Intel® FPGA SDK for OpenCL™ Pro Edition: Programming Guide

ID 683846
Date 3/28/2022

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Document Table of Contents RTL Reset and Clock Signals

Resets and clocks of RTL modules are connected to the same clock and reset drivers as the rest of the OpenCL™ pipeline.

Because of the common clock and reset drivers, an RTL module runs in the same clock domain as the OpenCL kernel. The module is reset only when the OpenCL kernel is first loaded onto the FPGA, either via Intel® FPGA SDK for OpenCL™ program utility or the clCreateProgramwithBinary host function. In particular, if the host restarts a kernel via successive clEnqueueNDRangeKernel or clEnqueueTask invocations, the associated RTL modules does not reset in between these restarts.

The following steps outline the process of setting the kernel clock frequency:

  1. The Intel® Quartus® Prime software's Fitter applies an aggressive constraint on the kernel clock.
  2. The Intel® Quartus® Prime software's Timing Analyzer performs static timing analysis to determine the frequency that the Fitter actually achieves.
  3. The phase-locked loop (PLL) that drives the kernel clock sets the frequency determined in Step 2 to be the kernel clock frequency.

Optionally, an RTL module can access a system-wide clock that runs at twice the frequency of the OpenCL™ kernel clock. This system-wide clock can be connected to an input signal of the RTL module by including an AVALON element of type clock2x. The phase relationship between the clock and clock2x signals is such that the rising and falling edges of clock are coincident with rising edges of clock2x.


Timing failures may occur if one or more signals in your design are not able to satisfy all of the timing requirements of the device. All timing small timing failure can cause problems, so binaries that failed timing should not be used for development and production builds.

If your design failed timing, you have the following options:

  • Timing failures can depend on how a design is placed on the FPGA, so running a sweep of different seeds (which results in different component placements) might lead to a passing binary.
  • Decreasing the size of the design makes the component placement easier and timing failures less likely.
  • Timing failures may be indicative of BSP problems, so if you are using a custom BSP, discuss with your BSP vendor. If you want to investigate it further, the Intel® Quartus® Prime Static Timing Analyzer outputs a *.sta.rpt file that contains more details about the timing analysis performed.