PIC18单片机UART发送字符异常问题求助

Let’s walk through the most likely causes and fixes for your problem—you’re sending "hello" (plus a newline) via PIC18 UART, see correct values in TXREG during debugging, but get garbled data on the receiving end.

1. Verify UART Configuration Match with RN4678 #

First, double-check that your PIC18’s UART settings exactly align with the RN4678’s default configuration:

• Baud rate: 115200 bps (don’t just assume this works—validate the calculation)
  Use the PIC18’s asynchronous mode baud rate formula:

  Baud Rate = FOSC / (64*(SPBRG+1))  ; BRGH=0 (low-speed mode)
  Baud Rate = FOSC / (16*(SPBRG+1))  ; BRGH=1 (high-speed mode)
  

  Ensure you’ve set BRGH correctly in the TXSTA register—RN4678’s 115200 baud usually requires high-speed mode for common PIC18 clock speeds (like 8MHz or 16MHz). Don’t forget to configure SPBRGH if your clock speed needs the extra bits for accurate baud rate generation.
• Frame format: 8 data bits, 1 stop bit, no parity (8N1)—this is the RN4678’s default. Confirm your PIC’s RCSTA and TXSTA registers reflect this (e.g., TX9=0, RX9=0, SPEN=1, SYNC=0).

2. Check the UART_Write_Text Implementation #

Even if TXREG looks correct during single-step debugging, full-speed execution might skip critical wait states:

• Your function must wait for TXREG to empty before writing the next character. The TXIF flag in PIR1 signals when TXREG is ready for new data. A reliable implementation would look like:

  void UART_Write_Text(const char *text) {
      while(*text) {
          while(!PIR1bits.TXIF);  // Wait until TXREG is available
          TXREG = *text++;
      }
      // Explicitly add the newline if your function doesn't include it
      while(!PIR1bits.TXIF);
      TXREG = '\n';
  }
  

  Skipping this wait can cause characters to overwrite each other in TXREG during full-speed execution, leading to corrupted transmission—even if single-step debugging shows correct writes.

3. Validate Hardware Connections & Level Compatibility #

• Pin mapping: Make sure your PIC18’s TX pin connects to the RN4678’s RX pin (crossed connection: TX → RX, RX → TX). Reversing these will result in no or garbled data.
• Level shifting: RN4678 operates at 3.3V, while many PIC18s are 5V devices. Directly connecting a 5V TX pin to a 3.3V RX pin can damage the module or cause unreliable signal detection. Use a resistor divider or dedicated level shifter (like a TXB0104) to convert the PIC’s 5V output to 3.3V.
• Power stability: The RN4678 needs a clean 3.3V supply with up to 100mA of current during transmission. Check for voltage drops or excessive ripple—add a 100nF decoupling capacitor near the module’s power pins to stabilize the supply.

4. Confirm RN4678’s Actual Configuration #

It’s possible the module’s default settings were modified accidentally. Use a USB-to-UART adapter to send AT commands to verify:

• Send AT+BAUD? to check the current baud rate. If it’s not 115200, set it back with AT+BAUD7 (7 corresponds to 115200 in the RN4678’s baud code list) followed by AT&W to save the setting.
• Send AT+PARITY? to confirm no parity is enabled, and AT+STOP? to check for 1 stop bit.

5. Debug with Hardware Tools #

If the above steps don’t fix the issue, use physical debugging to isolate the problem:

• Oscilloscope/Logic Analyzer: Probe the PIC18’s TX pin to check the signal waveform. Verify:
  • Each character has a valid start bit (low), 8 data bits, and 1 stop bit (high).
  • The baud rate matches 115200 (each bit should be ~8.68µs long).
  • There’s no noise or signal distortion on the line.
• If the PIC’s TX waveform looks correct, the issue is likely with the RN4678 (e.g., faulty module, incorrect configuration). If the waveform is corrupted, recheck your PIC’s UART settings and clock configuration.

内容的提问来源于stack exchange，提问作者Ignacio Neuhaus