Pins
Since understanding which GPIO pins are safe to use in projects is among the most frequently needed information, this information is placed at the beginning of this article.
6 Safe ESP32 Pins
Below ESP32 pins are safe to use for whatever purpose. Whether your microcontroller board actually exposes a particular pin depends on the board design.
| GPIO | Label | Remark |
|---|---|---|
| 4 | D4 | general purpose input/output GPIO |
| 13 | D13 | general purpose input/output GPIO |
| 14 | D14 | general purpose input/output GPIO |
| 16 | RX2 | general purpose input/output GPIO |
| 17 | TX2 | general purpose input/output GPIO |
| 20 | D20 | general purpose input/output GPIO |
4 Safe ESP32 Input-Only Pins
The pins below are safe to use for inputs only. These pins also do not have built-in pullup/pulldown resistors, so if you use them as input pin, you need to add an external resistor in order to keep the pin from floating and producing random input values.
| GPIO | Label | Remark |
|---|---|---|
| 34-35 | D34-D35 | input only, no pullup/pulldown resistor |
| 36 | VP | input only, no pullup/pulldown resistor |
| 39 | VN | input only, no pullup/pulldown resistor |
5 Interface Pins (I2C and SPI)
The pins below are used for I2C and SPI communications. If you do not require these interfaces, you are free to use their pins for other things.
| GPIO | Label | Remark |
|---|---|---|
| 18-19 | D18-D19 | SPI: 18=SCLK, 19=MISO |
| 21-22 | D21-D22 | I2C: 21=SDA, 22=SCL |
| 23 | D23 | SPI: MOSI |
4 Strapping Pins
If you need even more pins, you can use the additional four pins listed below - provided you do not use these pins during boot time.
| GPIO | Label | Remark |
|---|---|---|
| 0 | 0 | low to enter firmware upload mode |
| 2 | D2 | pin is attached to internal LED (if present), must be low during boot |
| 5 | D5 | must be high during boot |
| 12 | D12 | must be low during boot |
| 15 | D15 | prevents boot log if pulled low |
Strapping pin states are read and set by the microcontroller during boot and determine things like boot mode (normal boot vs. firmware upload boot) and boot logging. Do not pull these pins high or low during boot, or else your microcontroller may not start correctly.
2 Serial Comm Pins
The serial communications interface is used to upload new firmware, and often also to communicate with it: Serial.print() commands in your firmware code can output information, i.e. sensor data, that show in the terminal window of an IDE.
During normal operation, and if you don’t need serial communications yourself, its two pins can be used for other purposes as well:
| GPIO | Label | Remark |
|---|---|---|
| 1 | TX0 | serial comm (transmit) |
| 3 | RX0 | serial comm (receive) |
However, using these pins may require prerequisites to not interfere with required serial comm during boot or firmware upload.
Since your firmware code can not run at these sensitive instances, your code is always fine. What you do need to consider is your hardware design and schematics: do not physically pull-up or pull-down these pins or connect components to these pins that can alter their state during boot.
ESP32 Microcontroller Types
The ESP32 processor family is the ESP8266 successor and surfaced in 2016 with the ESP32 WROOM and ESP32 WROVER.
The ESP32 family has since evolved with many more variants. The Expressif Product Comparison provides a complete list of models and specs.
Here is a quick reference covering the most commonly used ESP32 types, and their most important specs:
| Feature | ESP32 | S2 | S3 | C3 | C6 | H2 | ESP8266 |
|---|---|---|---|---|---|---|---|
| Launch | 2016 | 2019 | 2020 | 2020 | 2021 | 2021 | 2014 |
| Core Type | LX6 | LX7 | LX7 | RISC-V | RISC-V | RISC-V | L106 |
| Cores | 2 | 1 | 2 | 1 | 1 | 1 | 1 |
| Frequency | 240 | 240 | 240 | 160 | 160 | 96 | 160 |
| Voltage | 2.3-3.6 | 3.0-3.6 | 3.0-3.6 | 3.0-3.6 | 3.0-3.6 | 3.3-3.6 | 2.5-3.6 |
| GPIOs | 34 | 43 | 45 | 22 | 30 | 19 | 17 |
