NSS-30017: Register Output Driving Its Own Control Signal Directly or
Through Combinational Logic
Combinational loops can cause significant stability and reliability
problems in a design. For example, because the behavior of a combinational loop often
depends on the relative propagation delays of the combinational loop's logic, and
because design tools experience difficulties when handling combinational loops, the
combinational loop after fitting may not function as it was originally intended to
function in the design.
Recommendation
A design should not contain any combinational loops where the output of a
register:
Directly drives one of its own control signals (for example, the
register's preset signal or asynchronous load signal). For example:Figure 1. Combinational Loop Example
Drives combinational logic that drives one of the register's
control signals. For example: