DHT11 Sensor Driver into Yocto : When I first got my BeagleBone board, I was super excited. I wanted to connect real-world sensors and see live data flowing into my board. One of the simplest yet powerful sensors I picked was the DHT11, a low-cost digital temperature and humidity sensor.
At first glance, I thought: “It should be as easy as connecting a wire and running a Python script, right?” But then came the challenge — I wanted to integrate it properly at the kernel driver level and make it part of my Yocto-built image, so that every time I flashed my SD card, my BeagleBone would already have the driver and I could directly fetch temperature readings.
If you’ve ever felt this curiosity, this guide is for you. Let’s go step by step.
Manual Way: Adding DHT11 Sensor Driver into Yocto for BeagleBone
Why DHT11 Sensor Driver into Yocto ?
- Consistency: Every image build includes your driver — no need to manually insert modules each time.
- Customization: You can control how the driver behaves and integrate it with other services.
- Scalability: If tomorrow you move to another sensor or a production board, your Yocto setup scales with you.
Step 1: Set Up Yocto Project Environment
- Download Poky and layers
git clone git://git.yoctoproject.org/poky.git cd poky git checkout <yocto-version> # e.g., dunfell, honister - Add necessary layers for BeagleBone
git clone git://git.yoctoproject.org/meta-ti.git git clone git://git.yoctoproject.org/meta-openembedded.git - Create a build directory
source oe-init-build-env build - Set your machine in
conf/local.confMACHINE ?= "beaglebone"
Step 2: Understand Your DHT11 Sensor
- DHT11 uses 1-wire digital communication.
- Pins:
- VCC: 3.3V or 5V
- GND: Ground
- DATA: Connected to a GPIO pin on BeagleBone
- The driver needs to read the digital pulses and convert them to temperature and humidity.
Step 3: Create a Custom Layer
- Create a new layer for your driver:
bitbake-layers create-layer meta-dht11 - Add this layer to your build:
bitbake-layers add-layer ../meta-dht11
Step 4: Write the Kernel Driver
4.1: Create Driver Skeleton
- Go to your layer:
meta-dht11/recipes-kernel/dht11/ - Create
dht11.c:
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/gpio.h>
#include <linux/delay.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#define DHT11_PIN 60 // Example: GPIO1_28 on BeagleBone
static int major;
static char temp_str[16];
static int dht11_open(struct inode *inode, struct file *file) {
return 0;
}
static int dht11_release(struct inode *inode, struct file *file) {
return 0;
}
static ssize_t dht11_read(struct file *file, char __user *buf, size_t len, loff_t *offset) {
int temperature = 25; // Mock value (we’ll implement real reading later)
snprintf(temp_str, sizeof(temp_str), "%d\n", temperature);
if (copy_to_user(buf, temp_str, strlen(temp_str))) {
return -EFAULT;
}
return strlen(temp_str);
}
static struct file_operations fops = {
.owner = THIS_MODULE,
.open = dht11_open,
.release = dht11_release,
.read = dht11_read,
};
static int __init dht11_init(void) {
major = register_chrdev(0, "dht11", &fops);
printk(KERN_INFO "DHT11 driver loaded. Major=%d\n", major);
return 0;
}
static void __exit dht11_exit(void) {
unregister_chrdev(major, "dht11");
printk(KERN_INFO "DHT11 driver unloaded\n");
}
module_init(dht11_init);
module_exit(dht11_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Nish");
MODULE_DESCRIPTION("DHT11 temperature driver for BeagleBone");
4.2: Create the Recipe (.bb file)
- File:
meta-dht11/recipes-kernel/dht11/dht11.bb
DESCRIPTION = "DHT11 temperature driver"
LICENSE = "GPLv2"
SRC_URI = "file://dht11.c"
S = "${WORKDIR}"
do_compile() {
${CC} -Wall -O2 -c ${WORKDIR}/dht11.c -o ${WORKDIR}/dht11.o
${CC} -Wall -O2 -shared -o ${WORKDIR}/dht11.ko ${WORKDIR}/dht11.o
}
do_install() {
install -d ${D}${sysconfdir}/modules
install -m 0644 ${WORKDIR}/dht11.ko ${D}${sysconfdir}/modules/
}
Step 5: Build and Add Driver to Image
- Build the driver:
bitbake dht11 - Include it in your image (
local.conf):IMAGE_INSTALL_append = " dht11" - Rebuild your image:
bitbake core-image-minimal
Step 6: Boot BeagleBone and Test
- Flash
.wicimage to SD card. - Boot BeagleBone.
- Load driver:
insmod /etc/modules/dht11.ko - Check temperature:
cat /dev/dht11
Step 7: Implement Actual DHT11 Reading
- Replace mock temperature with real GPIO reading.
- Use bit-banging with
gpio_get_value()and precise timing withudelay().
Tips
- Make sure GPIO pin numbers match your BeagleBone header.
