mixed signal design methodology-Bottom-up design approach

The traditional approach to design is referred to as bottom-up design. In this approach, the design process starts with the design of the individual blocks, which are then combined to form the system. The design of the blocks starts with a set of specifications and ends with a transistor level implementation. At this point, each block is verified as a stand-alone unit against specifications and not in the context of the overall system. Once verified individually, the blocks are then combined and verified together. At this point the entire system is represented at the transistor level
While bottom-up design continues to be effective for small designs, large designs expose
several important problems with this approach [:
• Once the blocks are combined, simulation takes a long time so verification becomes
difficult and perhaps impossible. The amount of verification must be reduced to meet
time-to-market goals.
• For complex designs, the greatest impact on the performance, cost and functionality is
typically found at the architectural level. With a bottom-up design style, little if any
architectural exploration is performed so these types of improvements are often
• Any errors or problems found when assembling the system are expensive to fix
because they involve redesign of the transistor-level blocks.
• Communication between designers is critical, yet an informal and error-prone
approach to communication is usually employed.
• Several important and expensive steps in the bottom-up design process must be
performed serially, which stretches the time required to complete the design.
Examples include system level verification and test development