In a negative logic system, a voltage near ground represents a logical one, while the supply voltage represents a logical zero. This approach is common in active-low control signals like resets or chip selects, often because pulling a line to ground is more robust against noise. It flips the standard truth tables, turning AND gates into OR gates from the perspective of the voltage levels.
The interrupt pin uses negative logic, so you need a pull-up resistor to keep it from firing randomly.
When reading schematics, negative logic signals are usually denoted by a bar over the signal name or a trailing asterisk.
Related terms
Positive logic
A digital design convention where the higher voltage level represents a binary one.
Logic level
A specific voltage range that represents a distinct binary state in a digital circuit.
Logic low
The lower of two voltage states used to represent a binary zero in a digital circuit.
Logic high
The higher of two voltage states used to represent a binary one in a digital circuit.