Arduino Serial.flush Hangs the Program

In this tutorial, you'll learn about Arduino Serial.flush Hangs the Program. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Calling Serial.flush() causes the program to freeze or behave unexpectedly.

Quick Fix

Wrong

Serial.print("Data");
Serial.flush();  // Waits for tx buffer to empty

Program appears frozen until all bytes are transmitted.

Right

Serial.print("Data");
Serial.flush();  // Safe - waits for TX to complete
// On AVR: flush() waits for TX shift register too
// On SAMD/ESP32: flush() waits for TX FIFO
Serial.println("Done");

DataDone  (flush ensures Data is fully sent before Done prints).

Prevention

Use Serial.flush() only when you must confirm all data is sent before continuing (e.g., before going to sleep). On older Arduino cores, flush() also discards received data — check the core documentation. On modern cores, flush() only waits for TX completion.

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

FAQ