1-Wire Driver - Raspberry Pi Hardware Reference - page 160

Chapter 11

1-Wire Driver

The 1-Wire protocol was developed by Dallas Semiconductor Corp. initially for the
iButton. 37 This communication protocol was attractive enough to be applied to other
devices and soon adopted by other manufacturers. This chapter provides an overview of
the 1-Wire protocol and how it is supported in the Raspberry Pi.

1-Wire Line and Power
The 1-Wire protocol actually uses two wires:

• Data : The single wire used for data communication

• Ground : The ground or “return” wire

The 1-Wire protocol was designed for communication with low-data content devices
like temperature sensors. It provides for low-cost remote sensing by supplying power
over the same wire used for data communications. Each sensor can accept power from
the data line while the data line is in the high state (which is also the line's idle state). The
small amount of power that is siphoned off charges the chip's internal capacitor (usually
about 800 pF). 37
When the data line is active (going low), the sensor chips continue to run off of their
internal capacitors (in parasitic mode). Data communications cause the data line to
fluctuate between low and high. So whenever the line level returns high again, even for an
instant, the capacitor recharges.
The device also provides an optional V DD pin, allowing power to be supplied to it
directly. This is sometimes used when parasitic mode doesn't work well enough. This, of
course, requires an added wire, which adds to the cost of the circuit. We'll be focusing on
the parasitic mode in this chapter. In parasitic mode, V DD is connected to the ground.

Line Driving
The data line is driven by open collector transistors in the master and slave devices.
The line is held high by a pull-up resistor when the driver transistors are all in the Off
state. To initiate a signal, one transistor turns on and thus pulls the line down to
ground potential.