{"id":76623,"date":"2022-06-17T17:00:36","date_gmt":"2022-06-17T21:00:36","guid":{"rendered":"https:\/\/www.engineersgarage.com\/?p=76623"},"modified":"2023-08-23T12:47:44","modified_gmt":"2023-08-23T16:47:44","slug":"what-is-the-1-wire-protocol","status":"publish","type":"post","link":"https:\/\/www.engineersgarage.com\/what-is-the-1-wire-protocol\/","title":{"rendered":"What is the 1-Wire protocol?"},"content":{"rendered":"

The 1-Wire protocol is a single-wire interface, half-duplex, bidirectional, low-speed and power, long-distance serial-data communication protocol. <\/span>Although this protocol is classified as a 1-wire standard, at least two wires are required in the 1-wire standard bus \u2014 one for the data and\/or power supply and another for the ground return. An additional wire might be necessary, depending on the power mode.\u00a0<\/span><\/p>\n

A 1-wire standard has a master and slave configuration, where there can only be one master device, a computer or microcontroller, and several slave devices. It\u2019s possible to connect up to 100-slave, 1-wire devices, with a 1-wire standard bus. However, as slave devices are added to the bus, their polling by the master device might take more time.\u00a0<\/span><\/p>\n

This protocol does not use a clock signal. Instead, the slave devices are internally clocked and synchronized with a signal from the master device. The master device is solely responsible for the read and write operations of the slave devices, so they cannot initiate a data transfer on their own. What they can do is indicate their presence over the bus when the master resets.\u00a0<\/span>Every master device is identified by a 64-bit address, stored within each one-wire slave device\u2019s ROM.\u00a0<\/span><\/p>\n

This is a low-speed serial communication standard, with a typical data speed of 15.4 kbps. The bus can be overdriven to a maximum data speed of 125 kbps. The data speeds in the 1-Wire protocol are low compared to other standard serial-data communication protocols (like UART, I2C, and SPI), but the 1-wire bus is extremely economical in production and operation. It offers a simple hardware implementation and an extremely low-power footprint.\u00a0<\/span><\/p>\n

Although the hardware is simple, the software implementation on the microcontroller side is highly complex. And despite a low power consumption, it can communicate data over relatively long distances.\u00a0<\/span><\/p>\n

The 1-Wire protocol is used by temperature sensors, real-time clocks, timers, EEPROMs, and the popular iButton. Most of these 1-wire slave devices are products of (what\u2019s now) Maxim Integrated.\u00a0<\/span><\/p>\n

Let’s discuss in more detail.<\/p>\n

What is the 1-Wire protocol?
\n<\/span><\/strong>The 1-Wire protocol is a single-wire interface for low-speed data communication in microcontrollers and computers. The protocol operates over a single data line without a clock signal. It\u2019s a master-slave serial communication protocol where the half-duplex bidirectional data communication with multiple slaves is solely managed and controlled by a single master.\u00a0\u00a0<\/span><\/p>\n

The 1-wire standard bus
\n<\/span><\/strong>The 1-wire standard bus has at least two wires. One is the data line and the other is the ground return. Both master and slaves have an open-drain (open-collector) connection with the data line. This is why a 4.7K resistor typically pulls the data line up. There are two possible powering modes for the 1-wire slave devices: parasitic and conventional.\u00a0<\/span><\/p>\n

In parasitic mode, only the data line and ground return must be traced to the 1-wire slave device. If the conventional power mode is used, an additional positive supply line must be traced to each 1-wire slave connected to the bus.\u00a0<\/span><\/p>\n

Therefore, the 1-wire bus on a PCB might have two or three lines. The conventional powering with three lines in the 1-wire bus is more reliable.\u00a0\u00a0<\/span><\/p>\n

Parasitic Vs. conventional power
\n<\/span><\/strong>As mentioned, 1-wire slave devices can be powered in parasitic and conventional modes. All 1-wire slave devices have three terminals: VDD, GND and data. In the parasitic mode, the VDD and GND pins are connected to the ground, so the signal and power are supplied to the slave devices on the same line (i.e. the data line).\u00a0<\/span><\/p>\n

The slave devices have an internal capacitor of 800 pF, which gets charged when the data line is HIGH. The stored charge keeps the slave live when the data line is LOW. The data line is typically pulled up by a 4.7K resistor.\u00a0<\/span><\/p>\n

Parasitic powering requires strict timing and supply to the exact specification to maintain the slave device live without fail. This is why this mode is less reliable. Often, an additional hard pull-up is used to ascertain the power supply.\u00a0<\/span><\/p>\n

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The parasitic powering of 1-wire devices.<\/p><\/div>\n

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The parasitic powering of 1-wire devices with an additional hard pull-up.<\/p><\/div>\n

In a conventional powering mode, 1-wire slave devices are externally powered. An additional wire is traced to each 1-wire slave. The external power supply to the slaves ensures safe operation even in harsh, high-temperature conditions.<\/p>\n

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The conventional powering of 1-wire devices.<\/p><\/div>\n

Typical 1-wire devices operate in voltages from – 1.71~1.89V, 1.71~3.63V, 2.97~6.63V, and 2.97~5.25V. The current drawn ranges between 1.06~5mA. The pull-up resistor sets the current level whether devices are supplied parasitic or conventional power.\u00a0\u00a0<\/span><\/p>\n

How the 1-Wire protocol works
\n<\/span><\/strong>This interface is not usually used in microcontrollers or microcomputers. It\u2019s often implemented by software using bit-banging or a universal asynchronous receiver-transmitter (UART).\u00a0<\/span><\/p>\n

Communication over the data line is initiated by the master using a reset. It pulls down the data line for 480 us and then releases it, allowing the typical pull-up resistor to pull the data line HIGH. If slave devices are connected to the bus, they respond to the reset signal by pulling the data line LOW for 60~240 us. If the line is pulled down by the slave(s), the master confirms their presence over the bus. After 60~240 us, the slave(s) release the data line, so the master can start writing.\u00a0<\/span><\/p>\n

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After a reset, the master can write and read data with the slave devices. Initially, it sends ROM commands, like the search ROM command (0xF0), to access the ROM address of the slave devices. After reading the ROM addresses of all the connected 1-wire slave devices, the master device can access one by sending the Match ROM command (0x55). The ROM commands are followed by the function ones.\u00a0<\/span><\/p>\n

For example, if a 1-wire temperature sensor is connected to the bus, the microcontroller can send the function commands to start the temperature conversion, read the temperature, etc. The ROM and function commands are 8-bit long.\u00a0<\/span><\/p>\n

Since the 1-Wire standard does not use any clock signal, communication of the \u20180\u2019 and \u20181\u2019 bits occur by setting the logical level of the data line for a specific time slot. Usually, the time slot is 60 us long. There\u2019s also an interval of 1us between each time slot, so that the data line is pulled HIGH again by the pull-up resistor. During each 60 us time slot, 1 bit is communicated between the master and slave. The time slot can be up to 10 times shorter if the bus is overdriven.\u00a0<\/span><\/p>\n

When the master has to write bits over the data line, it pulls the data line down.<\/span><\/p>\n