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ESP32 family
CPU » ESP32 SoC Series

Specific properties of ESP32 variant (family)

Specific properties of ESP32 variant (family)

Author
Gunar Schorcht gunar.nosp@m.@sch.nosp@m.orcht.nosp@m..net

Specific properties of ESP32 variant (family)

GPIO pins

ESP32 has 34 GPIO pins, where only a subset can be used as output, as ADC channel, as DAC channel and as GPIOs in deep-sleep mode, the so-called RTC GPIOs. Some of them are used by special SoC components, e.g., as touch sensors. The following table gives a short overview.

Pin Type ADC / RTC PU / PD Special function Remarks
GPIO0 In/Out yes yes Touch sensor Bootstrapping, pulled up
GPIO1 In/Out - yes UART0 TxD Console
GPIO2 In/Out yes yes Touch sensor Bootstrapping, pulled down
GPIO3 In/Out - yes UART0 RxD Console
GPIO4 In/Out yes yes Touch sensor -
GPIO5 In/Out - yes - -
GPIO6 In/Out - yes Flash SD_CLK -
GPIO7 In/Out - yes Flash SD_DATA0 -
GPIO8 In/Out - yes Flash SD_DATA1 -
GPIO9 In/Out - yes Flash SD_DATA2 only in qoutand qiomode, see section Flash Modes
GPIO10 In/Out - yes Flash SD_DATA3 only in qoutand qiomode, see section Flash Modes
GPIO11 In/Out - yes Flash SD_CMD -
GPIO12 In/Out yes yes MTDI / Touch sensor JTAG interface / Bootstrapping, pulled down
GPIO13 In/Out yes yes MTCK / Touch sensor JTAG interface
GPIO14 In/Out yes yes MTMS / Touch sensor JTAG interface
GPIO15 In/Out yes yes MTDO / Touch sensor JTAG interface / Bootstrapping, pulled up
GPIO16 In/Out - yes - usually not available when SPI RAM is used
GPIO17 In/Out - yes - usually not available when SPI RAM is used
GPIO18 In/Out - yes - -
GPIO19 In/Out - yes - -
GPIO21 In/Out - yes - -
GPIO22 In/Out - yes - -
GPIO23 In/Out - yes - -
GPIO25 In/Out yes yes DAC1 -
GPIO26 In/Out yes yes DAC2 -
GPIO27 In/Out yes yes Touch sensor -
GPIO32 In/Out yes yes XTAL32_P can be used to connect an external 32 kHz crystal
GPIO33 In/Out yes - XTAL32_N can be used to connect an external 32 kHz crystal
GPIO34 In yes - VDET -
GPIO35 In yes - VDET -
GPIO36 In yes - SENSOR_VP -
GPIO37 In yes - SENSOR_CAPP usually not broken out
GPIO38 In yes - SENSOR_CAPN usually not broken out
GPIO39 In yes - SENSOR_VN -

ADC: these pins can be used as ADC inputs
RTC: these pins are RTC GPIOs and can be used in deep-sleep mode
PU/PD: these pins have software configurable pull-up/pull-down functionality.

Note
GPIOs that can be used as ADC channels are also available as low power digital inputs/outputs in deep sleep mode.

GPIO0, GPIO2 are bootstrapping pins which are used to boot ESP32 in different modes:

GPIO0 GPIO2 Mode
1 X boot in FLASH mode to boot the firmware from flash (default mode)
0 0 boot in UART mode for flashing the firmware


Note
GPIO2 becomes the SPI MISO signal for boards that use the HSPI interface as SD-Card interface in 4-bit SD mode. On these boards all signals of the SD-Card interface are pulled up. Because of the bootstrapping functionality of GPIO2, it can become necessary to either press the Boot** button, remove the SD card or remove the peripheral hardware to flash RIOT.

