NEC Infrared Protocol Decoding and LCD Display Based on PIC16F877A in Proteus Simulation

This example is a solution for NEC infrared protocol decoding and LCD display based on the PIC16F877A, written using CCS C Compiler V5.015, and supports Proteus simulation (the compressed package includes the remote control model).

Detailed Explanation of NEC Infrared Protocol

1. Protocol Overview

The NEC protocol is a widely used infrared remote control protocol standard in consumer electronics (TVs, air conditioners, set-top boxes, etc.), characterized by the following features:

Carrier Frequency: 38kHz (duty cycle 1/3)
Modulation Method: Pulse Position Modulation (PPM)
Data Format: 32-bit frame structure (including address, command, and inverse code)
Transmission Distance: Typical 5-8 meters
Advantages: Simple hardware implementation, strong anti-interference capability

2. Protocol Frame Structure

A complete frame of data includes:

Component	Bits	Description
Lead Code	–	9ms high level + 4.5ms low level
User Code (Address)	8	Device identification (e.g., TV brand code)
User Code Inverse	8	User code bitwise inverse (for verification)
Command Code	8	Key function code (e.g., volume +)
Command Inverse	8	Command code bitwise inverse (for verification)

Example Frame (Hexadecimal):

Address: 0x12 → Inverse: 0xED

Command: 0x34 → Inverse: 0xCB

Complete Frame: 0x12ED34CB

3. Encoding Logic

Logic 0: 560μs high level + 560μs low level (total period 1.125ms)
Logic 1: 560μs high level + 1.69ms low level (total period 2.25ms)

4. Special Signals

Repeat Code (sent when the button is pressed for a long time):

9ms high level + 2.25ms low level + 560μs high level
Does not carry data, indicates the previous command is repeated
Sent every 110ms

Lead Code Function:

Helps the receiver synchronize the clock
Distinguishes noise from valid signals

5. Transmission Timing

Starting Phase: Send lead code
Data Transmission: Send 32-bit data in LSB (least significant bit first) order
End Flag: Maintain high level for 560μs after the last bit of data
Repeat Code: If the button is held down, send the repeat code

6. Protocol Variants

Extended NEC: Uses 16-bit address (original address + extended address)
NECx2: Doubles the carrier frequency (76kHz) for high-speed transmission
Non-standard NEC: Some manufacturers modify timing parameters (decoding threshold needs adjustment)

7. Key Points for Hardware Implementation

Transmitter:

38kHz carrier emitted through infrared LED
Use timer to generate precise timing

Receiver:

Integrated receiver head such as VS1838B/HS0038
Outputs demodulated digital signal (no carrier)

Design Approach

Infrared Signal Acquisition: Use RB0 as an external interrupt pin to capture the falling edge signal of the NEC protocol
Protocol Decoding: Identify the lead code, logic 0/1, and repeat code through time measurement
Multi-remote Switching: Single-pole triple-throw switch connects different remotes
LCD Display: 4-bit data mode drives LM016L, displaying the decoded key values

Main Program Code

void main(){

output_b(0); // PORTB initial state

set_tris_b(1);

lcd_init(); // Initialize LCD module

lcd_putc(‘\f’); // LCD clear

lcd_gotoxy(3, 1); // Go to column 3 row 1

lcd_putc(“NEC Decoding”);

delay_ms(2000);

lcd_putc(‘\f’);

lcd_gotoxy(1, 1); // Go to column 1 row 1

printf(lcd_putc, “Address:0x0000”);

lcd_gotoxy(1, 2); // Go to column 1 row 2

printf(lcd_putc, “Com:0x00 In:0x00”);

while(TRUE){

while(input(PIN_B0)); // Wait until RB0 pin falls

if(nec_remote_read()){

address = ir_code >> 16;

command = ir_code >> 8;

inv_command = ir_code;

lcd_gotoxy(11, 1);

printf(lcd_putc,”%4LX”,address);

lcd_gotoxy(7, 2);

printf(lcd_putc,”%2X”,command);

lcd_gotoxy(15, 2);

printf(lcd_putc,”%2X”,inv_command);

delay_ms(200);}

}

Key Configuration for Proteus Simulation

Microcontroller Settings:

PIC16F877A @ 4MHz
Configuration word: HS oscillator, disable watchdog

Infrared Receiving Circuit:

Three IR Receivers (VS1838B) outputs connected to a single-pole triple-throw switch
Common terminal of the single-pole triple-throw switch connected to RB0

LCD Connection:

PIC16F877A LM016L

RD0 ——> RS

RD1 ——> RW

RD2 ——> E

RD3 ——> D4

RD4 ——> D5

RD5 ——> D6

RD6 ——> D7

Function Description

Remote Control Switching:

Toggle the switch to select different remotes
LCD displays the current remote control number

Decoding Display:

When decoded correctly, display “Address:0x00XX”, “Com:0xXX In:0xXX” (XX is the hexadecimal key value).

Protocol Handling:

Automatically identify NEC standard frames and repeat frames
Verify the inverse check of address and command
Timeout detection to prevent deadlock

Debugging Key Points

If decoding fails, check:

Power supply of the infrared receiver (needs 5V)
Interrupt trigger edge settings
Timing measurement threshold (can be adjusted ±10%)

If LCD display is abnormal:

Check control line connections
Adjust initialization delay
Confirm 4-bit data mode configuration

This solution fully implements multi-remote selection, NEC protocol decoding, and LCD display functions, and can be directly used for Proteus simulation and actual hardware deployment.

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