Logic Design for Array-Based Circuits
Copyright © 1996, 2001, 2002 Donnamaie E. White
Last Edit July 22, 2001
Internal Net Delays
The delay in a heavily loaded net tends to be longer than the macro intrinsic delay, the delay through the macro that drives the net. The net delay is a result of the electrical effects of fan-out load, wire-OR load and the capacitance of the metal interconnect length. Wire-ORs if allowed in the library will add to the delay in the net with both an electrical load and with additional metal.
Front-Annotation delays (pre-place and route) due to metal length for a large array are on the average larger than for a small array. This is reasonable since the side to side distances are larger for the larger array. The break-up of heavily loaded paths into identical parallel paths can result in significant propagation delay improvement, regardless of array size.
Table 4-5 Components Of Internal Net Delays
Fan-out loading is the same regardless of the array size. A macro driving 6 loads on a large array would see the same load if the circuit were placed on a smaller array.
Wire-ORs when allowed
Some array libraries allowed dot-connects such as wire-ORs or wire-ANDs. These may save gate delays but add a wire or metal length penalty. A wire-OR driven by four macros and outputting to six other macros has the metal equivalent of a ten output net - using lumped Front-Annotation computations. This is considered to be a heavily loaded net. The added net delay will probably exceed the "saved" gate delay. The use of dot-connects should be carefully evaluated.
Verify that they are allowed on the schematics before evaluating their usefullness. They may be allowed on some arrays and not on others from the same vendor. For example, the AMCC Q5000 has wire-ORs but the Q20000 bipolar and Q24000 BiCMOS arrays do not allow their use.
Figure 4-2 Optimization - Speed
Optimization - Speed Considerations