Serial

ESPxxx come with UART controllers (universal asynchronous serial receiver/transmitter). UART is a simple two-wire protocol for exchanging serial data.

One of its UARTs typically is connected to an external USB-to-TTL converter chip which in turn is wired to the USB connector. This way, the ESPxxxx development board can be connected to a computer using a simple USB cable.

This bidirectional communication is very helpful both for uploading new firmware and for establishing a simple communication with the user. Most users (and many sketches) make use of this.

Taking advantage of the serial interface is very simple and straight-forward, however it is often the little things that get in the way. That’s why below you find code examples for the most important use cases.

Basic Serial Communication

The serial interface connected to USB is a convenient way of communicating with the microcontroller and i.e. return status information or sensor data.

Here are the basic steps in code to output information to your development environment (like Arduino IDE or platformio):

#include <Arduino.h>


int c = 0;

void setup() {
  // initialize serial output with the appropriate baud rate:
  Serial.begin(115200);
  // wait for the driver to fully initialize:
  while(!Serial);
  // add a small delay IF YOU MUST OUTPUT data in setup() already:
  delay(500);
  Serial.println("Start");
}

void loop() {
  // output an incrementing counter each second:
  c++;
  Serial.println(c);
  delay(1000);
}

To run the sketch, follow these steps:

1. Compile and upload the sketch. Depending on the IDE you are using, choose Upload and Monitor instead of Upload (to open the serial monitor), choose Monitor if your sketch is already running, or (in Arduino IDE) open the Serial Monitor.
2. Make sure the Serial Monitor and your sketch use the same baud rate (Speed) or else you receive nothing or garbled characters.

When the sketch runs, it first outputs the text Start, then outputs an incrementing number, one per second.

Before you can use the serial interface on an ESPxxxx, you need to include the Arduino libraries:

#include <Arduino.h>

Initializing Serial Port

To successfully send information to the IDE, the sketch first needs to initialize the serial interface and set it to the desired baud rate. This is typically done in setup();:

  Serial.begin(115200);

Waiting for Serial Port To Be Ready

It takes a moment for the serial port to be ready. If you start to output information right away, especially inside setup();, most likely you will lose this information unless you actively wait for the serial interface to be ready:

  // initialize serial output with the appropriate baud rate:
  Serial.begin(115200);
  // wait for the driver to fully initialize:
  while(!Serial);
  // add a small delay IF YOU MUST OUTPUT data in setup() already:
  delay(500);

This is a two-step process: while(!Serial); waits for the drivers to be initialized. The serial interface is now *about to be ready but not yet ready at this point.

If you output information immediately thereafter (i.e. inside setup();), you most likely still lose your data. To work around this, you’d have to add a very small delay, i.e. 500ms.

Use this only if you must ensure that all data is received by the terminal: adding checks and delays in setup(); will delay the start of the sketch when you power on the microcontroller. Since serial data is often used just occasionally during debugging, and since it might not be a problem to miss a few pieces of data at the beginning, skipping these checks and delays results in a much prompter start of your sketch after power-on.

Outputting Information

Data can be sent to the serial interface using Serial.print(); (no linefeed), Serial.println(); (adding a line feed), or one of the other print methods:

  Serial.println(c);

The output information is sent to the Terminal Window/Serial Monitor of your IDE:

• Arduino IDE: The Serial Monitor dialog window needs to be manually opened via Tools/Serial Monitor or Control+Shift+M. Make sure you select the correct baud rate in the Serial Monitor status bar.
• PlatformIO: The Terminal tab in the console pane is automatically opened. It is the same pane that shows the progress information during firmware uploads.

Interacting With User

The serial port is bidirectional: you can output information to the IDE terminal window, and you can receive information sent by the user via keypresses.

When responding to user input, the particular behavior depends on the IDE and its Serial Terminal as well. Some terminals echo the user input while others do not. Some terminals allow just one keypress to be sent, others accept many consecutive keypresses.

Here is a sketch demonstrating basic user interaction: the sketch sends a user prompt to the terminal window, asking for user input. The user can then either press 1 or 0 to turn the built-in LED either on or off.

#include <Arduino.h>

void setup() {
  // initialize serial interface:
  Serial.begin(115200);
  while(!Serial);
  delay(500);

  // setup built-in led
  pinMode(LED_BUILTIN, OUTPUT);

  // report back internal LED pin
  Serial.print("Using built-in LED on GPIO ");
  Serial.print(LED_BUILTIN);
  Serial.println(".");
  Serial.println("Make sure this GPIO is correct, and your board in fact has a built-in LED.");

  // write initial user prompt:
  Serial.print("Enter Command (1 or 0)> ");
}

void loop() {
  // did the user input anything?
  if(Serial.available()) 
  {
    // read user input until user presses ENTER (and adds a newline (\n))
    char command = Serial.read();

    // echo user input
    Serial.println(command);

    // process user input:
    if(command == '1') {
      digitalWrite(LED_BUILTIN, HIGH); 
      Serial.println("[LED is ON]");
    } else if(command == '0')  {
      digitalWrite(LED_BUILTIN, LOW); 
      Serial.println("[LED is OFF]");
    } else {
      Serial.print("[Command not recognized: ");
      Serial.print(command);
      Serial.print(" Enter 1 or 0]");
    }

    // write prompt again:
    Serial.print("Enter Command (ON or OFF)> ");
  }
}

Depending on your IDE, the user input is entered differently:

• Arduino IDE: In the Serial Monitor dialog window, at the top there is an input box where you can enter information. Once you press the button Send at its right end, the entered information is sent.
• PlatformIO: Simply click into the Terminal tab inside the console pane (where the serial output is shown), then press the key you want to send. By default, platformio sends each keypress immediately, making user input especially convenient.

