Linux Kernel Driver for CST816x Touchscreen Controller

The Linux kernel provides native support for the CST816X touchscreen driver (introduced in kernel version 6.18), located at drivers/input/touchscreen/ and managed through the CONFIG_TOUCHSCREEN_CST816X configuration. This tutorial details how to enable the integrated driver and implement a Device Tree overlay to operate a 1.28-inch round LCD utilizing the CST816S controller on a Raspberry Pi.

• Driver Source Code: Available in the Linux tree under drivers/input/touchscreen/
• DT-Binding Documentation: Available under Documentation/devicetree/bindings/input/touchscreen/hynitron,cst816s.yaml

---

Documentation

Raspberry Pi 4B to Display Pinout

Module Pin	GPIO(n)
VCC	3.3V
GND	GND
MISO	GPIO9
MOSI	GPIO10
SCLK	GPIO11
LCD_CS	GPIO8
LCD_DC	GPIO25
LCD_RST	GPIO27
LCD_BL	GPIO18
TP_SDA	GPIO2
TP_SCL	GPIO3
TP_INT	GPIO4
TP_RST	GPIO17

Refer to the Waveshare guide for hardware connection details.

Input Device Capabilities

Event Code	Description
BTN_LEFT	Gesture Left
BTN_RIGHT	Gesture Right
BTN_FORWARD	Gesture Up
BTN_BACK	Gesture Down
BTN_TOUCH	Single Touch
BTN_TOOL_TRIPLETAP	Long Press (Hold)

---

Kernel Driver Setup

Enable In-Tree Kernel Driver

1. Open the kernel configuration menu:

   make menuconfig

2. Navigate to the touchscreen section:

   Device Drivers
     └─> Input device support
           └─> Touchscreens
                 └─> Hynitron CST816X touchscreen support (<M> or <*> )
   

3. Alternatively, enable it directly in your .config:

   CONFIG_TOUCHSCREEN_CST816X=m

4. Rebuild and install kernel modules:

   make modules
   sudo make modules_install

---

Device Tree Setup

DTS Snippet

#include <dt-bindings/gpio/gpio.h>
#include <dt-bindings/interrupt-controller/irq.h>

i2c {
    #address-cells = <1>;
    #size-cells = <0>;

    touchscreen@15 {
        compatible = "hynitron,cst816s";
        reg = <0x15>;
        interrupt-parent = <&gpio0>;
        interrupts = <4 IRQ_TYPE_EDGE_RISING>;
        reset-gpios = <&gpio 17 GPIO_ACTIVE_LOW>;
    };
};

Compile & Apply Overlay (Raspberry Pi 4B)

cd rpi4b/
dtc -@ -I dts -O dtb -o cst816s.dtbo hynitron-cst816s.dts
sudo cp cst816s.dtbo /boot/overlays/
echo "dtoverlay=cst816s" | sudo tee -a /boot/config.txt
sudo reboot

---

Verification

Verify that the input device is recognized and registering events:

evtest /dev/input/eventN

Expected output:

Input device name: "Hynitron CST816X Touchscreen"
Supported events:
  EV_SYN
  EV_KEY: BTN_LEFT, BTN_RIGHT, BTN_FORWARD, BTN_BACK, BTN_TOUCH, BTN_TOOL_TRIPLETAP
  EV_ABS: ABS_X (0-240), ABS_Y (0-240)