ADC Channels

ESP32 integrates two 12-bit ADCs (ADC1 and ADC2) capable of measuring up to 18 analog signals in total. Most of these ADC channels are either connected to a number of integrated sensors like a Hall sensors, touch sensors and a temperature sensor or can be connected with certain GPIOs. Integrated sensors are disabled in RIOT's implementation and are not accessible. Thus, up to 18 GPIOs, can be used as ADC inputs:

These GPIOs are realized by the RTC unit and are therefore also called RTC GPIOs or RTCIO GPIOs.

The maximum number of ADC channels ADC_NUMOF_MAX is 18

Note
  • GPIO37 and GPIO38 are usually not broken out on ESP32 boards and can not be used therefore.
  • ADC2 is also used by the WiFi module. The GPIOs connected to ADC2 are therefore not available as ADC channels if the modules esp_wifi or esp_now are used.
  • Vref can be read with function adc_line_vref_to_gpio at GPIO25.

DAC Channels

ESP32 SoC supports 2 DAC lines at GPIO25 and GPIO26.

I2C Interfaces

ESP32 has two built-in I2C interfaces.

The following table shows the default configuration of I2C interfaces used for a large number of ESP32 boards. It can be overridden by application-specific configurations.

Device Signal Pin Symbol Remarks
I2C_DEV(0) #I2C0_SPEED default is I2C_SPEED_FAST
I2C_DEV(0) SCL GPIO22 #I2C0_SCL -
I2C_DEV(0) SDA GPIO21 #I2C0_SDA -


PWM Channels

The ESP32 LEDC module has 2 channel groups with 8 channels each. Each of these channels can be clocked by one of the 4 timers.

SDMMC Interfaces

The ESP32 variant uses the direct I/O (i.e. SOC_SDMMC_USE_IOMUX is defined in the SoC capabilities file). The GPIOs used for SDMMC signals are therefore fixed for each slot. Since the GPIOs used for slot 0 are the same as those used for the Flash, slot 0 cannot be used. Therefore, only slot 1 can be used.

The GPIOs used by ESP32 for slot 1 are:

Signal GPIO Slot 1
CLK GPIO14
CMD GPIO15
DAT0 GPIO2
DAT1 GPIO4
DAT2 GPIO12
DAT3 GPIO13

The board-specific configuration is realized by defining the sdmmc_config array, for example:

static const sdmmc_conf_t sdmmc_config[] = {
{
.slot = SDMMC_SLOT_1,
.cd = GPIO21,
.wp = GPIO_UNDEF,
.bus_width = 1,
},
};
#define SDMMC_NUMOF 1
GPIO_UNDEF
#define GPIO_UNDEF
Definition of a fitting UNDEF value.
Definition periph_cpu_common.h:54
sdmmc_config
static const sdmmc_conf_t sdmmc_config[]
SDMMC devices.
Definition periph_conf.h:150
SDMMC_SLOT_1
@ SDMMC_SLOT_1
SD/MMC host controller slot 1.
Definition periph_cpu.h:690
sdmmc_conf_t
SDMMC slot configuration.
Definition periph_cpu.h:704
sdmmc_conf_t::slot
sdmmc_slot_t slot
SDMMC slot used [ SDMMC_SLOT_0 | SDMMC_SLOT_1].
Definition periph_cpu.h:705

Used data bus width has to be defined in sdmmc_conf_t::bus_width in addition to the slot, where 1 and 4 are valid values.

Note
The slot must be SDMMC_SLOT_1 for ESP32.

If the board supports a Card Detect pin or a Write Protect pin, the corresponding GPIOs have to be defined in sdmmc_conf_t::cd and sdmmc_conf_t::wp. Otherwise they have to be set to undefined (GPIO_UNDEF).

SPI Interfaces

ESP32 has four SPI controllers where SPI0 and SPI1 share the same bus and are used as interface for external memory:

Thus, only SPI2 (HSPI) and SPI3 (VSPI) can be used as general purpose SPI in RIOT as SPI_DEV(0) and SPI_DEV(1) in arbitrary order.

