Quartus® Prime Standard Edition User Guide: Design Compilation

ID 683283
Date 10/22/2021
Document Table of Contents Allocate Global Routing Signals and Clock Networks if Required

In most cases, you do not have to allocate global routing signals because the 
 Quartus® Prime software finds the best solution for the global signals. However, if your design is complex and has multiple clocks, especially for a partition developed by a third-party IP designer, you may have to allocate global routing resources between various partitions.

Global routing signals can cause conflicts when independent partitions are integrated into a top-level design. The Quartus® Prime software automatically promotes high fan-out signals to use global routing resources available in the device. Third-party partitions can use the same global routing resources, thus causing conflicts in the top-level design. Additionally, LAB placement depends on whether the inputs to the logic cells within the LAB use a global clock signal. Problems can occur if a design does not use a global signal in a lower-level partition, but does use a global signal in the top-level design.

If the exported IP core is small, you can reduce the potential for problems by using constraints to promote clock and high fan-out signals to regional routing signals that cover only part of the device, instead of global routing signals. In this case, the Quartus® Prime software is likely to find a routing solution in the top-level design because there are many regional routing signals available on most Altera devices, and designs do not typically overuse regional resources.

To ensure that an IP block can utilize a regional clock signal, view the resource coverage of regional clocks in the Chip Planner, and then align LogicLock regions that constrain partition placement with available global clock routing resources. For example, if the LogicLock region for a particular partition is limited to one device quadrant, that partition’s clock can use a regional clock routing type that covers only one device quadrant. When all partition logic is available, the project lead can compile the entire design at the top level with floorplan assignments to allow the use of regional clocks that span only a part of the device.

If global resources are heavily used in the overall design, or the IP designer requires global clocks for their partition, you can set up constraints to avoid signal overuse at the top-level by assigning the appropriate type of global signals or setting a maximum number of clock signals for the partition.

You can use the Global Signal assignment to force or prevent the use of a global routing line, making the assignment to a clock source node or signal. You can also assign certain types of global clock resources in some device families, such as regional clocks. For example, if you have an IP core, such as a memory interface that specifies the use of a dual regional clock, you can constrain the IP to part of the device covered by a regional clock and change the Global Signal assignment to use a regional clock. This type of assignment can reduce clocking congestion and conflicts.

Alternatively, partition designers can specify the number of clocks allowed in the project using the maximum clocks allowed options in the Advanced Settings (Fitter) dialog box. Specify Maximum number of clocks of any type allowed, or use the Maximum number of global clocks allowed, Maximum number of regional clocks allowed, and Maximum number of periphery clocks allowed options to restrict the number of clock resources of a particular type in your design.

If you require more control when planning a design with integrated partitions, you can assign a specific signal to use a particular clock network in newer device families by assigning the clock control block instance called CLKCTRL. You can make a point-to-point assignment from a clock source node to a destination node, or a single-point assignment to a clock source node with the Global Clock CLKCTRL Location logic option. Set the assignment value to the name of the clock control block: CLKCTRL_G <global network number> for a global routing network, or CLKCTRL_R <regional network number> for a dedicated regional routing network in the device.

If you want to disable the automatic global promotion performed in the Fitter to prevent other signals from being placed on global (or regional) routing networks, turn off the Auto Global Clock and Auto Global Register Control Signals options in the Advanced Settings (Fitter) dialog box.

If you are using design partition scripts for independent partitions, the Quartus® Prime software can automatically write the commands to pass global constraints and turn off automatic options.

Alternatively, to avoid problems when integrating partitions into the top-level design, you can direct the Fitter to discard the placement and routing of the partition netlist by using the post-synthesis netlist, which forces the Fitter to reassign all the global signals for the partition when compiling the top-level design.