Using Latency Constraints - 2021.2 English

Vitis HLS supports the use of a latency constraint on any scope. Latency constraints are specified using the LATENCY directive.

When a maximum and/or minimum LATENCY constraint is placed on a scope, Vitis HLS tries to ensure all operations in the function complete within the range of clock cycles specified.

The latency directive applied to a loop specifies the required latency for a single iteration of the loop: it specifies the latency for the loop body, as the following examples shows:

Loop_A: for (i=0; i<N; i++) { 
#pragma HLS latency max=10
  ..Loop Body...  
}

If the intention is to limit the total latency of all loop iterations, the latency directive should be applied to a region that encompasses the entire loop, as in this example:

Region_All_Loop_A: {
#pragma HLS latency max=10
Loop_A: for (i=0; i<N; i++) 
  { 
  ..Loop Body... 
  }
}

In this case, even if the loop is unrolled, the latency directive sets a maximum limit on all loop operations.

If Vitis HLS cannot meet a maximum latency constraint it relaxes the latency constraint and tries to achieve the best possible result.

If a minimum latency constraint is set and Vitis HLS can produce a design with a lower latency than the minimum required it inserts dummy clock cycles to meet the minimum latency.