Analyzing RTL Simulations - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2021.2 English

When the C/RTL co-simulation completes, the simulation report opens and shows the measured latency and II. These results may differ from values reported after HLS synthesis, which are based on the absolute shortest and longest paths through the design. The results provided after C/RTL co-simulation show the actual values of latency and II for the given simulation data set (and may change if different input stimuli is used).

In non-pipelined designs, C/RTL co-simulation measures latency between ap_start and ap_done signals. The II is 1 more than the latency, because the design reads new inputs 1 cycle after all operations are complete. The design only starts the next transaction after the current transaction is complete.

In pipelined designs, the design might read new inputs before the first transaction completes, and there might be multiple ap_start and ap_ready signals before a transaction completes. In this case, C/RTL co-simulation measures the latency as the number of cycles between data input values and data output values. The II is the number of cycles between ap_ready signals, which the design uses to requests new inputs.

Note: For pipelined designs, the II value for C/RTL co-simulation is only determined if the design is simulated for multiple transactions.