Connecting Modules to the ESP32

I have been working with the Arduino and Raspberry Pi for a few years now, but I have only recently started to explore the ESP32. I have been impressed with the capabilities of the ESP32, and I have been experimenting with a few different modules to see what I can do with them.

The ESP32 is a powerful microcontroller that has built-in WiFi and Bluetooth capabilities. This makes it ideal for IoT projects, as it can connect to the internet and communicate with other devices wirelessly. Unfortunately it isn’t as simple to work with as some of the other microcontroller dev boards, so I wanted to share some of the things I have learned so far.

Interfaces

There are a number of interfaces that are typically used to connect modules to the ESP32. Let’s take a look at the most common ones:

• GPIO: General Purpose Input/Output pins are the most common way to connect modules to the ESP32. These pins can be used for digital input or output, and can also be used for analog input or output with the use of an ADC or DAC.
• I2C: Inter-Integrated Circuit is a two-wire serial bus that is commonly used to connect sensors and other modules to microcontrollers. The ESP32 has two I2C buses, which can be used to connect multiple devices.
• SPI: Serial Peripheral Interface is a synchronous serial communication interface that is commonly used to connect devices like displays, SD cards, and other peripherals to microcontrollers. The ESP32 has two SPI buses, which can be used to connect multiple devices.
• I2S: Inter-IC Sound is a serial bus that is commonly used to connect audio devices to microcontrollers. The ESP32 has two I2S buses, which can be used to connect audio devices like microphones and speakers.

GPIO

This is the more simple of the interfaces to use, as it is just a digital pin that can be set to input or output. Most of the pins are avaible to use as a GPIO, but there are some, such as the ones used for the flash memory, that are not available.

For more information take a look at this GPIO reference where you can see GPIO 6 to 11 are used for the integrated SPI flash.

In some development boards these are not broken out, but others have them available, so be careful.

I2C

I2C is a two-wire serial bus that is commonly used to connect sensors and other modules to microcontrollers. The ESP32 has two I2C buses, which can be used to connect multiple devices.

Typically I2C requires 4 pins:

• VCC (3.3V)
• GND
• SDA (Data)
• SCL (Clock)

Multiple devices can be connected to a single I2C bus, as long as they have different addresses. This is usually set by a jumper or solder bridge on the module if it is available to set.

For more information in I2C addresses take a look at this complete guide to i2c.

SPI

SPI is a synchronous serial communication interface that is commonly used to connect devices like displays, SD cards, and other peripherals to microcontrollers. The ESP32 has two SPI buses, which can be used to connect multiple devices.

Typically SPI requires the following pins:

• VCC (3.3V)
• GND
• MOSI (Master Out Slave In):
• MISO (Master In Slave Out)
• SCK (Clock)
• CS (Chip Select)

Chip Select is used to select the device you want to communicate with by setting high or low when multiple SPI devices are connected to the same bus.

There are specific pins for the available SPI busses:

SPI	MOSI	MISO	CLK	CS
VSPI	GPIO 23	GPIO 19	GPIO 18	GPIO 5
HSPI	GPIO 13	GPIO 12	GPIO 14	GPIO 15

Some devices (such as the display modules ST7789 or GA9A01) have additional pins for the backlight or touch screen. They may also have the CS pin grounded internally so you don’t need to connect it.

• RES (Reset)
• DC (Data/Command)
• BL (Backlight)

Example: ST7789 display

Here is an example of how to connect an ST7789 display to the ESP32:

SPI PIN	ST7789 LABEL	ESP32 PIN	Notes
VCC	VCC	3.3V
GND	GND	GND
SCLK	SCK / SCL	GPIO 18	Clock
MOSI	SDA / DIN	GPIO 23	Data
MISO	N/A	N/A	Not used
CS	N/A	N/A	Grounded internally
-	DC	GPIO 2	Data/Command
-	RES / RST	GPIO 4	Reset
-	BLK	3.3V / GND	Ground to disable

I2S

Probably the most confusing of the interfaces I have been working with, I2S allows you to connect audio devices to the ESP32. This is a serial bus that is used to transmit audio data between devices.

Typically I2S requires the following pins:

• VCC (3.3V)
• GND
• BCLK (Bit Clock)
• LRCLK (Left/Right Clock)
• DATA (Data)
• MCLK (Master Clock)

Also you may have the following pins depending on the device:

• LRSEL or L/R (Left/Right Select): For stereo audio
• SCLK or SCK (Serial Clock): Serial-Data Clock
• WS (Word Select): Serial Data Word Select
• GAIN: For devices that have an amplifier
• SD: allows you to mute the output on the MAX98357A amplifier, and on the INMP441 mems mic, it is used to output the data.

The pins can be set in code to use any of the available pins, but my testing has shown the following will work:

INMP441 MEMS microphone

I2S PIN	INMP441 LABEL	ESP32 PIN	Notes
VCC	VCC	3.3V
GND	GND	GND
WS	WS	GPIO 23	Word Select
SD	SD	GPIO 34	Serial Data
SCK	SCK	GPIO 32	Serial Clock

MAX98357A amplifier

I2S PIN	MAX98357A LABEL	ESP32 PIN	Notes
VCC	VCC	3.3V
GND	GND	GND
DOUT	DIN	GPIO 5	Data Input to amplifier
BCLK	BCLK	GPIO 26	Bit Clock
LRC	LRC	GPIO 25	Left/Right Clock
GAIN	GAIN	N/A	Amplifier gain control
SD	SD	33	Mute control (optional)

Gain can be controlled as followed reference:

• 15dB if a 100K resistor is connected between GAIN and GND
• 12dB if GAIN is connected directly to GND
• 9dB if GAIN is not connected to anything (this is the default)
• 6dB if GAIN is connected directly to Vin
• 3dB if a 100K resistor is connected between GAIN and Vin

Code examples

We have examples of the code for use with the modules in this guide in the GitHub repository:

ESP32 component examples

Help contribute

I’m always looking to improve the information in this article, so if you have any suggestions or corrections, please let me know in the discord community