Logic Design for Array-Based Circuits
Structured Design Methodology
Introduction To The Overview
The Structured Design Methodology, as developed here for the design of Bipolar, CMOS or BiCMOS logic arrays, applies to any array design effort regardless of technology or vendor. The designer who follows this methodology will ensure a smooth design flow between milestones that will help ensure a successful design the first time.
The design flow is presented in this chapter at the introductory level. Following chapters will detail specific areas such as timing analysis, simulation and power computation.
Design Sequence - Pre-Capture
The Structured Design Methodology stresses a certain design flow sequence of events, developed for use by the beginning array designer, the beginning user of an Engineering Workstation (EWS) or the designer experienced in both. Each step will be discussed in more detail after the design flow is fully outlined.
Circuit Functional Specification
The circuit functional specification is the target specification; it describes what it is that is to be implemented on one or more arrays. This includes: a block diagram of the system or circuit, overall performance requirements, I/O interface, testability, environmental and packaging requirements. (See Table 2-1.)
Once the functional specification identifies the need for more than one array, partitioning of the overall circuit modules to ensure proper boundary conditions must be made and then the functional specifications of the individual array circuits must be created. The specifications must be defined to be independent of each other to allow parallel circuit development. Note that there is no constraint at this point as to the product to be used beyond operating specifications.
The technology of the array is defined by the performance requirements. As a basic guideline, high speed requires ECL bipolar, slower speeds and low power require CMOS, and moderate speeds and bipolar drive capability without the price of bipolar power dissipation require BiCMOS. Where the boundaries are is subjective and subject to continual evolution and change.
Table 2-1 Components Of The Target Specification
Circuit Hardware Specification
The circuit hardware specification is the planned hardware approach to satisfying the target functional specification. For multiple array designs, this may involve another level of specification, one specification for each circuit intended for a different array. This implies that project partitioning has been completed, and defines all required I/O and throughput performance. (See Table 2-2.)
Table 2-2 Components Of The Hardware Specification
A hardware architecture specification equates to PDL (program description language) for software. It identifies modules and closely defines how the modules will work together. HDL (hardware description language) and VHDL have been developed to formalize this specification.
From this level of specification it is possible to estimate I/O signal requirements and internal cell utilization. At this point, the estimates are very rough and will only serve to allow a first cut at reducing the number of arrays that need to be considered. Some compromises or engineering tradeoffs may have been made, refining the functional specification.
For today's designs, the I/O estimates and the equivaent logic cell and memory space requirements serve to provie an estimate of the die size, to be refined later. If the die size exceeds expected limits, then the design will be either scrapped (and they do get cancelled), or adjustments are made to the expectations and requirements issued by the marketing department. If the die requires to much area, or the design draws too much power, these are critical issues in wireless, space-born, and hand-held devices.