Linux Kernel Driver for CST816x Touchscreen Controller

This guide helps you build, install, and set up a kernel module and device tree overlay for the CST816S touchscreen controller on a Raspberry Pi with a 1.28-inch round LCD.

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📌 Documentation

Raspberry Pi 4B to Display Pinout

Module Pin	Raspberry Pi (BCM)
VCC	3.3V
GND	GND
MISO	9
MOSI	10
SCLK	11
LCS_CS	8
LCS_DC	25
LCS_RST	27
LCS_BL	18
TP_SDA	2
TP_SCL	3
TP_INT	4
TP_RST	17

Refer to the Waveshare guide for hardware connection.

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)

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🧪 Testing Environment

• GC9A01 FBTFT Overlay
• LVGL Demo

Enable Frame Buffer in lv_conf.h

#define LV_USE_LINUX_FBDEV 1

Initialize Frame Buffer

const char *device = getenv_default("LV_LINUX_FBDEV_DEVICE", "/dev/fbN");
lv_display_t *disp = lv_linux_fbdev_create();
lv_linux_fbdev_set_file(disp, device);

Initialize Input Device

lv_indev_t *input = lv_evdev_create(LV_INDEV_TYPE_POINTER, "/dev/input/eventN");
lv_indev_set_display(input, disp);

Main Loop

ui_init();
while (1) {
    format_linux_date();
    lv_timer_handler();
    usleep(1000);
}

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🧱 Kernel Module Setup

Build as External Module

cd cst816x-driver/
make
# Or manually specify:
make -C <path_to_kernel_sources> M=$PWD

Install the Module

make install
# Or manually specify:
make -C <path_to_kernel_sources> M=$PWD install

Clean Build Files

make clean
# Or manually specify:
make -C /home/pi/linux-6.9.y M=$PWD clean

Load the Module

sudo insmod hynitron-cst816x.ko
dmesg

Example output:

[1593.107] hynitron_cst816x: loading out-of-tree module taints kernel.
[1593.283] input: Hynitron CST816X Touchscreen as /devices/...

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🔧 Built-in Kernel Driver

Kconfig Addition

config TOUCHSCREEN_HYNITRON_CST816X
    tristate "Hynitron CST816X touchscreen support"
    depends on I2C
    help
      Say Y if you have a Hynitron CST816X touchscreen.

Makefile Addition

obj-$(CONFIG_TOUCHSCREEN_HYNITRON_CST816X) += hynitron-cst816x.o

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🌱 Device Tree Setup

Example 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>;
    };
};

Raspberry Pi DTS Overlay

cd cst816x-driver/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

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🧪 Verify Input Device

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)