6.3 Hint 3 - Using Cascade Logic for Design Performance

If system performance is a concern, cascade logic can be used to borrow product terms from adjacent macrocells. This approach is particularly beneficial for high-performance state machines. Cascade logic incurs a much lower propagation delay than using foldback nodes or buried macrocell nodes. By default, the fitter uses cascade logic to optimize performance. If the design fails to fit, the fitter then attempts to implement logic using foldback nodes or buried macrocell feedback nodes. If output propagation delay is critical to meeting system timing requirements, the fitter can be customized to use cascade logic for specific outputs. Proper pin assignment is important to ensure that cascade logic is implemented on macrocells capable of borrowing product terms from adjacent macrocells. Each macrocell, except corner macrocells, can borrow up to five product terms from an adjacent macrocell. Multiple macrocells can be cascaded to provide up to 40 product terms for the macrocell at the bottom of the cascade chain.