12.4 Adjusting Clock Information

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After you define all required constraints, including clock information, enter any information related to clock noise when such information is not or cannot be generated automatically by the tools. This is generally the case for external clock jitter information occurring at the primary inputs of the device. There can be multiple sources of clock jitter including:

Global jitter (or internal system jitter)
PLL/DLL jitter
External jitter

For more information about these types of jitter, see the appropriate product datasheets and documents.

Starting with Libero SoC v2022.1, the automatic computation of system clock jitter for Place & Route and Static Timing Analysis was introduced to the design flow for PolarFire and PolarFire SoC devices. With this feature, you need only specify the clock jitter of any FPGA primary input using the set_input_jitter constraint. These jitter components are analyzed automatically, and a specific jitter value computed for each clock is passed on to the appropriate design flow steps. With the automatic clock jitter flow, you can add a margin to the estimated jitter of a clock using the set_clock_uncertainty constraint.

In rare cases or when instructed by user guides, you can use the set_system_jitter constraint to override the automatically computed global jitter value. System jitter is estimated based on the count of registers and RAM blocks in the synthesized netlist. A report similar to the following, detailing the jitter estimate, is generated upon completing the Place & Route and Verify Timing steps.

Figure 12-23. Sample Report Detailing Jitter Estimation

The expanded timing analysis path reports include the clock jitter as an extra line item.

Figure 12-24. Clock Jitter Shown as an Extra Line Item

You can also provide the latency of a clock from its source outside the device to the primary input using the set_clock_latency constraint.

For more information about clock jitters, see the PolarFire Family Clocking Resources User Guide .

12.4.1 Set a Clock Input Jitter Value

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Use the set_input_jitter constraint to specify the peak-to-peak jitter value at the primary input of the clock (see section set_input_jitter).

12.4.2 Set a Clock Uncertainty Constraint

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Use the Set Clock Uncertainty Constraint dialog box to set either the Simple Clock Uncertainty constraint or the Clock To Clock Uncertainty constraint from the Type drop-down menu. The default is Simple Clock Uncertainty.

12.4.2.1 Set Simple Clock Uncertainty Constraint

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To open the Set Clock Uncertainty Constraint dialog box from the Constraints menu, choose Clock Uncertainty and select Simple Clock Uncertainty from the Type drop-down menu in Set Clock Uncertainty Constraint dialog box.

Figure 12-25. Set Clock Uncertainty Constraint dialog box

The following table describes the Set Clock Uncertainty Constraint dialog box options.

Table 12-17. Set Clock Uncertainty Constraint Options
Option	Description
Type	Select Simple Clock Uncertainty (default) to set clock uncertainty constraint on a single clock.
Source	Specifies the clock name as the uncertainty source. To set the source clock, click the Browse button to open the Select Source for Simple Uncertainty Constraint dialog box. The following options are available in the Select Source for Simple Uncertainty Constraint dialog box: Type: Displays the type of available pins in the design. The available Pin Type options are: Clock Pins All Pins All Ports Pattern: Default is *, which is a wild-card match for all. You can specify any string value. Click Search to filter the available pins based on the specified Pin Type and Pattern. Available Pins: Displays the available clock pins or ports (ports are defined as clock signal in the design). If you change the pattern value, the list box displays the available pins or ports based on the filter. Use Add/Add All to add the clock pins from the Available Pins list to the Assigned Pins list or Remove/Remove All to delete the clock pins from the Assigned Pins list. Assigned Pins: Displays pins selected from the Available Pins list. Select Pins from this list and click OK to add the source to the constraint.
Uncertainity	Enter the time in `ns` that represents the amount of deviation between two clock edges.
Use uncertainty for	Enables you to select whether the uncertainty constraint applies to setup, hold, or all checks.
Comment	Enables you to save a single line of text that describes this constraint.

12.4.2.2 Set Clock-to-Clock Uncertainty Constraint

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To open the Set Clock-to-clock Uncertainty Constraint dialog box from the Constraints menu, choose Clock Uncertainty, and select Clock To Clock Uncertainty from the Type drop-down menu.

Figure 12-26. Set Clock-to-Clock Uncertainty Constraint Dialog Box

The following table describes the Set Clock-to-clock Uncertainty Constraint dialog box options.

