Mastering MicroPython 1-Wire Communication

Table of Contents#

1. Fundamental Concepts of MicroPython 1-Wire
2. Usage Methods
3. Common Practices
4. Best Practices
5. Conclusion
6. References

Fundamental Concepts of MicroPython 1-Wire#

1-Wire Protocol Basics#

The 1-Wire protocol uses a single bidirectional data line to transfer data between a master device (usually a microcontroller) and one or more slave devices. Communication on the 1-Wire bus is asynchronous, and data is transmitted bit by bit.

The protocol has a unique addressing scheme. Each 1-Wire device has a 64-bit unique identifier (ROM code), which consists of a 8-bit family code, 48-bit serial number, and an 8-bit CRC (Cyclic Redundancy Check) value. This allows the master to identify and communicate with specific devices on the bus.

MicroPython's 1-Wire Module#

MicroPython provides a onewire module to interact with 1-Wire devices. This module abstracts the low - level details of the 1-Wire protocol, making it easier for developers to use. The main classes and functions in the onewire module include:

• OneWire(pin): Initializes a 1-Wire bus on the specified GPIO pin.
• scan(): Scans the 1-Wire bus and returns a list of ROM codes of all the connected devices.
• reset(): Resets the 1-Wire bus, which is required before any communication with the devices.
• writebit(bit): Writes a single bit to the 1-Wire bus.
• readbit(): Reads a single bit from the 1-Wire bus.

Usage Methods#

Setting up the 1-Wire Bus#

First, you need to import the onewire module and initialize the 1-Wire bus on a specific GPIO pin. Here is an example using the Raspberry Pi Pico:

import machine
import onewire
 
# Initialize the 1-Wire bus on GPIO pin 2
pin = machine.Pin(2)
ow = onewire.OneWire(pin)

Scanning for Devices#

After initializing the 1-Wire bus, you can scan for connected devices using the scan() method.

# Scan for devices on the 1-Wire bus
devices = ow.scan()
print('Found devices:', devices)

Communicating with a Device#

Once you have identified the connected devices, you can communicate with them. For example, to send a command to a device, you first need to reset the bus and then write the appropriate command bytes.

# Reset the 1-Wire bus
ow.reset()
 
# Select the first device
device_rom = devices[0]
ow.select_rom(device_rom)
 
# Write a command (e.g., start a temperature conversion for a DS18B20 sensor)
ow.writebyte(0x44)

Common Practices#

Reading Temperature from a DS18B20 Sensor#

The DS18B20 is a popular 1-Wire temperature sensor. Here is a complete example of reading the temperature from a DS18B20 sensor using MicroPython:

import machine
import onewire
import ds18x20
import time
 
# Initialize the 1-Wire bus on GPIO pin 2
pin = machine.Pin(2)
ow = onewire.OneWire(pin)
ds = ds18x20.DS18X20(ow)
 
# Scan for devices on the 1-Wire bus
roms = ds.scan()
print('Found DS18B20 devices:', roms)
 
while True:
    # Start temperature conversion
    ds.convert_temp()
    time.sleep_ms(750)  # Wait for conversion to complete
 
    for rom in roms:
        temp = ds.read_temp(rom)
        print('Temperature:', temp, '°C')
 
    time.sleep(1)

Error Handling#

When working with the 1-Wire protocol, errors can occur due to various reasons such as loose connections or device failures. It is important to handle these errors properly. For example, you can check the return value of the reset() method to ensure that the bus was successfully reset.

if not ow.reset():
    print('1-Wire bus reset failed')
else:
    # Proceed with communication
    pass

Best Practices#

Power Management#

1-Wire devices can be powered in different ways: parasitically or externally. When using parasitic power, the device draws power from the data line during data transfer. However, this may not be sufficient for some operations, such as temperature conversion in a DS18B20 sensor. In such cases, it is recommended to use external power to ensure reliable operation.

Timing Considerations#

The 1-Wire protocol has strict timing requirements. Make sure to follow the recommended timing intervals between different operations, such as waiting for the temperature conversion to complete in a DS18B20 sensor.

Code Optimization#

If you are working with multiple 1-Wire devices, try to optimize your code to reduce the number of bus resets and device selections. This can improve the overall performance of your system.

Conclusion#

MicroPython's support for the 1-Wire protocol provides a convenient way for developers to interface with various 1-Wire devices. By understanding the fundamental concepts, usage methods, common practices, and best practices, you can effectively use the 1-Wire protocol in your MicroPython projects. Whether you are building a temperature monitoring system or a data logging application, the 1-Wire protocol can be a valuable tool in your embedded systems toolkit.

References#

• MicroPython Documentation
• Maxim Integrated 1-Wire Protocol Documentation
• DS18B20 Datasheet