6.6 Hint 6 - Prevent Certain Nodes from Collapsing

In some designs, it may be necessary to prevent the fitter from collapsing specific nodes. When collapsed, these nodes can generate excessive product terms, which may prevent the design from fitting into the ATF15XX/ATF1500 device. Designers can assist the fitter by explicitly defining nodes at strategic points in the design and preventing those nodes from collapsing. The following guidelines identify situations where inserting nodes may be beneficial:

• Outputs of complex intermediate combinatorial logic expressions
• Intermediate logic expressions which feed multiple inputs
• Multiple product-term register clock, reset or preset functions
• Multiple product-term output enables