The following table shows the pin configuration used for most boards, even though it can vary from board to board.

Device Signal Pin Symbol Remarks
SPI0_HOST/SPI1_HOST SCK GPIO6 - reserved for flash and PSRAM
SPI0_HOST/SPI1_HOST CMD GPIO11 - reserved for flash and PSRAM
SPI0_HOST/SPI1_HOST SD0 GPIO7 - reserved for flash and PSRAM
SPI0_HOST/SPI1_HOST SD1 GPIO8 - reserved for flash and PSRAM
SPI0_HOST/SPI1_HOST SD2 GPIO9 - reserved for flash and PSRAM (only in qio or qout mode)
SPI0_HOST/SPI1_HOST SD3 GPIO10 - reserved for flash and PSRAM (only in qio or qout mode)
SPI2_HOST (HSPI) SCK GPIO14 #SPI1_SCK can be used
SPI2_HOST (HSPI) MISO GPIO12 #SPI1_MISO can be used
SPI2_HOST (HSPI) MOSI GPIO13 #SPI1_MOSI can be used
SPI2_HOST (HSPI) CS0 GPIO15 #SPI1_CS0 can be used
SPI3_HOST (VSPI) SCK GPIO18 #SPI0_SCK can be used
SPI3_HOST (VSPI) MISO GPIO19 #SPI0_MISO can be used
SPI3_HOST (VSPI) MOSI GPIO23 #SPI0_MOSI can be used
SPI3_HOST (VSPI) CS0 GPIO5 #SPI0_CS0 can be used


Some boards use the HSPI (SPI2_HOST) as SD-Card interface (SDIO) in 4-bit SD mode.

Device Pin SD 4-bit mode SPI mode
SPI2_HOST (HSPI) GPIO14 CLK SCK
SPI2_HOST (HSPI) GPIO15 CMD CS0
SPI2_HOST (HSPI) GPIO2 DAT0 MISO
SPI2_HOST (HSPI) GPIO4 DAT1 -
SPI2_HOST (HSPI) GPIO12 DAT2 -
SPI2_HOST (HSPI) GPIO13 DAT3 MOSI


Note
On these boards, all these signals are pulled up. This may cause flashing problems due to the bootstrap function of the GPIO2 pin, see section GPIO pins. Therefore, it can be necessary to either press the Boot button, to remove the SD card or to remove the peripheral hardware to flash RIOT.

Timers

ESP32 has two timer groups with two timers each, resulting in a total of four timers. Since one timer is used as system timer, up to three timers with one channel each can be used in RIOT as timer devices TIMER_DEV(0) ... TIMER_DEV(2).

Additionally ESP32 has three CCOMPARE registers which can be used alternatively as timer devices TIMER_DEV(0) ... TIMER_DEV(2) can be used in RIOT if the module esp_hw_counter is enabled.

UART Interfaces

ESP32 integrates three UART interfaces. The following default pin configuration of UART interfaces as used by a most boards can be overridden by the application, see section [Application-Specific Configurations] (#esp32_application_specific_configurations).

Device Signal Pin Symbol Remarks
UART_DEV(0) TxD GPIO1 #UART0_TXD cannot be changed
UART_DEV(0) RxD GPIO3 #UART0_RXD cannot be changed
UART_DEV(1) TxD GPIO10 #UART1_TXD optional, can be overridden
UART_DEV(1) RxD GPIO9 #UART1_RXD optional, can be overridden
UART_DEV(2) TxD GPIO17 UART2_TXD optional, can be overridden
UART_DEV(2) RxD GPIO16 UART2_RXD optional, can be overridden


JTAG Interface

The JTAG interface on ESP32 is connected to the following GPIOs:

JTAG Signal ESP32 Pin
TRST_N CHIP_PU
TDO GPIO15 (MTDO)
TDI GPIO12 (MTDI)
TCK GPIO13 (MTCK)
TMS GPIO14 (MTMS)
GND GND