Support for the DS1821 Thermometer on Raspberry Pi
The Dallas DS1821 is an unusual 1-Wire “compatible” device. It functions as both a thermometer and a standalone thermostat, but it does not conform to the standard 1-Wire ROM protocol. As a result, the Linux kernel’s w1_therm driver cannot detect or communicate with it.
DS1821-Pi provides a set of C utilities and shell scripts that communicate with the DS1821 by bit-banging the 1-Wire protocol directly over GPIO, without requiring a kernel driver.
The problem
Standard 1-Wire thermometers such as the DS18B20 and DS18S20 each carry a unique 64-bit ROM code. The bus master uses this code to address individual devices, and the Linux w1_therm driver manages the process automatically — each sensor appears as a directory under /sys/bus/w1/devices/.
The DS1821 in thermostat mode does not follow this convention. It lacks a ROM code, does not respond to Search ROM or Match ROM commands, and treats the first byte received after a bus reset as a function command. Consequently, it cannot coexist with other 1-Wire devices, and the kernel is unable to discover it.
The solution
DS1821-Pi uses the pigpio library to implement the 1-Wire protocol at microsecond precision directly on a GPIO pin. The entire solution runs in userspace — no kernel module or device tree overlay is required.
Quick start
Wiring
Connect the DS1821 to the Raspberry Pi as follows:
DS1821 DQ → GPIO 17 (with a 4.7kΩ pull-up to 3.3V)
DS1821 GND → GND
DS1821 VDD → GPIO 4
Build
sudo apt install libpigpio-dev
git clone https://github.com/edwbaker/ds1821-pi
cd ds1821-pi
make
Read the temperature
sudo ./ds1821-program temp -q --power-gpio 4 --gpio 17
The command returns the temperature in degrees Celsius:
20.34
Omitting the -q flag produces a verbose output that includes the raw register values, counter data, and high-resolution calculation:
sudo ./ds1821-program temp
Example: periodic temperature logging
The following example demonstrates continuous temperature logging using the included systemd timer.
1. Install:
sudo make install
2. Edit the sensor config at /etc/ds1821/sensors.conf:
# <name> <data-gpio> [power-gpio]
indoor 17 4
3. Enable the timer:
sudo systemctl enable --now ds1821-update.timer
The timer invokes ds1821-update every 60 seconds. Each reading is written to files under /run/ds1821/indoor/:
cat /run/ds1821/indoor/temperature # millidegrees, e.g. 20340
cat /run/ds1821/indoor/thresholds # "th=25 tl=18"
cat /run/ds1821/indoor/alarms # "thf=0 tlf=0"
The file layout intentionally mirrors the kernel’s w1_therm sysfs structure. Existing scripts that read /sys/bus/w1/devices/.../temperature can be redirected to /run/ds1821/ with minimal modification.
Thermostat control
In addition to temperature measurement, the DS1821 provides hardware thermostat functionality with programmable high and low trip points. These thresholds can be configured from the command line:
# Set high threshold to 25°C and low threshold to 18°C
sudo ./ds1821-program set-th 25 set-tl 18
When the measured temperature crosses either threshold, the DS1821 asserts its TOUT output. The current TOUT state can be read as follows:
sudo ./ds1821-program --read-tout status
Links
- Source code: GitHub
- DS1821 datasheet: Maxim Integrated (PDF)