Macro options When an array provides macro options, those options should be reviewed for applicability to the design problem. Each option may have a different fan-out load or drive capability. Each option will have a different power dissipation. In some case, the different options may have different cell counts. This may be necessitated by an internal current limit for the array or from the early estimates of the junction temperature.
Drivers typically carry a power dissipation penalty.
They should be used in clock distribution lines and for heavily loaded paths that have tight timing specifications. Macro functionality Another reason to review the macro library before finalizing a design is that speed is usually a function of density. A hard macro, where the routing is always in the same pattern, can guaranty its worst-case speed. A soft macro has placement require ments and priority routing that must be used if it is to meet its specifications. Other design techniques to increase the circuit density include replacing gate structures with multiplexors where the speed and gate count would be reduced.