Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

ID 683323
Date 9/24/2018
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1.4.2. Use Global Clock Network Resources

Intel FPGAs provide device-wide global clock routing resources and dedicated inputs. Use the FPGA’s low-skew, high fan-out dedicated routing where available.

By assigning a clock input to one of these dedicated clock pins or with an Intel® Quartus® Prime assignment to assign global routing, you can take advantage of the dedicated routing available for clock signals.

In an ASIC design, you must balance the clock delay distributed across the device. Because Intel FPGAs provide device-wide global clock routing resources and dedicated inputs, there is no need to manually balance delays on the clock network.

Limit the number of clocks in the design to the number of dedicated global clock resources available in the FPGA. Clocks feeding multiple locations that do not use global routing may exhibit clock skew across the device leading to timing problems. In addition, generating internal clocks with combinational logic adds delays on the clock path. Delay on a clock line can result in a clock skew greater than the data path length between two registers. If the clock skew is greater than the data delay, you violate the timing parameters of the register (such as hold time requirements) and the design does not function correctly.

FPGAs offer low-skew global routing resources to distribute high fan-out signals. These resources help with the implementation of large designs with multiple clock domains. Many large FPGA devices provide dedicated global clock networks, regional clock networks, and dedicated fast regional clock networks. These clocks are organized into a hierarchical clock structure that allows multiple clocks in each device region with low skew and delay. There are typically several dedicated clock pins to drive either global or regional clock networks, and both PLL outputs and internal clocks can drive various clock networks.

To reduce clock skew in a given clock domain and ensure that hold times are met in that clock domain, assign each clock signal to one of the global high fan-out, low-skew clock networks in the FPGA device. The Intel® Quartus® Prime software automatically assigns global routing resources for high fan-out control signals, PLL outputs, and signals feeding the global clock pins on the device. To direct the software to assign global routing for a signal, turn on the Global Signal option in the Assignment Editor.

Note: Global Signal assignments only controls whether a signal is promoted using the specified dedicated resources or not, but does not control which or how many resources are used.

To take full advantage of the routing resources in a design, make sure that the sources of clock signals (input clock pins or internally-generated clocks) drive only the clock input ports of registers. In older Intel device families, if a clock signal feeds the data ports of a register, the signal may not be able to use dedicated routing, which can lead to decreased performance and clock skew problems. In general, allowing clock signals to drive the data ports of registers is not considered synchronous design and can complicate timing closure.