1-Wire FAQ

     

  • Does 1-Wire really use One Wire?
    • A 1-Wire bus has at least two wires. The data and power lines are integrated (parasitic power) but a ground connection is still necessary. Integrating the power and data line puts some limitations on the network so some 1-Wire buses will have three wires (Vcc, Gnd, Data)
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  • What is CRC?
    • A Cyclic Redundancy Check (CRC) ensures that data read from from a 1-Wire bus is not corrupted. 1-Wire uses an 8-bit CRC. When transmitting message through 1-Wire an additional byte is sent which is computed by XORing all other bytes together. The host and client both generate this value and compare. If it does not match an error has occurred.
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  • What is a 1-Wire Lasered ROM code.
    • A 1-Wire rom code consists of a unique 6-byte serial number, a 1-byte family code (see family code table), and 1-byte CRC. These values are defined in manufacturing, they can not be changed. During the discovery stage a 1-Wire controller generates a list of the ROM codes of all devices on the bus. Subsequent read and write operations are sent to specific 1-Wire devices by addressing their ROM codes.
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  • What is Parasitic Power?
    • Parasitic power allows a device to receive data and power over the same line. When the line is not transmitting data it goes to a high state. 1-Wire devices charge an internal capacitor (typically 800pf) from this source. Many 1-wire devices support an external power supply in addition to parasitic power.
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  • What are Parasitic Power design constraints.
    • You must check the maximum current output of the 1-Wire bus controller and the maximum combined instantaneous current consumption of all devices on it. When a DS18B20’s capacitor is completely drained it will draw close to 1.5 mAs from the OneWire bus. If a 1-Wire controller can supply no more than 15the voltage will become unstable if more than 10 DS18B20 devices charge at the same time. There can be more than 10 DS18B20 devices on the bus, but they can not all charge at the same time.
    • Circuits like this one can be used to increase the available current on a 1-Wire bus.
    • The capacitor of a 1-wire device holds a limited charge (typically 800pf). Devices need to rest between operations to recharge it. High frequency polling of one-wire sensors is not possible. Device dartasheets will indicate the required rest periods.
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  • How long can a 1-wire bus be?
    • The limiting factor is the bus capacitance. If capacitance is too high the signal line can not rise and fall fast enough to transmit data properly. The capacitance of a 1-Wire bus can not exceed 10 nanofarads (nf) aka 10,000 picofarads (pf). Twisted pair Cat5 wires have a typical capacitance of 50 pF per meter. Flat Phone cable has a typical capacitance of 60 pF per meter. PCB traces have a typical capacitance of 24p pF per meter. Every 1-wire slave device adds 30pF to the bus.
    • Example: A 1-Wire bus has 20 devices and uses phone cable. What is it’s maximum length? (10,000 pf – 20 devices * (30pf / device)) / (60pf / meter) = 156.67 meters
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  • What pull up resistor value should I use on the 1-Wire bus?
    • Pull up resistance must be low enough that there is sufficient current to charge any parasitic power devices, but high enough that the line can be pulled low for data communication. 1k – 5k is normally used.
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  • What is an iButton?
    • An iButton is a special package for 1-Wire devices resembling a watch battery. It takes advantage of the fact that a parasitic powered 1-Wire device only needs two contacts to make an extremely simple and reliable connection to a terminal. The steel cases are extremely durable making iButtons suitable for transit passes, building keys and industrial equipment tagging.
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  • How many devices can exist on a 1-wire bus?

 

    • What is MicroLAN.
      • MicroLAN is the former brand name of 1-Wire.
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