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Principles of Top-Down Mixed-Signal Design

A Rigorous Top-Down Design Process

essential departure from the digital design methodology. Mixed-level simulation is required to establish that the blocks function as designed in the overall system.

To verify a block with mixed-level simulation, the model of the block in the top-level schematic is replaced with the transistor level schematic of the block before running the simulation. For this reason, all of the blocks in the architectural description of the sys- tem must be “pin-accurate”, meaning that they must have the right number of pins and characteristics of each pin must be representative of the expected signal levels, polari- ties, impedances, etc.

The pin-accurate system description, described at a high level, acts as a test bench for the block, which is described at the transistor level. Thus, the block is verified in the context of the system, and it is easy to see the effect of imperfections in the block on the performance of the system. Mixed-level simulation requires that both the system and the block designers use the same simulator and that it be well suited for both system- and transistor-level simulation.

Mixed-level simulation allows a natural sharing of information between the system and block designers. When the system-level model is passed to the block designer, the behavioral model of a block becomes an executable specification and the description of the system becomes an executable test bench for the block. When the transistor level design of the block is complete, it is easily included in the system-level simulation.

Mixed-level simulation is the only feasible approach currently available for verifying large complex mixed-signal systems. Some propose to use either timing simulators (sometimes referred to as fast or reduced accuracy circuit simulators) or circuit simula- tors running on parallel processors. However, both approaches defer system-level verifi- cation until the whole system is available at transistor level, and neither provide the performance nor the generality needed to thoroughly verify most mixed-signal systems. They do, however, have roles to play both within the mixed-level simulation process and during final verification.

Successful use of mixed-level simulation requires careful planning and forethought (provided during the verification planning process). And even then, there is no guaran- tee that it will find all the problems with a design. However, it will find many problems, and it will find them much earlier in the design process, before full-chip simulations, when they are much less costly to fix. And with mixed-level simulation, it is possible to run tests that are much too expensive to run with full-chip simulation.

5.3.1

Mixed-Level Simulation Example

Though this example is several years old, it is representative of the type of circuit com- plexity that are mainstream today. It is a PRML channel chip that is difficult to simulate for two reasons. First, it is a relatively large circuit that involves both analog and digital sections that are closely coupled. Second, the architecture involves complex feedback loops and adaptive circuits that take many cycles to settle. The combination of many transistors and many cycles combines with the result being a simulation that is so expen- sive as to be impractical. In this case, the expected simulation time was predicted to be greater than a month.

The traditional approach to simulating a complex circuit like this is to simulate the blocks individually. Of course this verifies that the blocks work individually, but not together. In addition, for this circuit it is difficult to verify the blocks when operating

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