1-Wire

1-Wire is a computer bus system designed by Dallas Semiconductor that provides low-speed data, signaling and power over a single wire. 1-Wire is similar in concept to I²C, but with lower data rates and a much lower cost.

One of the attractive features of the bus is that a device only needs two wires, data and ground. To accomplish this, the IC includes an 800pf capacitor to power it from the data line. The devices are often delivered in tiny cans that look like small capacitors or watch crystals.

In mass-produced systems 1-Wire devices are on a printed circuit board with their controller.

Some laboratory systems connect them using RJ-11 wiring, with the devices themselves mounted in a socket.

Systems of sensors can be built by wiring together 1-Wire components, each including all of the logic needed to operate on the 1-Wire bus. Examples include temperature loggers, timers, voltage and current sensors, battery monitors, and memory. In a laboratory, these can connected to a PC using a bus converter, USB, or a printer port being popular solutions.

The iButton is a mechanical packaging standard that places a 1-Wire component inside a small stainless steel "button" similar to a disk-shaped battery. iButtons are connected to 1-Wire bus systems with an adaptor that connects the "lid" and "base" of the canister to an RJ-11 plug.

Use of the bus

The hardware-level of the protocol is usually performed by special software in the bus master, and a resistor. The resistor pulls the wire up to five volts, and incidentally provides the power to the network of devices.

The master starts a sequence of bits starts with a "reset" pulse, which pulls the wire to zero volts for a long time: 480 microseconds. This resets every slave device on the bus, probably by depriving them all of power. After that, any slave device, if present, shows that it exists with a "presence" pulse: it holds the wire at ground for at least 60 microseconds after the master stops driving the bus to ground.

To send bits, the software of the bus master places a sixty microsecond zero-volt pulse on the wire to send a zero, and a one to fifteen microsecond zero-volt pulse to send a one. If a parallel port is inconvenient or the operating system interferes with the timing, a UART run at 100K BAUD with some driver circuitry can be used to produce and sense acceptable pulses.

When receiving data, the master sends a one to fifteen microsecond zero-volt pulse to start each bit. If the transmitting slave unit wants to send a one, it does nothing, and the wire goes immediately up to five volts. If the transmitting slave wants to send a zero, it shorts the wire to ground for sixty microseconds.

The basic sequence is a reset pulse, an eight bit command, and then data is sent or received in groups of eight bits.

When a sequence of data is being transferred, errors can be detected with an 8-bit CRC.

Many devices can share the same bus. Each device on the bus has a unique 64-bit serial number. The most significant byte of the serial number is an 8-bit number that tells the type of the device. The least-significant byte is a standard (for the 1-wire bus) 8-bit CRC.

There are several standard broadcast commands, and commands addressed to particular devices. The master can send a selection command, and then the address of a particular device, and then the next command is executed only by the selected device.

The bus also has an algorithm to recover the address of every device on the bus. Since the address includes the device type and a CRC, recovering the addresses automatically produces a reliable inventory of the devices on the bus.

To find the devices, the master broadcasts an enumeration command, and then an address, "listening" after each bit of an address. If a slave has all the address bits so far, it returns a zero. The master uses this simple behavior to search systematically for valid sequences of address bits. The process is much faster than a brute force search of all possible 64-bit numbers because as soon as an invalid bit is detected, all subsequent address bits are known to be invalid. An enumeration of ten or fifteen devices finishes too quickly to be timed.

The location of devices on the bus is sometimes important as well. For these situations, the manufacturer has a special device that either passes-through the bus, or switches it off. Software can therefore explore sequential "bus domains".

External links

• How It Works (PDF) (http://pdfserv.maxim-ic.com/en/an/onewirebus.pdf)
• 1-Wire and iButton (http://www.maxim-ic.com/1-Wire.cfm)
• iButton (http://www.ibutton.com/)
• 1-Wire Weather Instrument Kit (http://www.aagelectronica.com/aag/index.html)