Open drain

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Open drain is one of the many different electrical input/output standards in digital designs today.

[edit] Definition

Open-drain refers to the drain terminal of a MOSFET transistor. (The analog for BJT devices is the open collector.) An open drain terminal is connected to ground in the low voltage (logic 0) state, but has high impedance in the logic 1 state. This prohibits current flow, but as a result, such a device requires an external pull-up resistor which is also connected to the positive voltage rail.

When a device is in the high-impedance state, the pull-up resistor keeps the line at logic 1. The line stays there until the device goes into the logic 0 state, and begins to sink current. This current flow creates a voltage drop across the pull-up resistor, and the line drops to the logic 0 voltage.

[edit] Applications

Wired-OR circuit using open-drain gates.

One useful property is that the resistor need not be connected to the same voltage as the chip supply voltage Vcc: a lower or higher voltage can be used instead. Open drain circuits are therefore sometimes used to interface two series of devices that have different operating logic levels (voltages).

Another advantage is that more than one open-drain output can be attached to a single wire. If all outputs attached to the wire are in the high-impedance/logic 1 state, the pull-up resistor will hold the wire in a high voltage state. If at least one of the device outputs is in the ground/logic 0 state, it will sink current and bring the line voltage low. This is analogous to a logical-OR function (assuming the output is being viewed as active-low logic) and is often referred to as a "wired-OR" circuit.

Open-drain devices are commonly used to connect multiple devices to a bus (i.e. one carrying interrupt or write-enable signals). This enables one device to drive the bus without interference from the other devices - if open-drain devices are not used, the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output .