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

ID 683323
Date 9/24/2018
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1.2.1.1. Avoid Combinational Loops

Combinational loops are among the most common causes of instability and unreliability in digital designs. Combinational loops generally violate synchronous design principles by establishing a direct feedback loop that contains no registers.

Avoid combinational loops whenever possible. In a synchronous design, feedback loops should include registers. For example, a combinational loop occurs when the left-hand side of an arithmetic expression also appears on the right-hand side in HDL code. A combinational loop also occurs when you feed back the output of a register to an asynchronous pin of the same register through combinational logic.

Figure 1. Combinational Loop Through Asynchronous Control Pin
Tip: Use recovery and removal analysis to perform timing analysis on asynchronous ports, such as clear or reset in the Intel® Quartus® Prime software.

Combinational loops are inherently high-risk design structures for the following reasons:

  • Combinational loop behavior generally depends on relative propagation delays through the logic involved in the loop. As discussed, propagation delays can change, which means the behavior of the loop is unpredictable.
  • In many design tools, combinational loops can cause endless computation loops . Most tools break open combinational loops to process the design. The various tools used in the design flow may open a given loop differently, and process it in a way inconsistent with the original design intent.