Logic Design for Array-Based Circuits

by Donnamaie E. White

Design Optimization

Last Edit July 22, 2001

Design To Reduce Internal Cell Utilization

Reduction of the internal cell utilization or equivalent gate count is also called logical circuit minimization. Factoring of common terms from the logical equation and the removal of redundant logic help reduce cell counts by reducing the logic that must be implemented. Minimization is critical when a high fault-grade score is desired since redundant logic will lower the potential test score (fault masking).

Use of the higher functionality MSI macros and the design approach discussed earlier, of selecting higher functionality macros first and working back towards the SSI macros in the library, will contribute to a cell-efficient design. The design approaches for internal cell minimization are shown in Table 4-6.

Table 4-6 Minimizing Internal Cell Utilization

Reducing Internal Cell Utilization
Logic minimization.
High-functionality internal macros.
Use shift counters instead of parallel counters.
Use ripple counters if the propagation delay meets specification delays.
Use ripple-carry adders (between MSI blocks).
Use single polarity between macros.
Use serial data transfer.
Use a scan-test F/F or latch to replace a MUX-F/F or MUX-LATCH combination.
Avoid extraneous invertors (those added just to invert signals). Many macros are available in complementary form or use DeMorgan's theorem.
When converting from TTL or ECL, do not implement unused functions. Keep the macro design application-specific.
Avoid: unused preset, clear multiple enables excess carry logic excess load logic

Internal cell minimization is not fully compatible with the approaches used to improve speed. While logic minimization does help speed, as does the use of high-functionality macros, serial operations are slower than parallel operations. The designer must be guided by the priority assigned to the conflicting design objectives.

Example

An experienced ECL designer chose the Q3500 array and converted a standard-part design into macros from the chosen library. He was careful to duplicate the parts exactly. When he was finished, he had 124% cell utilization. At the time, there was no larger array.

The solution came when the logic was minimized and the unused functionality of the individual standard parts was deleted from the design. By changing the design from a direct conversion to an application-specific implementation, the cell utilization was reduced to 98% and the circuit was built. {True story.}

Figure 4-3 Optimization - Cell Utilization (Sizing)

Optimization Issues - Cell Utilization