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  • JohnB389 commented on mosvath's instructable Temperature with DS18B203 years ago
    Temperature with DS18B20

    Actually MadScott, the schematic is correct. The OneWire devices can operate either with a power line, or in parasitic mode, where power is drawn from the data line. If you look at the schematic you will see pin 3 (VCC) on the devices is unconnected. This means they are using parasitic mode. The OneWire device will effectively "charge" itself from the data line, using VCC via the 4.7k resistor, and use use this charge to run during times when the data line is used to exchange data. Note that the charge is very small and only designed to last while transferring data. This places some constraints on the data transfers, such as how long the line must be idle before any communication to ensure all devices are charged. The reason the resistor must be so high is so that the data lin...

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    Actually MadScott, the schematic is correct. The OneWire devices can operate either with a power line, or in parasitic mode, where power is drawn from the data line. If you look at the schematic you will see pin 3 (VCC) on the devices is unconnected. This means they are using parasitic mode. The OneWire device will effectively "charge" itself from the data line, using VCC via the 4.7k resistor, and use use this charge to run during times when the data line is used to exchange data. Note that the charge is very small and only designed to last while transferring data. This places some constraints on the data transfers, such as how long the line must be idle before any communication to ensure all devices are charged. The reason the resistor must be so high is so that the data line can still be asserted low without drawing too much from the host power supply. The libraries take of this detail and in practice it works really well. Hope this helped.

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