Pin Reference

The ESP32-S3 provides a generous set of GPIOs by microcontroller standards, but the 16-bit RGB parallel display interface consumes nineteen of them in one stroke. Understanding exactly which pins are committed to which peripheral — and which remain available for your application — is essential for effective hardware design around this board. The CH422G IO expander partially compensates for this GPIO pressure by offloading several control signals to the I2C bus, but the allocation still requires careful planning.

Note

The ESP32-S3 has limited GPIO, and many are dedicated to the RGB LCD interface. The CH422G IO expander handles several control signals that would otherwise consume additional GPIOs. Always consult the tables below before assigning pins in your firmware.

RGB LCD Interface (16-bit RGB565)

The display uses a 16-bit RGB565 parallel interface, meaning five bits for red, six for green, and five for blue — plus synchronization and clock signals. The ESP32-S3’s LCD peripheral drives these pins directly at the pixel clock rate (21 MHz). Because there are not enough GPIOs to support full 24-bit RGB888 color, the lower bits of each color channel (R0-R2, G0-G1, B0-B2) are tied to GND on the PCB, fixing them at zero. The visible result is 65K colors, which is more than sufficient for LVGL interfaces and typical embedded UI work.

GPIO	Function	Description
IO0	R4	Red channel bit 4
IO1	B6	Blue channel bit 6
IO2	B5	Blue channel bit 5
IO3	VSYNC	Vertical sync
IO4	R3	Red channel bit 3 (LSB)
IO5	DE	Data enable
IO6	G3	Green channel bit 3 (LSB)
IO7	PCLK	Pixel clock (21 MHz)
IO14	B3	Blue channel bit 3 (LSB)
IO21	G7	Green channel bit 7 (MSB)
IO38	B4	Blue channel bit 4
IO39	G2	Green channel bit 2
IO40	R7	Red channel bit 7 (MSB)
IO41	R6	Red channel bit 6
IO42	R5	Red channel bit 5
IO45	G4	Green channel bit 4
IO46	HSYNC	Horizontal sync
IO47	B7	Blue channel bit 7 (MSB)
IO48	G5	Green channel bit 5

Tip

The missing color bits (R0-R2, G0-G1, B0-B2) are hardwired to GND on the PCB. This is a design trade-off — not a limitation of the ESP32-S3’s LCD peripheral itself, but of the available GPIO count. If your application requires higher color fidelity, external shift registers could theoretically expand the interface, though this is rarely practical.

I2C Bus (Shared)

The board routes a single I2C bus to three on-board devices and an external expansion header. All devices share the same SDA/SCL pair, and proper I2C address management is critical to avoid bus collisions.

GPIO	Function	Devices
IO8	SDA	GT911 touch, CH422G IO expander, PCF85063A RTC
IO9	SCL	GT911 touch, CH422G IO expander, PCF85063A RTC

I2C Address Map

Address	Device	Function
0x20-0x27	CH422G	IO expander (8-address range)
0x51	PCF85063A	Real-time clock (Type B only)
0x5D	GT911	Capacitive touch controller

Caution

I2C addresses 0x20-0x27 and 0x30-0x3F are reserved by the CH422G IO expander. Do not connect external I2C devices at these addresses — doing so will cause bus collisions and unpredictable behavior on both the conflicting device and the CH422G’s control of backlight, touch reset, SD card, and USB multiplexer.

Communication Interfaces

The board provides two industrial serial buses and a shared UART path for programming. The CAN and RS-485 interfaces use dedicated transceivers with screw terminal connections on the board edge.

Interface	GPIO	Function
RS-485 TX	IO43	UART TX to SP3485 transceiver
RS-485 RX	IO44	UART RX from SP3485 transceiver
CAN TX	IO15	TWAI TX to TJA1051T transceiver
CAN RX	IO16	TWAI RX from TJA1051T transceiver

Note that IO43 and IO44 are shared between the RS-485 transceiver and the CH343P USB-UART bridge. The FSUSB42 multiplexer (controlled by CH422G EXIO5) determines which path is active. During firmware upload, the USB-UART path is selected; during normal operation, the firmware can switch to the RS-485 path.

SPI Bus (SD Card)

The microSD card slot operates in SPI mode. Three of the four SPI signals use dedicated GPIOs, while the chip select routes through the CH422G expander to conserve a GPIO pin.

GPIO	Function
IO11	MOSI (Master Out, Slave In)
IO12	SCK (Serial Clock)
IO13	MISO (Master In, Slave Out)
CH422G EXIO4	CS (Chip Select, active low)

Note

Because the SD card chip select is managed by the CH422G rather than a direct GPIO, the I2C bus must be initialized before the SD card can be accessed. Ensure the CH422G driver is configured early in your initialization sequence.

USB

The dual USB Type-C topology uses an analog multiplexer to share the ESP32-S3’s USB data pins between two physical ports.

