Delta Cycles and Race Conditions - 2023.2 English

Vivado Design Suite User Guide: Logic Simulation (UG900)

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2023.2 English

This user guide describes event-based simulators. Event-based simulators can process multiple events at a given simulation time. While these events are being processed, the simulator cannot advance the simulation time. This event processing time is commonly referred to as delta cycles. There can be multiple delta cycles in a given simulation time step.

Simulation time is advanced only when there are no more transactions to process for the current simulation time. For this reason, simulators can give unexpected results, depending on when the events are scheduled within a time step. The following VHDL coding example shows how an unexpected result can occur.