Intel® Quartus® Prime Standard Edition User Guide: Timing Analyzer

ID 683068
Date 9/24/2018
Document Table of Contents Relaxing Setup with Multicycle (set_multicyle_path)

You can use a multicycle exception when the data transfer rate is slower than the clock cycle. Relaxing the setup relationship increases the window when timing analysis accepts data as valid.

In the following example, the source clock has a period of 10 ns, but the clock enables a group of registers, so the registers only enable every other cycle. Since the registers are fed by a 10 ns clock, the Timing Analyzer reports a setup of 10 ns and a hold of 0 ns. However, since the data is transferring every other cycle, the Timing Analyzer must analyze the relationships as if the clock is operating at 20 ns. That results in a setup of 20 ns, while the hold remains 0 ns, thus extending the window for data recognition.

The following pair of multicycle assignments relax the setup relationship by specifying the -setup value of N and the -hold value as N-1. You must specify the hold relationship with a -hold assignment to prevent a positive hold requirement.

Constraint to Relax Setup and Maintain Hold

set_multicycle_path -setup -from src_reg* -to dst_reg* 2
set_multicycle_path -hold -from src_reg* -to dst_reg* 1
Figure 59. Multicycle Setup Relationships

You can extend this pattern to create larger setup relationships to ease timing closure requirements. A common use for this exception is when writing to asynchronous RAM across an I/O interface. The delay between address, data, and a write enable may be several cycles. A multicycle exception to I/O ports allows extra time for the address and data to resolve before the enable occurs.

The following constraint relaxes the setup by three cycles:

Three Cycle I/O Interface Constraint

set_multicycle_path -setup -to [get_ports {SRAM_ADD[*] SRAM_DATA[*]} 3
set_multicycle_path -hold -to [get_ports {SRAM_ADD[*] SRAM_DATA[*]} 2

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