Table 12-18. Set Clock-to-Clock Uncertainty Constraint Options
Option	Description
Type	Select Clock To Clock Uncertainty.
From Clock	Specifies clock name as the uncertainty source. To set the From Clock, click the Browse button to open the Select Source Clock List for Clock-to-clock Uncertainty dialog box. The following options are available in the Select Source Clock List for Clock-to-clock Uncertainty dialog box: Type: Displays the type of available pins in the design. The only choice available for Pin Type is Clock Pins. Pattern: Default is *, which is a wild-card match for all. You can specify any string value. Click Search to filter the available pins based on the specified Pin Type and Pattern. Available Pins: Displays the available clock pins. If you change the pattern value, the list box displays the available pins based on the filter. Use Add/Add All to add the clock pins from the Available Pins list to the Assigned Pins list or Remove/Remove All to delete the clock pins from the Assigned Pins list. Assigned Pins: Displays pins selected from the Available Pins list. Select Pins from this list and click OK to add the source clock for the Clock-to-Clock Uncertainty constraint.
Edge	Enables you to select if the clock-to-clock uncertainty applies to rising, falling, or both edges.
To Clock	Specifies clock name as the uncertainty destination. To set the To Clock, click the Browse button to open the Select Destination Clock List for Clock-to-clock Uncertainty Constraint dialog box. The following options are available in the Select Destination Clock List for Clock-to-clock Uncertainty Constraint dialog box: Type: Displays the type of available pins in the design. The only valid selection is Clock Pins. Pattern: Default is *, which is a wild-card match for all. You can specify any string value. Click Search to filter the available pins based on the specified Pin Type and Pattern. Available Pins: Displays the available clock pins. If you change the pattern value, the list box displays the available pins based on the filter. Use Add/Add All to add the clock pins from the Available Pins list to the Assigned Pins list or Remove/Remove All to delete the clock pins from the Assigned Pins list. Assigned Pins: Displays pins selected from the Available Pins list. Select Pins from this list and click OK to add the destination clock for the Clock-to-Clock Uncertainty constraint.
Uncertainty	Enter the time in `ns` that represents the amount of deviation between two clock edges.
Use Uncertainty For	Enables you to select whether the uncertainty constraint applies to setup, hold, or all checks.
Comment	Enables you to save a single line of text that describes this constraint.

12.4.3 Set Clock Source Latency Constraint

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Use the clock source latency constraint to specify the delay from the clock generation point to the clock definition point in the design.

Clock source latency defines the delay between an external clock source and the definition pin of a clock.

You can specify both an "early" delay and a "late" delay for this latency, providing an uncertainty that the timing analyzer can use for propagating through its calculations. Rising and falling edges of the same clock can have different latencies. If only one clock source latency value is provided, the value is taken as the exact latency value for both rising and falling edges.

To specify a Clock Source Latency constraint, open the Set Clock Source Latency Constraint dialog box in one of the following four ways:

In the Constraints Browser, double-click Clock Source Latency.
Click the Clock Source Latency Constraint icon.
From the Constraints menu, click Clock Source Latency .
Right-click any row of Clock Latency Constraints Table and click Add Clock Source Latency. The Set Clock Source Latency Constraint dialog box appears.

Figure 12-27. Set Clock Source Latency Constraint Dialog Box

The following table describes the Set Clock Source Latency Constraint dialog box options.

Table 12-19. Set Clock Source Latency Constraint Options
Option	Description
Clock Name or Source	To select the clock source, click the Browse button to open the Choose the Clock Source Pin dialog box. The only choice available for Pin Type is Clock Pins.
Late Rise	Specifies the largest possible latency (in nanoseconds) of the rising clock edge at the clock port or pin selected with respect to its source. Negative values are acceptable, but may lead to overly optimistic analysis.
Late Fall	Specifies the largest possible latency (in nanoseconds) of the falling clock edge at the clock port or pin selected with respect to its source. Negative values are acceptable, but may lead to overly optimistic analysis.
Early Rise	Specifies the smallest possible latency (in nanoseconds) of the rising clock edge at the clock port or pin selected with respect to its source. Negative values are acceptable, but may lead to overly optimistic analysis.
Early Fall	Specifies the smallest possible latency (in nanoseconds) of the falling clock edge at the clock port or pin selected with respect to its source. Negative values are acceptable, but may lead to overly optimistic analysis.
Falling same as rising	Specifies that the rising and falling clock edges have the same latency.
Early same as Late	Specifies that the clock source latency must be considered a single value, not a range from early to late.
Comment	Enter a one-line comment to describe the clock source latency.

12.4.4 Set a Clock Group

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To add or delete a clock group constraint, open the Add Clock Groups Constraint dialog box in one of the following ways:

From the Constraints menu, click Clock Groups.
In the Constraints Browser, double-click Clock Groups.
Right-click any row of Clock Groups Constraints Table and click Add Clock Groups.

Figure 12-28. Add Clock Groups Constraints Dialog Box

The following table describes the Add Clock Groups Constraints dialog box options.

Table 12-20. Add Clock Groups Constraints Options
Option	Description
ClockGroupsName	Enter a name for the clock groups to be added.
Exclusive Flag	Choose one of the following three clock group attributes for the clock group: Logically Exclusive : Use this setting for clocks that can exist physically on the device at the same time, but are logically exclusive (for example, multiplexed clocks). Physically Exclusive : Use this setting for clocks that cannot exist physically on the device at the same time (for example, multiple clocks defined on the same pin). Asynchronous : Use this setting when there are valid timing paths between the two clock groups, but the two clocks have no frequency or phase relationship and therefore these timing paths can be excluded from timing analysis.
Add Group	Click Add to open a dialog box to add clocks to a clock group. In the Available Pins list, click the clocks, and then click Add to move them to the Assigned Pins list. Click OK.
Delete Group	Delete the clocks from a clock group. Select the group of clocks to be deleted and click Delete Group. This will delete the clock group.

12.4.5 Modifying a System Jitter Value

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Use the set_system_jitter constraint to adjust the global jitter that is automatically generated by Libero.