How to Fix UART Baud Rate Mismatches on LPC1769FBD100
Understanding the Problem:A UART baud rate mismatch occurs when the transmitting device and the receiving device are set to communicate at different speeds. In the case of the LPC1769FBD100 microcontroller, UART (Universal Asynchronous Receiver/Transmitter) is commonly used for serial Communication . If there’s a mismatch between the baud rates of the transmitter and receiver, communication will fail or result in corrupted data. The issue can be traced back to several factors.
Causes of UART Baud Rate Mismatches:Incorrect Baud Rate Settings: Both the transmitting and receiving devices must have the same baud rate for successful communication. If either side has a misconfigured baud rate, the data will not align correctly, causing errors.
Clock Source Mismatch: The baud rate is often derived from a clock signal (typically the system clock or a peripheral clock). If the clock source or its configuration differs between the devices, even if the baud rates appear to be the same, they might not be synchronized.
External Noise or Interference: Electrical noise on the line can affect the communication, causing bits to be misread and leading to baud rate mismatches.
Incorrect Microcontroller Configuration: The UART on the LPC1769FBD100 must be configured properly in terms of both the baud rate and the corresponding clock source. If the configuration registers (e.g., the LPC1769's U0LCR, U0DLL, and U0DLM) are set incorrectly, it can cause a mismatch.
Step-by-Step Solution: Verify the Baud Rate Settings: Check both the LPC1769 microcontroller and the communicating device for the baud rate configuration. Ensure that both devices are set to the same baud rate (e.g., 9600, 115200, etc.). This setting is usually found in the UART configuration of the software or hardware configuration files. Example: If you're using a serial terminal to communicate, ensure that the baud rate of the terminal matches the microcontroller's configuration. Confirm the Clock Source: Ensure that the system clock or the clock used by the UART peripheral is properly configured. For the LPC1769FBD100, the baud rate is derived from the system clock. Use the formula: [ Baud_Rate = \frac{Pclk}{16 \times (Divisor)} ] where Pclk is the peripheral clock and Divisor is the value in the U0DLL and U0DLM registers. Make sure this configuration matches on both sides of the communication link. Check for Noise or Interference: Ensure that the physical UART communication lines (TX, RX, GND) are properly shielded and not subject to external noise or interference. Use good-quality cables and connectors, and keep the UART lines as short as possible to minimize signal degradation. Recheck the Microcontroller’s UART Registers:On the LPC1769FBD100, use the U0LCR (Line Control Register), U0DLL, and U0DLM registers to configure the UART baud rate correctly.
The U0LCR register controls the line configuration (e.g., 8 data bits, no parity). The U0DLL and U0DLM registers hold the baud rate divisor.Ensure that these settings are correctly initialized in the firmware.
Example code to set baud rate on LPC1769:
// Set the baud rate to 9600 uint32_t baud_rate = 9600; uint32_t pclk = 12000000; // Example peripheral clock (12 MHz) uint32_t divisor = pclk / (16 * baud_rate); // Set the divisor in U0DLL and U0DLM LPC_UART0->LCR = 0x83; // Enable DLAB (Divisor Latch Access Bit) LPC_UART0->U0DLL = divisor & 0xFF; LPC_UART0->U0DLM = (divisor >> 8) & 0xFF; LPC_UART0->LCR = 0x03; // Disable DLAB Check for Software or Firmware Bugs: Ensure that there are no bugs in your code that might be changing the baud rate or misconfiguring UART settings during runtime. This can often be overlooked but is a common source of baud rate mismatch issues. Test Communication with Known Good Devices: To isolate the problem, test the LPC1769FBD100 UART communication with a known good device (such as a USB-to-serial adapter). This will help confirm whether the problem lies with the LPC1769 configuration or with the other device in the communication chain. Use an Oscilloscope for Diagnostics: If you still experience issues, use an oscilloscope or logic analyzer to check the waveform of the UART signals. This can help identify whether the signal is correctly transmitted and received or if there’s distortion. Conclusion:By ensuring that both devices are set to the correct baud rate, confirming the correct clock source, and checking the microcontroller’s UART settings, you can resolve most UART baud rate mismatch issues. Additionally, consider factors such as noise and hardware interference that might affect communication. Following these steps will help you get UART communication working smoothly on the LPC1769FBD100.