Use the GPIO pins of the ESP32

UART on ESP32

The UART is the serial protocol that allows easy data exchange between 2 devices. On the ESP32, 3 UART buses are available: UART0, UART1 and UART2. We can use them to communicate with a sensor, an Arduino, a Raspberry Pi, a computer …

• The UART0 is by default on pins GPIO1(TX0) and GPIO3(RX0) of the ESP32. It is used to communicate with the computer through the serial monitor. It is also the one used to flash the ESP32 board when uploading a new program. It generally displays messages in the console with Serial.println() .

• To use the UART2, simply add Serial2.begin() in the function setup() and to use the``Serial2.println()`` to send messages. By default, the UART2 bus is on pins GPIO16(RX2) and GPIO17(TX2), but you can change them (useful with a Wrover module) during setup. This simple code allows you to use the UART2 bus:

  void setup() {
    Serial2.begin(115200);
  }
  
  void loop() {
    Serial2.println("Hello from UART2");
    delay(100);
  }
  

  Note

  The message will not be displayed on the serial monitor since the UART2 is not connected to the computer’s USB.

  

• The UART1 is by default on the pins used by the ESP32 flash. However, We can use it by choosing the pins you want, thanks to the “GPIO matrix” of the ESP32. Thus this code allows having a serial interface on the pins GPIO14 and GPIO12 by using the UART1 bus.

  void setup() {
    /*
    * UART1  -> Serial1
    * RX Pin -> GPIO 14
    * TX Pin -> GPIO 12
    * UART Configuration -> SERIAL_8N1
    */
    Serial1.begin(115200,SERIAL_8N1,14,12);
  
  }
  
  void loop() {
    Serial1.println("Hello from UART1");
    delay(100);
  }
  

  

I2C on ESP32

The ESP32 has 2 I2C buses :

• The I2C0 bus is the one used by default by Arduino libraries. It is connected to the pins GPIO22 (SCL) and GPIO21 (SDA) of the ESP32. It can be used on any pin of the ESP32 when you use the Wire.h library by specifying the pins with the function Wire.begin(SDA_PIN, SCL_PIN) .

• The I2C1 bus can also be used on any pin (pay attention to the “limited pins” described previously). Here is an example that uses the second I2C bus:

  #include <Wire.h>
  
  TwoWire I2C1 = TwoWire(1);
  
  void setup() {
    I2C1.begin(14,12,400000); // SDA pin 14, SCL pin 12, 400kHz frequency
  }
  
  void loop() {
    I2C1.beginTransmission(0x42);
    I2C1.write(140);
    I2C1.endTransmission();
    delay(100);
  }
  

SPI on ESP32

Even if the ESP32 has 3 SPI buses, we can use only two because the internal flash memory uses one. The two available SPI buses are called VSPI and HSPI.

By default, the pin mapping is as follows:

As for the I2C, it is possible to manually choose the pins you want to use for the SPI by specifying the pins during the initialization of the SPI bus.begin(CLK_PIN, MISO_PIN, MOSI_PIN, SS_PIN);

PWM on ESP32

The ESP32 has 16 channels that can be used to generate PWM signals: you can have up to 16 different PWM outputs. All the pins configured as output can be used to output a PWM signal. The use of PWM differs from Arduino’s, and you can configure more parameters. I suggest you check the usage of PWM on ESP32 with Arduino code to use it in your projects.

Capacitive sensors on the ESP32

The ESP32 has ten capacitive sensors (9 if the GPIO0 pin is unavailable). They can be used as touch buttons. They are the famous TOUCH pins that can be found on some ESP32 pinouts. They can also be used to wake it up when the ESP32 is in DeepSleep mode (a power-saving mode). I invite you to read the use of capacitive sensors on the ESP32 with the Arduino code for more details.

The internal capacitive sensors are connected to the following GPIO pins:

ADC on ESP32

The ESP32 has two separate ADC: ADC1 with eight channels and ADC2 with ten channels for a total of 18 analog inputs. For example, on the GPIO34, you will find channel number 6 of the ADC1 (ADC1_CH6). But in practice, there are far fewer available because of Wi-Fi.

In general, the ESP32 ADC is not very reliable despite its 12-bit resolution. I invite you to read the article on the use and limitations of the ESP32 ADC for more information.

DAC on ESP32

The ESP32 has two 8-bit DAC outputs to convert a digital signal into an analog signal (generate a sine signal, for example). The pins used are the following: