UART Break/Autobaud

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Microchip UART modules support advanced communication features including Break character transmission/detection and Auto-Baud rate detection for robust communication in systems where baud rates are unknown or need synchronization.

Overview

Microchip UART modules support advanced communication features including Break character transmission/detection and Auto-Baud rate detection. These features enable robust communication in systems where baud rates are unknown or need synchronization, and provide special signaling capabilities for protocol implementations.

When to use:

• LIN bus communication — Break + Sync byte pattern for LIN header
• DMX512 protocol — Break signals frame start in DMX lighting control
• Auto-baud detection — Synchronize to unknown baud rate using 0x55 sync byte
• Multi-drop networks — Break as attention/wakeup signal for slave devices
• Protocol signaling — Break as special control character in custom protocols

When NOT to use:

• Standard UART communication — Use UART Tx/Rx blocks without Break/Autobaud
• Fixed baud rate — Configure baud in UART Config block (no autobaud needed)
• CAN/SPI/I2C — Use protocol-specific blocks (not UART-based)
• External Mode — XCP protocol doesn’t use Break/Autobaud features
• Already synchronized — Once baud rate established, no need for repeated autobaud

Device Support

Examples

Example 1: LIN Bus Communication

LIN (Local Interconnect Network) uses Break + Sync pattern:

Example 2: DMX512 Protocol

DMX512 uses Break for frame start signaling: // Send DMX Break (Mark-After-Break timing) U1CON |= 0x0200; // BRKOVR=1 delay_us(88); // Break: 88-1000 μs U1CON &= ~0x0200; // BRKOVR=0 delay_us(8); // MAB: 8-1000 μs U1TXB = 0x00; // Start code

Example 3: Unknown Baud Rate Detection

Troubleshooting

Auto-Baud Not Working

Problem: ABDEN set but no auto-baud completion.Solutions:

• Verify transmitter sends exactly 0x55 (alternating bit pattern)
• Check UART clock source (FUART) is correct
• Ensure no framing errors (check FERR flag)
• Verify ABDOVIF flag - if set, BRG rolled over (increase clock speed or lower baud range)

Break Not Detected

Problem: Break sent but RXBKIF never sets.Solutions:

• Check Break duration ≥11 bit times (use oscilloscope)
• Verify RXBIMD setting matches detection criteria needed
• Ensure UART RX pin correctly configured
• Check line polarity (RXPOL bit if available)

Incorrect Baud Rate After Auto-Baud

Problem: Auto-baud completes but calculated rate is wrong.Solutions:

• Verify peripheral clock (FCY/FUART) setting
• Check BRGH and CLKMOD bit settings
• Ensure Sync byte 0x55 transmitted correctly (verify with logic analyzer)
• Read UxBRG value after auto-baud to debug calculation

Break Character Feature

What is a Break Character?

A Break character is a special UART signal consisting of a continuous low (or high if polarity inverted) state for an extended period - typically 11 to 13 bit times. It’s used for:

• Communication synchronization and initialization
• Protocol signaling (LIN, DMX512, etc.)
• Error recovery and attention signaling
• Multi-drop bus addressing

Break Transmission

Method 1: Automatic Break (SENDB)

Set the SENDB bit (UxCON[8]) and write any value to UxTXB: U1CON |= 0x0100; // Set SENDB bit U1TXB = 0x00; // Trigger Break transmission (value ignored) // Break (13 bit times) is sent, followed by UxTXB contents

Method 2: Manual Override (BRKOVR)

Use BRKOVR bit (UxCON[9]) for custom-duration Break: U1CON |= 0x0200; // Set BRKOVR - manually drive TX line // TX line driven low (if TXPOL=0) or high (if TXPOL=1) delay_us(break_duration); U1CON &= ~0x0200; // Clear BRKOVR - release TX line

• TXPOL=0: TX line driven LOW
• TXPOL=1: TX line driven HIGH
• Any duration possible (not limited to 13 bit times)

Break Reception/Detection

The UART continuously monitors for Break sequences, even during normal reception:

Detection Modes (RXBIMD bit)

Break Interrupt Configuration

Auto-Baud Feature

What is Auto-Baud?

Auto-Baud (Automatic Baud Rate Detection) allows the UART receiver to automatically determine and synchronize to the transmitter’s baud rate without prior configuration. This is essential for:

• Systems where baud rate is unknown at startup
• Multi-device communication with varying speeds
• Plug-and-play serial interfaces
• LIN bus and other automotive protocols

Auto-Baud Detection Process

Step 1: Transmitter Sends Sync Byte

Transmitter sends 0x55 (binary: 0b01010101) - alternating bit pattern: // Transmitter (known baud rate) U1TXB = 0x55; // Sync byte for auto-baud

Step 2: Receiver Enables Auto-Baud

// Receiver (unknown baud rate) U1CON |= 0x0040; // Set ABDEN bit (UxCON[6]) // UART state machine enters Idle, waiting for Start bit

Step 3: Automatic BRG Calculation

On detecting the Start bit of 0x55, the UART:

• Counts BRG clock cycles between 5 falling edges (0x55 pattern)
• Calculates average bit time
• Writes appropriate value to UxBRG register
• Clears ABDEN bit (hardware auto-clear)
• Sets ABDIF flag (UxUIR[6])

Step 4: Completion Handling

Auto-Baud with Break Preamble

The Sync byte (0x55) may optionally be preceded by a Break character for additional synchronization: // Transmitter sequence U1CON |= 0x0100; // Set SENDB U1TXB = 0x55; // Send Break followed by Sync byte

Error Handling: BRG Overflow

If the 5th falling edge is not detected before BRG counter rolls over:

• ABDOVIF flag (UxSTAT[5]) sets - indicates overflow
• ABDOVIF cannot be cleared until ABDEN is cleared
• If ABDOVIE bit (UxSTAT[13]) set → error interrupt generated

Supported Baud Rate Modes

Auto-baud works in both baud rate generation modes:

• Legacy Mode (CLKMOD=1): Standard BRG calculation
• Fractional Mode (CLKMOD=0): Enhanced fractional BRG

Register Summary

UxCON (Mode Control Register)

UxSTAT (Status Register)

UxUIR (UART Interrupt Register)

Configuration in MCHP Blockset

While a dedicated Break/Autobaud block may be available, these features are typically configured through:

• UART Config block - Auto-baud enable option (device-dependent)
• Direct register access - Use embedded code blocks for manual configuration
• S-Function blocks - Custom implementations for protocol-specific needs

Related Documentation

• UART Config - Configure UART module
• UART Tx - Transmit data
• UART Rx - Receive data
• Device Datasheet - Detailed UART register descriptions
• LIN 2.2 Specification - LIN protocol implementation
• DMX512 Standard - DMX protocol timing requirements