Debugging Serial Output in Arduino Interrupt Service Routines

When working with Arduino, especially in projects that require precise timing or responsiveness like MIDI controllers, Interrupt Service Routines (ISRs) are invaluable. They allow your Arduino to react to external events in real-time without constantly polling for changes in your main loop. A common debugging technique in Arduino programming is using the serial printer to monitor variable values and program flow. However, using Serial.println() within ISRs can sometimes lead to unexpected behavior, such as incomplete or garbled output in the serial monitor. This article explores potential issues when using the serial printer in Arduino ISRs, specifically focusing on scenarios similar to the user’s experience with pin change interrupts and debugging output.

Pin Change Interrupts (PCIs) are a powerful feature of Arduino microcontrollers that allow you to trigger an interrupt when any pin within a specified port changes its state (from HIGH to LOW or LOW to HIGH). This is particularly useful for detecting button presses or other external events efficiently. In the provided code snippet, the user correctly sets up Pin Change Interrupts on Port D to detect button presses on digital pins D2 to D7.

The code uses Serial.println() inside the PCINT2_vect ISR to print “B2_Pressed” and “B2_Released” for debugging purposes. The reported issue is that only the first two characters, “B2”, are printed when the button is initially pressed. The rest of the message and the “B2_Released” message are printed only after the button is released. This points to a potential problem with how serial communication and interrupts interact, especially concerning timing and buffer management.

Here’s a breakdown of the possible reasons behind this incomplete serial printing issue and how to approach debugging such scenarios when using the Serial Printer Arduino in interrupt routines:

Potential Causes for Incomplete Serial Output in ISRs:

1. Timing and Serial Buffer: Serial communication in Arduino is relatively slow compared to the microcontroller’s processing speed. Serial.println() sends data to a serial buffer, which is then transmitted byte by byte to the serial monitor. ISRs should be kept as short and efficient as possible to avoid delaying the main program flow and other interrupts. If Serial.println() takes too long within the ISR, it might interfere with the interrupt handling itself or the serial communication process. It’s possible that the initial part of “B2_Pressed” gets into the buffer, but the ISR exits before the entire string can be processed and transmitted. Upon exiting the ISR and possibly when the button is released, the remaining buffer content gets flushed and printed.

2. Interrupt Latency and Blocking: While Serial.println() is non-blocking in the sense that it returns quickly after placing data in the buffer, the actual transmission is handled in the background. However, if the serial buffer fills up because of frequent Serial.println() calls within an ISR, subsequent calls might get delayed or even dropped. Furthermore, if the ISR itself is lengthy due to the while loop waiting for button release, it further exacerbates timing issues and can impact serial communication reliability.

3. Interaction with Main Loop: Although the provided code has an empty loop() function, in more complex programs, the main loop might also be using serial communication. Interrupts can preempt the execution of the main loop. If both the ISR and the main loop are trying to use the serial printer simultaneously, there could be contention or unexpected interactions leading to data corruption or incomplete output.

Debugging Strategies for Serial Printer Arduino Issues in ISRs:

1. Verify Baud Rate and Serial Monitor Settings: Double-check that the baud rate set in Serial.begin(9600) in the Arduino code matches the baud rate selected in the serial monitor. Mismatched baud rates are a common cause of garbled or unreadable serial output.

2. Simplify the ISR Code: ISRs should be as short and fast as possible. Avoid lengthy operations or delays within them. In the provided code, the while loop waiting for button release inside the ISR is generally not recommended practice. Consider using a different approach to detect button release outside of the ISR if precise timing is not critical within the ISR itself.

3. Minimize Serial Printing in ISRs: For debugging purposes, it’s often better to use minimal serial output within ISRs. Instead of printing entire strings, consider printing single characters or very short codes to indicate interrupt events. For example, print ‘P’ for “Pressed” and ‘R’ for “Released” instead of the full strings. This reduces the time spent in the ISR and minimizes potential buffering issues.

4. Use Flags and Print from the Main Loop: A more robust approach is to set flags within the ISR to indicate events (like button press and release) and then check and process these flags in the main loop() function. Perform the Serial.println() calls in the main loop based on the flag states. This moves the serial printing operations out of the time-critical ISR and into the main loop, which has more time to handle serial communication without disrupting interrupt handling.

   volatile bool button2Pressed = false;
   volatile bool button2Released = false;
   
   ISR (PCINT2_vect) {
     if(PIND & B00000100) { // Button 2 Pressed
       if (!button2Pressed) { // To avoid repeated triggers
         button2Pressed = true;
         button2Released = false;
       }
     } else { // Button 2 Released
       if (!button2Released) { // To avoid repeated triggers
         button2Released = true;
         button2Pressed = false;
       }
     }
   }
   
   void loop() {
     if (button2Pressed) {
       Serial.println("B2_Pressed");
       button2Pressed = false; // Reset flag after printing
     }
     if (button2Released) {
       Serial.println("B2_Released");
       button2Released = false; // Reset flag after printing
     }
     // ... rest of your loop code ...
   }

5. Alternative Debugging Methods: If serial printing in ISRs consistently causes issues, consider alternative debugging techniques. Using LEDs to signal events or states can be a faster and less intrusive way to debug interrupt-driven code. You could toggle an LED in the ISR to visually confirm that the interrupt is being triggered as expected.

Conclusion:

While Serial.println() is a convenient tool for debugging Arduino code, using it extensively within Interrupt Service Routines can lead to timing issues and incomplete or unreliable serial output. Understanding the limitations of serial communication within ISRs and employing strategies like minimizing print statements, using flags, and shifting printing to the main loop are crucial for robust and predictable Arduino programs, especially when dealing with time-sensitive applications like MIDI controllers or real-time event handling. When debugging “serial printer arduino” related problems in interrupts, always prioritize keeping your ISRs short and efficient and consider alternative debugging approaches to ensure the reliability of your interrupt handling and serial communication.