One Wire (1-Wire) Communication Protocol: Single Wire Interface Overview

The 1-Wire interface protocol is a communication protocol developed by Dallas Semiconductor (now part of Maxim Integrated) that uses a single wire to transmit and receive data, along with a ground connection. This protocol is designed for simplicity and cost-efficiency, making it suitable for a range of applications. Let us understand one wire protocol along with its features, advantages and disadvantages.

Single wire interface uses master and slave configuration in which single slave or multiple slaves are interfaced with single master on the bus. It is generally used for low power and low speed communication. The data communication is byte wise in which LSB (Least Significant Bit) is transmitted first.

Key features of one wire protocol

Following are the features of one wire interface protocol:
• It use single data line and no clock is needed. There are at least two wires (i.e. data and GND) used for 1 wire protocol.
• The clock signal is not required as slave devices make use of internal clock. This internal clock signal is synchronized with signal from master device.
• It has Half duplex communication mechanism.
• It has less hardware complexity but has more software complexity in implementing the algorithm.
• Due to less wiring, it is cheaper interface protocol and hence it is economical.
• It uses supply voltage between 2.8V to 5.25V .
• It uses 64 bit device addressing scheme.
• The multiple slaves are supported in multi-drop mode.
• It supports data rate of 16.3 Kbps in standard mode and 163 kbps in overdrive mode.
• It has less power consumption.

1 wire interface Bus Requirements

The figure-1 depicts simple 1 wire interface configuration between single master and multiple slaves in multi-drop mode. Following interface bus requirements and power modes are supported.

Bus Requirement:
All the output pins should be connected with weak pull up resistor and they must be open drain. The bus will have low state when any one device drives it low. The transfer of data occurs between two devices when others are in IDLE state. The device uses external power. Hence it requires pull up values 10K or higher for lower data rate with less trace length. It requires pull up less than 1K for higher data rate with long trace length.

Power Modes:
There are two power modes supported by slave devices.
• External mode: Slave requiring more power uses this mode. Power pin is used on the slave device to interface with external supply.
• Parasitic mode: Slave derives power from the data line. When the bus is idle, it utilizes capacitor to store the energy. It is pulled up by weak pull up resistor.

One wire protocol signaling modes

The figure-2 depicts write 0/1 bit signaling diagram. As shown, drive the bus low for about 60µs to write 0 bit. To write 1 bit, drive the bus low for less than 15 µs. Typically 6µs is used. Release the bus until 60 µs after falling edge.

The figure-3 depicts read 0/1 bit signaling diagram. The read bit signaling is same as write 1 signaling diagram. Here master will read instead of writing. The logic for reading is to drive the bus low from 1µs to 15µs. Here sampling of bus is done after falling edge to read the bit from slave usually at 15 µs.

Advantages of 1 Wire Protocol

1. Simplicity:
Single Wire: Only one data line and a ground line are needed, reducing the complexity and cost of wiring.
Ease of Implementation: The protocol is relatively simple to implement and integrate into various systems.
2. Cost Effective:
Reduced Wiring Costs: Fewer wires mean lower material and installation costs.
Low Cost Devices: Many 1-Wire devices are designed to be inexpensive, making the protocol suitable for large-scale applications.
3. Flexible Power Options:
Parasite Power Mode: Devices can operate without an external power supply, which is useful in battery operated or remote applications.
4. Multiple Devices: Multiple 1-Wire devices can be connected to the same bus, allowing for easy expansion and scalability.
5. Robustness: Built-in error checking mechanisms help ensure data integrity and reliability.

Disadvantages of 1 Wire Protocol

1. Limited Data Rate: The 1-Wire protocol has relatively low data transfer rates compared to other communication protocols, which may not be suitable for high speed applications.
2. Limited Distance: The communication range is limited, and performance can degrade over longer distances or with multiple devices connected to the same bus.
3. Power Constraints: In parasite power mode, power constraints can limit the number of devices on the bus and affect performance. For some applications, external power may be required.
4. Complex Timing Requirements: The protocol requires precise timing for communication, which can make implementation challenging in some systems.
5. Device Addressing: While each 1-Wire device has a unique address, managing and organizing a large number of devices on the same bus can become complex.

Summary

The 1-Wire protocol is a useful and cost-effective solution for simple communication and sensor networks, especially where wiring simplicity and low cost are priorities. However, it has limitations in terms of data rate, distance, and power constraints, which should be considered when designing systems that use this protocol.

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