count = 0;
    
    while(1) {
        // Detect KEY1 (increment)
        if(KEY1 == 0) {
            Delay(10);
            if(KEY1 == 0) {
                count++;      // Increment counter by 1
                P2 = ~count;  // Display binary
                while(KEY1 == 0);
                Delay(10);
            }
        }
        
        // Detect KEY2 (decrement)
        if(KEY2 == 0) {
            Delay(10);
            if(KEY2 == 0) {
                count--;      // Decrement counter by 1
                P2 = ~count;  // Display binary
                while(KEY2 == 0);
                Delay(10);
            }
        }
    }
}

Key Points Summary

1. Binary Mapping: Each bit of the counter directly corresponds to an LED

• bit 0 → P2.0 → D1
• bit 1 → P2.1 → D2
• …
• bit 7 → P2.7 → D8

Necessity of Inversion: Due to the common anode connection, the binary 1/0 needs to be inverted to low/high level

Automatic Overflow: When count reaches 255, adding 1 will reset it to 0, achieving cyclic counting

Real-time Display: The display updates immediately after each key press, without the need for additional display programs

Practical Learning Significance

This experiment helps you understand:

• ✅ The concept and representation of binary numbers
• ✅ The relationship between bits and bytes
• ✅ How variables are stored in memory
• ✅ Conversion between binary and decimal
• ✅ Bit manipulation of microcontroller I/O ports

Conclusion

The principle of independent key control of binary display using the Puzhong A2 microcontroller is to use a counter to record the number of key presses, and to send the inverted binary value of the counter to port P2, using 8 LEDs to visually display each bit of the binary number.