TM4C1294 LED Control via UART

Overview

This embedded C program provides a command-line interface for controlling LEDs on a Texas Instruments TM4C1294NCPDT microcontroller via UART2 communication. Users can send simple text commands through a serial terminal to turn individual LEDs on or off.

Hardware Requirements

• Microcontroller: TM4C1294NCPDT
• LEDs: Connected to Port M pins PM0-PM3 (4 LEDs total)
• UART Interface: UART2 on Port D
  • RX: PD4
  • TX: PD5
• Baud Rate: 115200 bps, 8 data bits, 1 stop bit, no parity

Features

1. LED Control: Turn on/off any of 4 LEDs individually
2. Command Validation: Strict syntax checking with graceful error handling
3. Interactive Prompt: Visual command prompt for user interaction
4. Echo Display: Received commands are echoed back via printf()
5. Continuous Operation: Infinite loop for repeated command processing

Command Syntax

Commands follow a strict 6-character format:

led<action><number>

• led: Command prefix (required, lowercase)
• action:
  • + to turn LED on
  • - to turn LED off
• number: LED identifier (0, 1, 2, or 3)

Valid Command Examples

• led+0 - Turn on LED 0
• led-0 - Turn off LED 0
• led+1 - Turn on LED 1
• led-3 - Turn off LED 3

Invalid Commands

Any deviation from the syntax is ignored:

• LED+0 (uppercase)
• led+4 (LED number out of range)
• ledon0 (invalid action character)
• led+0 (extra spaces or characters)

How to Use

1. Hardware Setup

• Connect LEDs to Port M pins PM0-PM3 with appropriate current-limiting resistors
• Connect UART2 pins (PD4/PD5) to a USB-to-serial adapter or debugger
• Power the TM4C1294 board

2. Terminal Configuration

Configure your serial terminal (PuTTY, Tera Term, minicom, etc.) with:

• Port: Check device manager for COM port
• Baud Rate: 115200
• Data Bits: 8
• Stop Bits: 1
• Parity: None
• Flow Control: None

3. Running the Program

1. Compile and flash the program to the microcontroller
2. Open your serial terminal
3. Wait for the prompt: > (preceded by carriage return and line feed)
4. Type a command and press Enter
5. The command will be echoed back
6. The corresponding LED will turn on or off
7. A new prompt appears for the next command

Example Session

>led+0
led+0
>led+1
led+1
>led-0
led-0
>led+2
led+2

Code Structure

Main Function (main)

• Initializes Port M (LEDs) and Port D (UART)
• Configures UART2 for 115200 baud communication
• Enters infinite loop:
  • Sends prompt
  • Receives command string (up to 50 characters)
  • Echoes command via printf()
  • Decodes and executes command

Port Initialization (init_port_m)

• Enables clock for Port M
• Configures PM0-PM3 as digital outputs
• Initializes all LEDs to off state

Command Processing (decode_command)

• Validates command syntax character by character
• Extracts action (on/off) and LED number
• Controls GPIO pins via bit manipulation
• Aborts silently on syntax violations

Prompt Display (send_prompt)

• Sends carriage return (0x0D)
• Sends line feed (0x0A)
• Sends '>' character
• Waits for UART transmit buffer availability

Technical Details

UART Configuration

• Clock Frequency: 120 MHz (assumed system clock)
• Baud Rate: 115200
• Integer Baud Rate Divisor (IBRD): 8
• Fractional Baud Rate Divisor (FBRD): 44
• Frame Format: 8-N-1 (8 data bits, no parity, 1 stop bit)

GPIO Pin Mapping

LED	Port M Pin	Bit Position
0	PM0	Bit 0
1	PM1	Bit 1
2	PM2	Bit 2
3	PM3	Bit 3

Buffer Management

• Maximum input buffer size: 50 characters
• Commands terminated by Enter key (0x0D)
• Buffer automatically null-terminated
• Overflow protection included

Error Handling

The program implements silent error handling:

• Invalid commands are ignored without error messages
• No LED state change occurs on invalid input
• System continues waiting for next valid command

Limitations

• Commands must be typed exactly as specified (case-sensitive)
• No feedback for invalid commands
• No command history or editing capabilities
• Maximum command length: 49 characters + null terminator
• Only controls 4 LEDs (expandable by modifying code)

Troubleshooting

LEDs don't respond:

• Verify LED connections to PM0-PM3
• Check LED polarity and current-limiting resistors
• Confirm power supply to microcontroller

No prompt appears:

• Check UART connections (TX/RX may be swapped)
• Verify terminal baud rate settings (115200)
• Ensure UART2 pins are properly configured

Commands don't work:

• Use lowercase only: led+0 not LED+0
• Ensure no spaces: led+0 not led + 0
• Press Enter after typing command
• Verify LED number is 0-3

Extension Ideas

• Add status display showing current LED states
• Implement error messages for invalid commands
• Support multiple LED control in one command
• Add LED blinking or pattern capabilities
• Include help command to display syntax

License

This code is provided for educational purposes. Modify and use as needed for your projects.