Logic Level - Logic Voltage Levels

Logic Voltage Levels

The two logical states of a wire are usually represented by two different voltages, but current is used in some logic families. A threshold is designed for each logic family. When below that threshold, the wire is "low," when above "high". Intermediate levels are undefined and the behavior of the connected circuits is highly implementation-specific. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that all results are predictable.

It is common 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 would occur only in a fault condition or during a logic level transition, as circuits cannot instantly change voltage levels. However, few logic circuits can detect such a fault, and most will end up interpreting the signal as either a 0 or a 1 input, unpredictably and possibly inconsistently.

Combinational circuit outputs also take longer to settle to a final state when an input is close to the invalid middle range, and in a synchronous circuit, this can lead to a propagation of metastability. A clock domain crossing is one situation commonly faced by digital designers where metastability is likely and must be handled carefully.

Nearly all digital circuits use a consistent logic level for all internal signals — however, that level varies widely from one system to another. A level shifter connects one digital circuit that uses one logic level to another digital circuit that uses another logic level. Often two level shifters are used, one at each system: A "line driver" converts from internal logic levels to standard interface line levels; a "line receiver" converts from interface levels to internal voltage levels. The most common agreed-upon voltage levels are the TTL logic levels; almost as common is the RS-232 voltage levels.

The voltage levels used internally are called the "logic level", while the voltage levels used externally are called the "line level". In particular, when connecting a system that uses TTL levels internally to a RS-232 cable, the TTL levels are the "logic level". When connecting a system that uses 3.3 V CMOS levels internally to a IEEE 1284 bus, the TTL levels are the "line level".