1-Wire
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1-Wire is a device communications bus system designed by Dallas Semiconductor that provides low-speed data, signaling and power over a single wire (a Ground wire is also needed). 1-Wire is similar in concept to I²C, but with lower data rates and a much lower cost. It is typically used to communicate with small inexpensive devices such as digital thermometers, weather instruments etc.
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 often mounted on a printed circuit board along with their controller.
Some laboratory systems and other data acquisition and control systems connect to 1-wire devices using cords with modular connectors or with CAT-5 cable, with the devices themselves mounted in a socket, a small enclosure, or attached to the object being monitored.
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. These can be connected to a PC using a bus converter. USB, RS-232 Serial, and parallel port interfaces are popular solutions for PC connectivity.
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 a 6P2C modular plug (commonly used for telephones).
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[edit] Use of the bus
The hardware level of the protocol is often 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. Specific 1-Wire driver and bridge chips are also available.
The master starts a sequence of bits with a "reset" pulse, which pulls the wire to zero volts for 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 releases the bus.
To send a 1 bit, the software of the bus master sends a very brief (one to fifteen microsecond) low pulse. To send a 0, the software sends a sixty microsecond low pulse. The falling edge of the pulse is used to start a monostable multivibrator in the slave device. The multivibrator in the slave clocks to read the data line about thirty microseconds after the falling edge. The slave's multivibrator unavoidably has analog tolerances that affect its timing accuracy, which is why the output pulses have to be sixty microseconds long, and the starting pulse can't be longer than fifteen.
If a parallel port is inconvenient or the operating system interferes with the timing, a UART running at 100 kbaud with a few resistors and special software can produce and sense acceptable 1-wire pulses. Serial or USB "bridge" chips are also available that handle the timing and waveform requirements of the 1-Wire bus, and are particularly useful in utilizing long (greater than 100 m) cables effectively.
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 the pulled-up voltage. If the transmitting slave wants to send a zero, it pulls the data line to ground for sixty microseconds.
The basic sequence is a reset pulse followed by an 8-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 address roster also 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 0. 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 very quickly. 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."
[edit] Example communication with the DS2432
The following signals were generated by a FPGA, which was the master for the communication with a DS2432 chip, and measured with a logic analyzer. High on the 1-wire output means, that the output of the FPGA is in tri-state mode and the 1-wire device can pull down the bus. Low means, that the FPGA pulls down the bus. The 1-wire input is the measured bus signal. On input sample time high, the FPGA samples the input for detecting the device response and receiving bits.
[edit] See also
- single wire earth return is a technique for electric power transmission with only "1 wire" (plus ground).
