During the compilation process (v++
-c
), you can specify a kernel frequency using the
--hls.clock
command. This lets you compile the kernel targeting the specified frequency, and lets
the Vitis HLS tool perform validation of the kernel
logic at the specified frequency. While this is just a target frequency for compilation,
it does provide optimization and feedback.
To implement the kernel using a different frequency from the platform default frequency, you can use the --clock Options for platforms with fixed clocks as described in Identifying Platform Clocks. When generating the device binary (.xclbin) you can connect multiple kernels to the platform using different clock frequencies. Each kernel, or unique instance (CU) of the kernel can connect to a specified clock frequency, or multiple clocks, and different kernels can use different clock frequencies.
v++ --kernel_frequency
option to specify the clock frequency for
one or both clocks on the platform, as described in v++ General Options. Therefore the process for managing clock frequencies for kernels is as follows:
- Compile the HLS code at a specified frequency using the Vitis
compiler:
v++ -c -k <krnl_name> --hls.clock freqHz:<krnl_name>
Tip:freqHz
must be in Hz (for example,250000000Hz
is 250 MHz). - During linking, specify the clock frequency or clock ID for each
clock signal in a kernel with the following command:
v++ -l ... --clock.freqHz <freqHz>:kernelName.clk_name
You can specify the --clock
option using
either a clock ID from the platform shell, or by specifying a frequency for the kernel
clock. When specifying the clock ID, the kernel frequency is defined by the frequency of
that clock ID on the platform. When specifying the kernel frequency, the platform
attempts to create the specified frequency by scaling one of the available fixed
platform clocks.
In some cases, the clock frequency can only be achieved in some
approximation, and you can specify the --clock.tolerance
or --clock.default_tolerance
to indicate an acceptable range. If the
available fixed clock cannot be scaled within the acceptable tolerance, a warning is
issued and the kernel is connected to the default clock.