GPIO	Function	Notes
IO43	ESP_TXD	USB-UART via CH343P (Type-C2, programming port)
IO44	ESP_RXD	USB-UART via CH343P (Type-C2, programming port)
IO19	USB D-	Native USB Host/OTG (Type-C1, side port)
IO20	USB D+	Native USB Host/OTG (Type-C1, side port)

The FSUSB42 multiplexer, controlled by CH422G EXIO5, switches between the UART bridge and native USB paths. Only one USB mode can be active at any given time.

Touch Panel

The GT911 capacitive touch controller connects through the shared I2C bus with an interrupt line and an expander-controlled reset.

GPIO	Function
IO4	TP_IRQ (touch interrupt, active low)
IO8	TP_SDA (shared I2C data)
IO9	TP_SCL (shared I2C clock)
CH422G EXIO1	TP_RST (touch controller reset)

The interrupt line (IO4) signals the ESP32-S3 when a touch event occurs, allowing event-driven rather than polled touch handling. The reset line through CH422G EXIO1 enables software-controlled touch controller initialization and recovery from fault states.

CH422G IO Expander Pin Map

The CH422G sits at the nexus of several board subsystems. Its outputs control critical functions that would otherwise consume scarce GPIOs. Understanding this mapping is essential for proper board initialization — the CH422G must be configured before the display, SD card, or USB multiplexer can function.

EXIO Pin	Connected To	Function
EXIO1 (OC0)	CTP_RST	Touch controller reset (active low)
EXIO2 (OC1)	LCD_BL	LCD backlight enable (high = on)
EXIO3 (OC2)	LCD_RST	LCD panel reset (active low)
EXIO4 (OC3)	SDCS	SD card chip select (active low)
EXIO5 (IO0)	USB_SEL	USB switch control (FSUSB42 mux select)
EXIO0 / EXIO5	DI0, DI1	Isolated digital inputs (Type B)
OD0 / OD1	DO0, DO1	Isolated digital outputs (Type B)

The open-collector outputs (OC0-OC3) can sink current but require external pullups, which are provided on the PCB for each controlled signal. The bidirectional IO pins and open-drain outputs handle the isolated I/O channels on the Type B variant.

Strapping Pins

The ESP32-S3 samples certain GPIO states during reset to determine boot mode and voltage configuration. These pins must be in the correct state during power-on or reset, and care should be taken if connecting external circuits to them.

GPIO	Default	Pull	Function
IO0	HIGH	10K pullup to 3.3V	Boot mode: HIGH = normal boot, LOW = download mode
IO45	LOW	Internal pulldown	VDD_SPI voltage: LOW = 3.3V (default)
IO46	LOW	Internal pulldown	Boot mode: LOW = SPI boot (default)

The BOOT button on the board pulls IO0 low when pressed. To enter download mode for firmware flashing, hold BOOT while pressing RESET (or during power-on), then release BOOT after the chip has sampled the pin state.

Available GPIOs for User Expansion

After accounting for all on-board peripherals, the following GPIOs remain available for user applications.

Fully Available

These GPIOs have no on-board connections and are fully available for user applications via the expansion headers:

GPIO	ADC Channel	Notes
IO17	ADC2_CH6	General purpose
IO18	ADC2_CH7	General purpose
IO35	—	General purpose
IO36	—	General purpose
IO37	—	General purpose

Conditionally Available

These GPIOs are assigned to on-board peripherals but can be repurposed if their peripheral is not needed:

GPIO	Default Assignment	Available If
IO15	CAN TX (TWAI)	CAN bus interface not used
IO16	CAN RX (TWAI)	CAN bus interface not used
IO19	USB D- (native)	Native USB Host/OTG not used
IO20	USB D+ (native)	Native USB Host/OTG not used

Caution

If you repurpose IO15/IO16 (CAN) or IO19/IO20 (USB), ensure the corresponding transceiver or multiplexer is not driving those lines. The TJA1051T and FSUSB42 will still be electrically connected — floating transceiver outputs can cause unexpected behavior or increased current draw.

Physical Connectors

The board exposes its interfaces through a variety of connectors suited to their intended use — FPC for display and touch ribbon cables, screw terminals for field-wired industrial buses, and pin headers for prototyping.

Designator	Type	Function
PORT1	40-pin FPC (0.5mm pitch)	Display panel (RGB565 + backlight)
—	40-pin FPC (0.5mm pitch)	Touch panel (GT911 I2C + interrupt)
Type-C1	USB Type-C (side)	Native USB Host/OTG
Type-C2	USB Type-C (top)	USB-UART programming (CH343P)
J1	3-pin screw terminal	RS-485 (A, B, GND)
J7	3-pin screw terminal	CAN bus (CANH, CANL, GND)
J5	MX1.25 / PH2.0 2-pin	Li-Po battery (3.7V)
H1	8-pin header	Touch panel breakout
H7	Pin header	Expansion (I2C, ADC, GPIO)
H8	Pin header	Expansion (power, GPIO)
—	TF card slot	microSD (push-push, SPI mode)
—	CR927 holder	RTC backup battery (Type B)