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Logic level
In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. The range of voltage levels that represent each state depends on the logic family being used. A logic-level shifter can be used to allow compatibility between different circuits.
In binary logic the two levels are logical high and logical low, which generally correspond to binary numbers 1 and 0 respectively or truth values true and false respectively. Signals with one of these two levels can be used in Boolean algebra for digital circuit design or analysis.
The use of either the higher or the lower voltage level to represent either logic state is arbitrary. The two options are active high (positive logic) and active low (negative logic). Active-high and active-low states can be mixed at will: for example, a read only memory integrated circuit may have a chip-select signal that is active-low, but the data and address bits are conventionally active-high. Occasionally a logic design is simplified by inverting the choice of active level (see De Morgan's laws).
The name of an active-low signal is historically written with a bar above it to distinguish it from an active-high signal. For example, the name Q, read Q bar or Q not, represents an active-low signal. The conventions commonly used are:
Many control signals in electronics are active-low signals (usually reset lines, chip-select lines and so on). Logic families such as TTL can sink more current than they can source, so fanout and noise immunity increase. It also allows for wired-OR logic if the logic gates are open-collector/open-drain with a pull-up resistor. Examples of this are the I²C bus, CAN bus, and PCI bus.
Some signals have a meaning in both states and notation may indicate such. For example, it is common to have a read/write line designated R/W, indicating that the signal is high in case of a read and low in case of a write.
The two logical states are usually represented by two different voltages, but two different currents are used in some logic signaling, like digital current loop interface and current-mode logic. High and low thresholds are specified for each logic family. When below the low threshold, the signal is low. When above the high threshold, the signal is high. Intermediate levels are undefined, resulting in highly implementation-specific circuit behavior.
It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic-level transition. However, few logic circuits can detect such a condition, and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporate Schmitt trigger inputs, whose behavior is much better defined in the threshold region and have increased resilience to small variations in the input voltage. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that the circuit behaves predictably.
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Logic level
In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. The range of voltage levels that represent each state depends on the logic family being used. A logic-level shifter can be used to allow compatibility between different circuits.
In binary logic the two levels are logical high and logical low, which generally correspond to binary numbers 1 and 0 respectively or truth values true and false respectively. Signals with one of these two levels can be used in Boolean algebra for digital circuit design or analysis.
The use of either the higher or the lower voltage level to represent either logic state is arbitrary. The two options are active high (positive logic) and active low (negative logic). Active-high and active-low states can be mixed at will: for example, a read only memory integrated circuit may have a chip-select signal that is active-low, but the data and address bits are conventionally active-high. Occasionally a logic design is simplified by inverting the choice of active level (see De Morgan's laws).
The name of an active-low signal is historically written with a bar above it to distinguish it from an active-high signal. For example, the name Q, read Q bar or Q not, represents an active-low signal. The conventions commonly used are:
Many control signals in electronics are active-low signals (usually reset lines, chip-select lines and so on). Logic families such as TTL can sink more current than they can source, so fanout and noise immunity increase. It also allows for wired-OR logic if the logic gates are open-collector/open-drain with a pull-up resistor. Examples of this are the I²C bus, CAN bus, and PCI bus.
Some signals have a meaning in both states and notation may indicate such. For example, it is common to have a read/write line designated R/W, indicating that the signal is high in case of a read and low in case of a write.
The two logical states are usually represented by two different voltages, but two different currents are used in some logic signaling, like digital current loop interface and current-mode logic. High and low thresholds are specified for each logic family. When below the low threshold, the signal is low. When above the high threshold, the signal is high. Intermediate levels are undefined, resulting in highly implementation-specific circuit behavior.
It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic-level transition. However, few logic circuits can detect such a condition, and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporate Schmitt trigger inputs, whose behavior is much better defined in the threshold region and have increased resilience to small variations in the input voltage. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that the circuit behaves predictably.