- For production, consider interrupt-based reading for better accuracy.
- Use
dmesgto debug kernel messages.
Auto Temperature Reading Without Manual Module Loading
Step 1: Setup Yocto Kirkstone Environment
- Clone Poky (Kirkstone branch):
git clone -b kirkstone git://git.yoctoproject.org/poky.git
cd poky
- Clone meta layers:
git clone -b kirkstone git://git.yoctoproject.org/meta-openembedded.git
git clone -b kirkstone git://git.yoctoproject.org/meta-ti.git
- Initialize the build environment:
source oe-init-build-env
- Set your target machine in
conf/local.conf:
MACHINE ?= "beaglebone"
Step 2: Create a Custom Layer for DHT11
- Create layer:
bitbake-layers create-layer meta-dht11
- Add it to your build:
bitbake-layers add-layer ../meta-dht11
Step 3: Write the DHT11 Driver
- Go to your layer folder:
meta-dht11/recipes-kernel/dht11/ - Create
dht11.c:
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/gpio.h>
#include <linux/delay.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#define DHT11_PIN 60 // Adjust according to BeagleBone header
static int major;
static char buf[32];
static int dht11_open(struct inode *inode, struct file *file) { return 0; }
static int dht11_release(struct inode *inode, struct file *file) { return 0; }
static int read_dht11(int *temperature, int *humidity) {
int bits[40] = {0}, i, j=0, data[5] = {0};
// Send start signal
gpio_direction_output(DHT11_PIN, 0);
mdelay(20);
gpio_direction_input(DHT11_PIN);
udelay(30);
// Wait for response
while(gpio_get_value(DHT11_PIN) == 1);
while(gpio_get_value(DHT11_PIN) == 0);
while(gpio_get_value(DHT11_PIN) == 1);
// Read 40 bits
for(i=0;i<40;i++){
while(gpio_get_value(DHT11_PIN) == 0);
j=0;
while(gpio_get_value(DHT11_PIN) == 1){ udelay(1); j++; }
bits[i] = (j > 40) ? 1 : 0;
}
// Convert bits to bytes
for(i=0;i<5;i++){
data[i]=0;
for(j=0;j<8;j++){
data[i]<<=1;
data[i] |= bits[i*8+j];
}
}
if(data[4] != ((data[0]+data[1]+data[2]+data[3]) & 0xFF)) return -1;
*humidity = data[0];
*temperature = data[2];
return 0;
}
static ssize_t dht11_read(struct file *file, char __user *user_buf, size_t len, loff_t *offset){
int temp=0, hum=0;
if(read_dht11(&temp,&hum)!=0) return -EIO;
snprintf(buf,sizeof(buf),"Temp:%dC Hum:%d%%\n",temp,hum);
if(copy_to_user(user_buf,buf,strlen(buf))) return -EFAULT;
return strlen(buf);
}
static struct file_operations fops = {
.owner=THIS_MODULE,
.open=dht11_open,
.release=dht11_release,
.read=dht11_read,
};
static int __init dht11_init(void){
major = register_chrdev(0,"dht11",&fops);
printk(KERN_INFO "DHT11 driver loaded. Major=%d\n",major);
return 0;
}
static void __exit dht11_exit(void){
unregister_chrdev(major,"dht11");
printk(KERN_INFO "DHT11 driver unloaded\n");
}
module_init(dht11_init);
module_exit(dht11_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Nish");
MODULE_DESCRIPTION("DHT11 driver for BeagleBone with real readings");
Step 4: Create the BitBake Recipe
Create dht11.bb in the same folder:
DESCRIPTION = "DHT11 Temperature & Humidity Kernel Driver"
LICENSE = "GPLv2"
SRC_URI = "file://dht11.c"
S = "${WORKDIR}"
do_compile() {
${CC} -Wall -O2 -c ${WORKDIR}/dht11.c -o ${WORKDIR}/dht11.o
${CC} -Wall -O2 -shared -o ${WORKDIR}/dht11.ko ${WORKDIR}/dht11.o
}
do_install() {
install -d ${D}${nonarch_libdir}/modules
install -m 0644 ${WORKDIR}/dht11.ko ${D}${nonarch_libdir}/modules/
# Auto-load at boot
install -d ${D}/etc/modules-load.d
echo "dht11" > ${D}/etc/modules-load.d/dht11.conf
}
Step 5: Include Driver in Yocto Image
Edit conf/local.conf and add:
IMAGE_INSTALL_append = " dht11"
Step 6: Build the Image
bitbake core-image-minimal
After completion, your image already includes the DHT11 driver and will auto-load at boot.
Step 7: Flash SD Card and Test
- Flash
.wicimage to SD card. - Boot BeagleBone.
- Check temperature and humidity:
cat /dev/dht11
# Example output: Temp:27C Hum:55%
✅ You don’t need to insmod anymore. The driver is now part of the image.
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