1-Wire

1-Wire or One-Wire or wire bus describes a serial interface Dallas Semiconductor Corp.. , Which uses just one data wire ( DQ), which is used both as a power supply as well as send and receive line. The term 1-Wire is misleading because in addition, a ground connection (GND ) is required. This ground connection is achieved arrangements on mutual isolation of the housing halves with knob -shaped. In fact, always two physical fibers are used (GND, DQ).

Available blocks are integrated to measure the temperature, battery monitoring, real-time clock, etc. The small memory technique has been developed for the communication between the components of a device, e.g. for detecting the battery condition in a mobile data recording device (PDA).

The coin -shaped devices are used as a particularly simple mechanically related to the wearer's authentication factors especially at point of sale terminals.

Survey

  • The serial connection operates and bidirectional, i.e., to a data line for transmission and reception.
  • The data transmission is asynchronous, that is, it will not transmit clock signal.
  • But not both is transmitted in half duplex mode, ie either sending a block, or a block is received, at the same time.
  • The transmission takes place according to the principle One-Master/Multi-Slave, i.e. it can only be one master ( controller ), but up to 100 slaves (sensors, memories, etc. ) for each bus can be used.
  • Each slave is addressed by a 64- bit ROM ID. This consists of an 8 -bit family code, a 48- bit serial number (unique device ID ) and an 8-bit CRC checksum.
  • The slaves to provide an internal capacitor that is charged from the data line. An external power supply is not necessary.
  • 1-Wire is a voltage interface. The voltage depends on the device from 2.8 to 6 volts.

Electrical properties

Many 1-Wire devices operate with a supply voltage, which can be between 2.8V and 5.5V. For some devices, however, only 3.0 V to 3.7 V are allowed, so necessarily the data sheet should be observed. The current consumption of 1-Wire devices, depending on the block between 2 uA and a few mA.

A special feature of 1-Wire devices, the associated power supply from the remote station. This is done to supply the slaves via the data line: With an inactive communication (idle state), the data is on 5 V High - level and charges a storage capacitor which is integrated in each 1-Wire slave. During the communication, the bus through the devices (devices) is pulsed low. During the low pulses the slave is fed by its capacitor. Depending on the charge state, the capacitor low times over until about 960 microseconds. This time is sufficient to measure with a commercially available DS18S20 temperature sensor temperatures up to about 100 ° C. At higher temperatures, the internal leakage currents rise, whereby the power supply from the built-in capacitor can no longer be maintained. In order to exploit the full measuring range up to 125 ° C, the continuous supply of the sensor via the separate power connection is required.

The 1- wire bus, is pulled through a pull -up resistor, which is usually 1 to 2.2 kOhm to high level. The size should be adjusted accordingly depending on the number of devices and cable length. To avoid disturbances and problems, the values ​​of the Pullupwiderstände are necessarily read in the data sheets.

Timing

Since the 1-Wire bus is in contrast to many other bus does not have a separate line for the clock signal, a predetermined timing must be maintained. Synchronization is performed at each bit with the falling edge generated by the master. To write a logic 1, the bus by the master for 1 to 15 microseconds is pulled to a low level, with a logic 0 for 60 to 120 microseconds. To read the master pulls as in " Write 1 " signal the bus for 1 to 15 microseconds at a low level and the slave holds for the transmission of a logic 0 the bus in addition to low. For a reset, the master sends a low level with a duration of 480 microseconds. A slave indicates its presence by then pulls the bus for at least 60 microseconds to low within 60 microseconds The 1-Wire devices still have an additional overdrive mode, which makes it possible to achieve much higher transmission rates. To create a logical 1 in overdrive mode, a low level must be applied only for only 1-2 microseconds, for a logic 0 are in overdrive mode already 6μs sufficient. To generate a reset are enough 48 microseconds. If the reset signal is longer than 80 microseconds, so the 1-Wire devices go into the regular operating mode, otherwise they remain in Overdrive mode.

In normal operation mode, data rates up to 16.3 kbit / s are possible by timing conditions listed above. The Overdrive mode speeds up to 142 Kbps on this / s

Cable Requirements

The wiring can be a simple cable or a single line on a printed circuit board. A passive pull- up resistor as cable lengths of up to 100 m with 150 1 -wire devices are also possible. By controlling the slew rate, and by an active pull- up resistor, it is even possible to realize cable lengths of up to 300 m with 500 1 -wire devices. The quality of the earth connection as the return line on the general ground potential must be tested under operating conditions.

For example a one -wire communication

The signals in the diagram have been created by an FPGA as the master and one DS2432 ( EEPROM), as a slave, and measured by a logic analyzer. The 1-wire output signal represents the isolated output signal of the master; 1-wire input is the measured one -wire bus signal; with a high signal of the input sample time measurement of the 1-Wire bus signal by the master will take place.

Other standards

IButton ®: The registered default is based on the same technology. He describes the communication to devices in button cell size.

Bidirectional (disambiguation)
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