Identifying Platform Clocks - 2024.1 English

The kernels can have any number of independent and edge-aligned clocks, and platforms can have multiple kernels running at different clock frequencies under user control. Platforms have a variety of clocking: processor, PL, and AI Engine clocking. The following table explains the clocking for each type.

Vitis clocking automation differentiates between three types of platform clock: scalable, fixed, and fixed_non_ref, which are specified in Vivado during platform capture, and define how automation can employ clocks to meet v++ clocking directives.

Scalable Clocks (AMD Alveo™ Platforms only)

An AMD Alveo platform provides a frequency scalable kernel clock with ID (or Index) 0 that drives all XRT managed kernels. XRT can set the clock frequency of this clock according to the metadata contained in the xclbin file, when loading the file. An Alveo platform also provides a second scalable clock with ID 1 that is also controllable based on xclbin metadata. You do not need to provide an option to connect to scalable clocks, but you can specify the clock frequency during v++ linking using the --freqhz option or --kernel_frequency, as described in v++ General Options. The v++ linker automatically connects PL kernel clocks ap_clk to clock ID 0, and ap_clk2 to clock ID 1.

Tip: In practice, ap_clk2 is primarily found on RTL kernels, because the HLS compiler does not generate kernels with multiple clocks.

Fixed Clocks

Platform clocks declared as 'fixed' can be used to drive kernels at their specified frequency, or as a reference clock for v++ clock automation to derive clocks as needed.

Fixed clocks use MMCMs to generate frequencies other than the fixed frequencies defined on the platform. For example, if you specify clock frequencies: 60, 200, and 450, the Vitis compiler adds all the necessary logic to generate the required clocks from the available platform fixed clocks.

Use the --freqhz option to specify the clock frequency for a kernel. The --clock Options can also be used to specify PL kernel connections to specific platform clocks, or to specify clock frequencies that are generated from fixed clocks on the platform.

fixed_non_ref

Platform clocks declared as fixed_non_ref can be used to drive kernels at their specified frequency, but cannot be used as a reference clock by v++ clock automation.

The canonical justification for declaring a platform clock fixed_non_ref is because it is driven by an MBUFG logical clock resource.

Refer to --clock Options for more details.

You can determine the clocks available in the target platform by using the platforminfo command.

=================
Clock Information
=================
  Default Clock Index: 2
  Default Clock Frequency: 312.499712
  Default Clock Pretty Name: PL 2
  Clock Index:         0
    Frequency:         156.249856
    Status:            fixed
    Name:              clk_wizard_0_clk_out2
    Pretty Name:       PL 0
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            6.400006
    Normalized Period: .006400
  Clock Index:         1
    Frequency:         104.166570
    Status:            fixed
    Name:              clk_wizard_0_clk_out1
    Pretty Name:       PL 1
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            9.600009
    Normalized Period: .009600
  Clock Index:         2
    Frequency:         312.499712
    Status:            fixed
    Name:              clk_wizard_0_clk_out3
    Pretty Name:       PL 2
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            3.200003
    Normalized Period: .003200
  Clock Index:         3
    Frequency:         78.124928
    Status:            fixed
    Name:              clk_wizard_0_clk_out4
    Pretty Name:       PL 3
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            12.800012
    Normalized Period: .012800
  Clock Index:         4
    Frequency:         208.333141
    Status:            fixed
    Name:              clk_wizard_0_clk_out5
    Pretty Name:       PL 4
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            4.800004
    Normalized Period: .004800
  Clock Index:         5
    Frequency:         416.666283
    Status:            fixed
    Name:              clk_wizard_0_clk_out6
    Pretty Name:       PL 5
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            2.400002
    Normalized Period: .002400
  Clock Index:         6
    Frequency:         624.999425
    Status:            fixed
    Name:              clk_wizard_0_clk_out7
    Pretty Name:       PL 6
    Inst Ref:          clk_wizard_0
    Comp Ref:          clk_wizard
    Period:            1.600001
    Normalized Period: .001600

For Versal devices, the --part option can be used instead of --platform with the v++ --link and v++ --package commands. With --part the tool generates a base design for use on the device, and can generally be used while waiting for the development of a full platform specification. However, the v++ linker generated base platform design employs PLRAM only, so is unsuitable for running Linux applications on the target.