The sketch uses the constant LED_BUILTIN and reports back the assigned GPIO. If the assigned GPIO is wrong, or if the constant LED_BUILTIN is not defined for your board, you can replace it with the actual GPIO number that is connected to the LED. Some boards turn the LED off when the GPIO is set to HIGH, effectively turning the LED off when it should be on, and vice versa. If that is the case, switch LOW and HIGH in the sketch. And finally: there are development boards (like the DevKitC V4) that have no built-in user-controllable LED, just a power LED that is constantly on and not controllable.

Here is the output from the Terminal window in platformio:

Using built-in LED on GPIO 15.
Make sure this GPIO is correct, and your board in fact has a built-in LED.
Enter Command (1 or 0)> 1
[LED is ON]
Enter Command (ON or OFF)> 0
[LED is OFF]
Enter Command (ON or OFF)> 0
[LED is OFF]
Enter Command (ON or OFF)> 1
[LED is ON]
Enter Command (ON or OFF)>

Accepting String Commands

Reading and processing a single character via Serial.Read(); is simple to code, simple to input, and clear: numbers are interpreted the same in any culture, and there is no potential for confusing lower and upper case.

Still, there are valid use cases where accepting more complex user input is required. Maybe you want to allow the user to not only switch the LED on and off, but also to dim it using various levels. In a nutshell, you may require more capable user commands.

Let’s assume a sketch should support these user commands:

• on: turns the LED on
• off: turns the LED off
• dim 1-254: turns LED on at the specified PWM brightness (1-255)

Here is a revised sketch that does this:

#include <Arduino.h>

void setup() {
  // initialize serial interface:
  Serial.begin(115200);
  while(!Serial);
  delay(500);

  // setup built-in led
  pinMode(LED_BUILTIN, OUTPUT);

  // report back internal LED pin
  Serial.print("Using built-in LED on GPIO ");
  Serial.print(LED_BUILTIN);
  Serial.println(".");
  Serial.println("Make sure this GPIO is correct, and your board in fact has a built-in LED.");

  // write initial user prompt:
  Serial.print("Enter Command ('on', 'off', 'dim <1-254>')> ");
}

void loop() {
  // did the user input anything?
  if(Serial.available()) 
  {
    // read user input until user presses ENTER (and adds a newline (\n))
    String command = Serial.readStringUntil('\n');

    // remove whitespace and newline characters:
    command.trim();

    // process user input:
    if(command == "on") {
      digitalWrite(LED_BUILTIN, HIGH); 
      Serial.println("[LED is ON]");
    } else if(command == "off")  {
      digitalWrite(LED_BUILTIN, LOW); 
      Serial.println("[LED is OFF]");
    } else if(command.startsWith("dim ")) {
      int dimLevel = command.substring(4).toInt();
      if (dimLevel<1 || dimLevel>254) {
        Serial.println("[Invalid DIM level. Must be a value between 1 and 254.]");
      } else {
        analogWrite(LED_BUILTIN, dimLevel);
      }
    } else {
      Serial.print("[Command not recognized: ");
      Serial.print(command);
      Serial.println("]");
    }

    // write prompt again:
    Serial.print("Enter Command ('on', 'off', 'dim <1-254>')> ");
  }
}

Adjustments for platformio

If you use platformio as your IDE, add these options to your projects’ platformio.ini:

monitor_filters = send_on_enter
monitor_echo = yes

• monitor_filters = send_on_enter tells platformio that you do not want it to automatically send off each keystroke but instead want it to wait for ENTER. Without this setting, you would have to type each character no slower than 1000ms, or else incomplete input would be transferred automatically.
• monitor_echo = yes tells platformio to echo the user input while the user types it into the terminal window.

Playing With The Sketch

Click Upload and Monitor in platformio, or open the Serial Monitor in Arduino IDE.

You can now use the three commands to either turn the LED completely on or off, or set its brightness level in a range of 1-254:

Using built-in LED on GPIO 15.
Make sure this GPIO is correct, and your board in fact has a built-in LED.
Enter Command ('on', 'off', 'dim <1-254>')> on
[LED is ON]
Enter Command ('on', 'off', 'dim <1-254>')> off
[LED is OFF]
Enter Command ('on', 'off', 'dim <1-254>')> dim 60
Enter Command ('on', 'off', 'dim <1-254>')> dim 1
Enter Command ('on', 'off', 'dim <1-254>')> dim 300
[Invalid DIM level. Must be a value between 1 and 254.]
Enter Command ('on', 'off', 'dim <1-254>')> dim 222
Enter Command ('on', 'off', 'dim <1-254>')> dim 44
Enter Command ('on', 'off', 'dim <1-254>')

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