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An open collector is a common type of output found on many integrated circuits (IC).
Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized (open) on a pin of the IC. The emitter of the transistor is connected internally to the ground pin. If the output device is a MOSFET the output is called open drain and it functions in a similar way.
In the picture, the transistor base is labeled "IC Output". This is a signal from the internal IC logic to the transistor. This signal controls the transistor switching. The external output is the transistor collector, and the transistor forms an interface between the internal IC logic and parts external to the IC. On schematic component symbols, an open collector output may be annotated with an identifying symbol.
The output forms either an open circuit or a connection to ground. The output usually consists of an external pull-up resistor, which raises the output voltage when the transistor is turned off. When the transistor connected to this resistor is turned on, the output is forced to nearly 0 volts. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.
A three-state logic device is unlike an open collector device, because it comprises transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.
Because the pull-up resistor is external and need not be connected to the chip supply voltage, a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating voltage levels. The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Such devices are commonly used to drive devices such as Nixie tubes, and vacuum fluorescent displays, relays or motors which require higher operating voltages than the usual 5-volt logic supply.
Another advantage is that more than one open-collector output can connect to a single line. If all outputs attached to the line are in the high-impedance state, the pull-up resistor will hold the wire in a high voltage (logic 1) state. If one or more device outputs are in the logic 0 (ground) state, they will sink current and pull the line voltage toward ground. This wired logic connection has several uses.
Open-collector 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 inactive devices - if open-collector devices are not used, then the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output.
By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) gates in that it will be logic 1 whenever (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the Boolean OR for negative-true logic, where the output is LOW if any one of its inputs is low.
One problem with open-collector devices is power consumption, since they tend to require higher current minimums for correct operation. They also have a high static power as there is always a direct current path from Vdd to ground when the device is switched ON. Even in the 'off' state, they often have a few nanoamperes of leakage current (the exact amount varies with temperature).
The analogous connection used with MOS transistors is an open-drain connection. Open-drain outputs can be useful for analog weighting, summing, and limiting as well as digital logic. An open drain terminal is connected to ground when a high voltage (logic 1) is applied to the gate, yet presents a high impedance when a low voltage (logic 0) is applied to the gate. This high impedance state occurs because the terminal is at an undefined voltage (floating) thus such a device requires an external pull-up resistor connected to the positive voltage rail (logic 1) in order to provide a logic 1 as output.
Note that microelectronic devices using open drain signals (such as microcontrollers) may provide a 'weak' internal pull-up resistor to connect the terminal in question to a positive voltage source/rail like Vdd of the device. Such weak pullups, often on the order of 100 kΩ, reduce power usage by keeping input signals from floating. External pullups are stronger (perhaps 3 kΩ) to reduce signal rise times (like with I²C) or to minimize noise (like on system RESET inputs). Internal pullups can often be disabled for cases where there is an external one, or in other cases where they are not needed.
The Pseudo Open Drain (POD) drivers, have a strong pull-down strength but a weaker pullup strength. A pure open-drain driver, by comparison, has no pullup strength except for leakage current: all the pull-up action is on the external termination resistor. This is why the term "pseudo" has to be used here: there is some pull-up on the driver side when output is at high state, the remaining pull-up strength is provided by parallel-terminating the receiver at the far end to the HIGH voltage, often using a switchable, on-die terminator instead of a separate resistor. The purpose of all this is to reduce the overall power demand compared to using both strong pullup and strong pull-down, as in drivers such as HSTL. DDR4 memory uses POD drivers, replacing push-pull drivers in DDR3 that drove strongly in both High and Low states with a half-supply reference point on the termination (known as Center-Tab Termination or CTT).
JEDEC standardized the POD15, the POD135 and the POD12 for 1.5V, 1.35V and 1.2V interface supply voltages. A comparison of both DDR3 and DDR4 termination schemes in terms of skew, eye aperture and power consumption was published in late 2011.
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