| SRAM KB | 520 | 320 | 512 | 400 | 512 | 256 | 160 |
| RTC/LP SRAM KB | 16 | 16 | 16 | 8 | 16 | 4 | 1 |
| int Flash MB | 0/2/4 | 0/2/4 | 0/8 | 0/4 | 0/4 | 2/4 | 4 |
| Cache KB | 64 | 8/16 | 16 | 16 | 16 | 16 | 32 |
| ROM KB | 448 | 128 | 384 | 384 | 320 | 128 | 0 |
| ADC | 2x12bit 18ch | 2x13bit 20ch | 2x12bit 20ch | 2x12bit 6ch | 1x12bit 7ch | 1x12bit 5ch | 1x10bit 1ch |
| DAC | 2x8bit | 2x8bit | - | - | - | - | - |
| Timers | 4x64bit | 4x64bit | ? | 2x54bit/1x52bit | ? | 2x54bit | 2x23bit |
| Watchdog | 3 | 3 | 3 | 3 | 3 | 3 | 1 |
| I2C | 2 | 2 | 2 | 1 | 2 | 2 | 1 |
| I2S | 2 | 1 | 2 | 1 | 1 | 1 | 1 |
| SPI | 4 | 4 | 4 | 3 | 2 | 3 | 2 |
| Ethernet | yes | - | - | - | - | - | - |
| LCD Interface | 1 | 1 | 1 | - | - | - | - |
| Temperature | - | yes | yes | yes | yes | yes | - |
| Touch | 10 | 14 | 14 | - | - | - | - |
| UART | 3 | 2 | 3 | 2 | 3 | 2 | 2 |
| Ethernet | 1 | - | - | - | - | - | - |
| IR/RMT | 8 | 4 | 8 | 4 | 4 | 4 | 1 |
| Hall Sensor | 1 | - | - | - | - | - | - |
| LED PWM | 16 | 8 | 8 | 6 | 6 | 6 | 5 |
| Motor PWM | 6 | - | 2 | - | - | 1 | - |
| Wifi | 4 | 4 | 4 | 4 | 4/6 | no | 4 |
| Wifi Mbps | 150 | 150 | 150 | 150 | 150 | - | 75 |
| Bluetooth | 4.2 | - | BLE5.0 | BLE5.0 | BLE5.3 | BLE5.3 | 0 |
| Thread 1.3 | - | - | - | - | yes | yes | - |
| Zigbee 3.0 | - | - | - | - | yes | yes | - |
| Matter | - | - | - | - | yes | yes | - |
| HomeKit, MQTT, etc | - | - | - | - | yes | yes | - |
| USB OTG | - | yes | yes | - | - | - | - |
| Camera Interface | 1 | 1 | 1 | - | - | - | - |
| TWAI (CAN 2.0) | 1 | 1 | 1 | 1 | 2 | 1 | - |
| SD Host | 1 | - | 2 | - | - | - | - |
| SD Slave | 1 | - | - | - | 1 | - | 1 |
| JTAG | yes | yes | yes | yes | yes | - | |
| TOF | - | yes | - | - | - | - | - |
| AI Acceleration Support | - | - | yes | - | - | - | - |
| Deep Sleep | 100uA | 22uA | 7uA | 5uA | 7uA | 8uA | 20uA |
| Size mm | 5x5/6x6 | 7x7 | 7x7 | 5x5 | 5x5 | 4x4 | 5x5 |
What was previously referred to as RTC memory has been renamed to LP memory (low power memory). You might see both terms used interchangeably.
Quick Selection Guide
In a nutshell:
- if you want it all then get an ESP32 S3. They come in large boards (with all pins exposed) and super-tiny.
- if your focus is GPIO then get an ESP32 S2. They are super cheap with large memory.
- if you want matter support, get an ESP32 C3. They are super cheap as well.
Classic ESP32 WROOM are perfect generalists as well - as long as you don’t want to minimize energy consumption in battery projects or use the fancy new wireless techniques like matter.
Depending on your feature requirements, here is a more feature-based guidance:
| Category | You want to… | Recommendation |
|---|---|---|
| I/O | control as many external devices as possible | S2 or S3 (>40 GPIOs) |
| DAC | output analog voltages using a built-in DAC | ESP32 or S2 |
| Touch | use touch sensors | ESP32, S2, or S3 |
| USB Support | use USB devices such as keyboards and mice, or turn your microcontroller into a USB memory stick | S2 or S3 (USB-OTG required) |
| Camera Support | use a camera | ESP32, S2, or S3 |
| Low Voltage | work with the lowest-possible voltage (i.e. solar projects) | ESP32, all other versions require a minimum of 3.0-3.3V |
| Low Energy | run on battery for a long time | S3 or C3 |
| Bluetooth | use advanced Bluetooth LE features | C6 or H2 |
| Temperature | monitor the CPU temperature | any except ESP32 |
| Motor Control | control motors | ESP32 or S3 |
| Matter, Threads, etc. | build matter-compatible devices | C6 or H2 |
Overview
Espressif, the company producing ESPxxx microcontrollers, has never targeted the hpbbyist market: their processors were always targeting commercial IoT device applications.
The ESP32 process family therefore is driven by the needs of this industry, and DIY hobbyists happily benefit from the top notch innovations that become available for very little money (as the commercial market consumes high numbers of these microcontrollers and dictates low prices).
