Logic Design for Array-Based Circuits

by Donnamaie E. White

Sizing the Design - Selecting the Array

Functional Specification - A Closer Look

The functional or target specification is the first level of description of the project that may encompass one or more arrays when the design is partitioned. There may be a specification tree with the total project at the top node and individual circuit blocks or modules detailed underneath. Topics included in a functional specification are listed in Table 3-1.

At this early stage, a functional description of what is to be accomplished is created along with some of the top-level circuit requirements.

Array Interfacing

For the partitioned project (multiple arrays), the individual array specifications would include a description of array interfacing.

Interconnection between arrays is faster when done with ECL. When choosing single or dual (differential) rail ECL use the following guidelines:

If the arrays will be placed on the same board and will be adjacent to each other, single rail (non-differential) ECL may be acceptable.
If the arrays will communicate across a backplane or be remote on the board, differential ECL may be required.
Differential ECL is required if the operating speeds exceed the maximum frequency specifications for single rail ECL.

The potential need for differential ECL should be indicated at the functional specifica tion level.

Partitioned circuits should attempt to balance
the distribution of I/O and internal cell usage
between the different arrays while maintaining
critical paths within one array if possible.

This is still the rule to follow - no matter how big the arrays get.

It is also a good guideline for how to break up a 6-8 milllion gate array into top-level blocks - keep the critical paths inside the block if possible. Interblock connections today are what interarray connections were yesterday.

In today's very large designs (1 - 12 million equivalent gates, the blocks within the design (250K to 500K equivalent gates or what can be run overnight with Design Compiler) fall under the same rules as multiple-arrays did. The same care in partitioning the I/O (in this case the block interconnect), the grouping of the logic by function and to maintain the best options for the critical paths, and consideration of the external I/O ring requirements are still the rules.

Table 3-1 Components of The Functional specification

Functional Specification

Block diagram to the module level- including any partitioning into more than one array
Description of the boundaries between the modules and the rest of the system

Initial sizing of the I/O interface by type - ECL, TTL, etc.

Functional Description of the Modules

Description of the interface between the circuit modules - busses, control, critical interconnects

The overall performance requirements

- - - the maximum frequency of operation

- - - target clock speed (per clock)

- - - path propagation delay requirements set by modules external to this design
I/O toggle rates

Synchronous/asynchronous signals

Allowed or available power supplies

Power restrictions

Physical size restrictions

Environmental requirements -Commercial, Military, Industrial, Other

Packaging requirements

Derating for junction temperature

Prioritized design objectives

Hard Specifications

Design criteria that are considered as hard (inflexible) specifications should be clearly documented as such. Specifications that might be alterable should also be clearly identified. If a tradeoff or judgment call needs to be made during the remainder of the design project, such information can save time and possibly the project.

Design Objectives

Overall design objectives should be clearly identified and documented. These include optimization for speed, power or die size, which translates to minimized inter nal cell utilization and minimized I/O utilization. Since these objectives are in conflict, they should be prioritized.

As a last step, there should be a careful design review of the circuit and system functional specifications, and the partitioning.