posedge

A posedge, short for positive edge, refers to the rising transition of a digital signal from logic low to logic high, typically the clock signal in synchronous circuits. It denotes the exact moment when the signal crosses the threshold from 0 to 1, and many sequential elements are designed to respond to this event rather than to the steady state level.

In hardware description languages, posedge is used to specify actions that should occur on rising edges of a clock. For example, in Verilog and SystemVerilog, a common construct is always @(posedge clk) followed by statements that update state. In VHDL, the rising edge is detected with the rising_edge function (or a similar construct). These usages model edge-triggered behavior, where a flip-flop or other clocked element samples inputs and updates its outputs only on the moment of the rising edge.

Edge-triggered devices, such as positive-edge-triggered flip-flops, rely on the clock edge to synchronize data flow in a circuit. Correct operation requires meeting setup and hold time requirements around the posedge, as well as accounting for clock skew and propagation delays to prevent metastability and data corruption.

Negatives and variations: The opposite event is the negative edge (negedge), the falling transition from 1 to 0. Some designs employ both edges of a clock (dual-edge or double-data-rate systems) to double data capacity, or use negedge sensitivity for different timing characteristics, though this can complicate timing analysis.

Overall, posedge is a fundamental concept in synchronous digital design, used to define when state changes are allowed and when inputs are sampled.

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