As commercial IoT devices are increasingly interconnected, smartphone controlled, using cloud backends and requiring to run days and weeks on small batteries while providing complex and even AI-based services, the three major fields of innovation are lower energy consumption, support for all broadly used wireless technologies, and support for AI vector analysis math coprocessors.
Wireless Strategy
All ESPxxx-based microcontrollers come with classic WiFi support - with one exception: the new H2 is the first ESP32 not supporting WiFI. That’s for a reason because the H2 has a very special role.
- Matter over WiFi: Matter is a new wireless standard that makes it simple to add and to control matter-compliant devices to smartphones. Any WiFI-enabled ESPxxx can be used to build Matter WiFi devices.
- Matter Thread: Matter can use Thread as a transport as well. Only the C6 and H2 come with the 802.15.4-compliant (5GHz) technology for Thread.
- Thread Border Routers: A H2 can be used to build a Thread Border Router to bridge between Thread and the regular WiFi world (including all other ESPxxx that have no native Thread support).
- Matter Zigbee Routers: A H2 can also be used to build a Matter Zigbee Router to bridge between Zigbee and the regular WiFi world (including all other ESPxxx that have no native Zigbee support).
The original ESP32 (and its successors like the C3 or S3) are a great choice for almost any microcontroller-based DIY project: they are cheap and come with a wealth of useful features, including wireless support for many wireless standards and also sophisticated interfaces (such as I2S which enables high-frequency analog sampling and direct memory access).
In addition, ESP32 development boards come with decent voltage regulators (compared to ESP8266), a massive number of GPIOs, and generous memory sizes.
In a nutshell, there is no good reason today anymore to purchase ESP8266 as you get ESP32 development boards for the same price.
The same is true for Arduinos: they were indeed ground breaking 15 years ago. Today, almost all Arduino boards are massively underpowered and at the same time massively overpriced when you compare them to any of the ESP32 microcontrollers. Since you can continue to use your favorite IDE (Arduino IDE, platformio, etc), run the same sketches, enjoy ESP32-support for almost any library, and can continue to use 5V components, considering one of the ESP32 types for your next project might be a good idea.
If you can get your hands on really really cheap ESP8266 boards (below EUR 1.00), or if you still have a pile of them on stock, you can of course continue to use them, and they will continue to perform great for most typical DIY projects. Before you buy new boards, though, please compare offers: most likely you’ll quickly find ESP32 sales matching the price tag of ESP8266. At the time of writing, ESP32, C3, and S2 are all available for under EUR 2.00/piece.
Comparing to ESP8266
Compared to the ESP8266, in a nutshell ESP32 doubles everything :
Double the CPU cores, double the WiFi Speed, (roughly) double the internal memory, double the GPIO pins, double the I2C and SPI interfaces, double the ADCs, double the Infrared support
Plus, it adds many interfaces and capabilities: built-in support for Bluetooth, CAN, LCDs, SD Cards, Ethernet, Camera, Touch, you name it.
The ESP32 doubles everything, except the price. ESP32 breakout boards start under EUR 3.00. Even if you don’t need the power or extra features (like bluetooth) offered by ESP32, choosing ESP32 breakout boards over ESP8266 today is almost always the best choice. Unlike many ESP8266 breakout board, ESP32 boards come with decent power regulators.
| Feature | ESP32 | ESP8266 |
|---|---|---|
| Core | 2 | 1 |
| Frequency MHz | 240 | 160 |
| SRAM KB | 520 | 160 |
| GPIO | 36 | 17 |
| Touch | 10 | - |
| ADC | 16, 12bit | 1, 10bit |
| DAC | 1, 8bit | - |
| Bluetooth | BLE 1.3 | - |
| CAN | 1 | - |
| I2C | 2 | 1 |
| PWM | 16 | 8 |
| Hall | 1 | - |
| typ. Power Consumption | 260mA | 80mA |
The only good reason for picking ESP8266 might appear to be its much lower normal energy consumption. This lower consumption though is a direct effect of its lower performance: a higher frequency and two cores cost energy. At the same time, they can solve tasks much faster. In real-world device design, when power consumption is an issue, the trick is to use the various sleep modes during idle times. In well-designed solutions, ESP32 almost always uses less energy over-all: it solves issues faster and can stay most of the time in power-saving sleep modes.
Same Form Factor
Despite its many additional features, ESP32 are not necessarily bigger in size. The raw chip size did not change from *ESP8266.
The ESP32 C3-based board from Seeed for example is tiny:
ESP32 boards can be extremely small yet very powerful
If you do not need a USB Connector, you can further cut board size. The picture below shows an ESP8266 on such a board. The same form factor is available for ESP32, effectively reducing the board size to the size of the actual microcontroller and a few supporting elements like a ceramic WiFI antenna:
Boards without USB connector and UART are even smaller but require an external programmer to transfer the firmware.
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(content created Mar 23, 2024)