A UART framing error is a type of receiver error that occurs when the Universal Asynchronous Receiver-Transmitter (UART) fails to detect a properly formed start-bit to stop-bit data frame during data reception. It signals that the incoming serial data stream did not conform to the expected structure of a data packet.
Understanding UART Communication Basics
UART (Universal Asynchronous Receiver-Transmitter) is a common hardware communication protocol that uses asynchronous serial communication, meaning it doesn't rely on a shared clock signal between devices. Instead, it relies on each device having its own clock and agreeing on a common data rate, known as the baud rate. Data is transmitted in frames, which typically consist of:
- Start Bit: A logic '0' that signals the beginning of a new data frame.
- Data Bits: The actual data (typically 5 to 9 bits).
- Parity Bit (Optional): Used for basic error checking.
- Stop Bit(s): Logic '1's that mark the end of the data frame and prepare the receiver for the next start bit.
What Specifically is a UART Framing Error?
As per the provided reference, "Framing means the USART did not see a clean start bit to stop bit data frame." This indicates that the receiving Universal Synchronous/Asynchronous Receiver/Transmitter (USART) module, which encompasses UART functionality, was unable to identify the expected sequence of start and stop bits enclosing the data.
Like an overrun error, a framing error is fundamentally a receiver error. The reference states, "Framing and overrun are receiver errors." This means the issue lies with the receiver's ability to correctly interpret the incoming signal's timing and structure. The receiver might have sampled the data at the wrong time, or the start/stop bits themselves were corrupted or missing.
Common Causes of UART Framing Errors
Several factors can lead to a UART framing error, disrupting the receiver's ability to properly delineate data frames:
- Mismatched Baud Rates: This is the most frequent cause. If the transmitting and receiving UARTs are configured for different baud rates (e.g., one at 9600 bps and the other at 115200 bps), the receiver will sample the data bits at incorrect intervals, causing it to misinterpret the start and stop bit positions.
- Clock Drift: Even if baud rates are initially matched, inaccuracies in the internal clocks of the two devices can cause them to drift apart over time. This timing misalignment can lead to sampling errors.
- Noise or Interference on the Line: Electrical noise, electromagnetic interference (EMI), or ground loops on the communication line can corrupt the signal, especially the critical start and stop bits. This makes it difficult for the receiver to distinguish them from data bits or idle line states.
- Poor Signal Integrity: Long cables, inadequate shielding, or improper termination can degrade the signal quality, leading to distorted bit transitions that the receiver struggles to correctly interpret.
- Improper Configuration of Data Bits or Parity: While less common for "framing" specifically, an incorrect setup of data bits or parity settings can contribute to overall communication failure and sometimes manifest in ways that confuse the receiver's frame detection.
Detecting and Clearing Framing Errors
UART modules in microcontrollers and other devices typically have a status register that flags communication errors. When a framing error occurs, a specific bit in this status register will be set.
To clear these errors and allow the receiver to resume normal operation, a specific sequence of actions is required, as outlined in the reference: "To clear these errors read the USART status, then the data register."
- Read the Status Register: This action acknowledges the error and typically clears the error flag itself.
- Read the Data Register: Even if the received data is corrupted, reading the data register is crucial to complete the error clearing process for the current received frame and prepare the receiver for new incoming data.
Practical Solutions and Best Practices
To ensure reliable UART communication and prevent framing errors, consider the following practical approaches:
- Baud Rate Verification: Always double-check that both the transmitting and receiving devices are configured for the exact same baud rate. This is fundamental for asynchronous communication.
- Stable Clock Sources: Utilize precise and stable clock sources for your UART peripherals to minimize clock drift between devices.
- Signal Integrity Improvement:
- Use shielded cables, particularly for longer communication distances, to reduce external interference.
- Ensure proper grounding between communicating devices to prevent ground loops.
- Keep communication lines away from noisy electrical components or power lines.
- Error Handling in Software: Implement robust error handling in your firmware or software.
- Regularly monitor the UART status register for framing error flags.
- Promptly clear error flags as described (read status, then data register).
- Consider re-synchronization strategies or data retransmission requests if critical data is lost due to errors.
Framing Error Quick Reference
| Aspect | Description |
|---|---|
| Definition | Receiver fails to detect a clean start-bit to stop-bit data frame. |
| Error Type | Receiver error (similar to overrun). |
| Common Causes | Mismatched baud rates, clock drift, noise, poor signal integrity, grounding issues. |
| Detection | UART status register flags. |
| Clearing Method | Read the USART status register, then read the data register. |
| Prevention | Match baud rates, use stable clocks, improve signal integrity, proper grounding, robust error handling. |
A UART framing error indicates a fundamental issue in the structure or timing of received data, preventing the receiver from correctly interpreting the start and end of a data byte. Addressing the underlying causes, such as baud rate mismatches or noise, and implementing proper error handling, are crucial for reliable UART communication.
