A Rigorous Top-Down Design Process
Principles of Top-Down Mixed-Signal Design
outside the system, and it is difficult to predict the performance of the system just know- ing the performance of the individual blocks.
When the architecture was simulated at a high level with each block represented by a pin-accurate behavioral model, the simulation time was less than 10 minutes. Then, when a single block was run at the transistor level, the simulation ran overnight. Even though the full system was never simulated at the transistor level, it worked the first time because this methodology verified the blocks in the context of the system and it verified the interfaces between the blocks.
Once a block is implemented, one could update the models that represent it to more closely mimic its actual behavior. This improves the effectiveness of mixed-level and system-level simulation and is referred to as bottom-up verification. To reduce the chance of errors, it is best done during the mixed-level simulation procedure. In this way, the verification of a block by mixed-level simulation becomes a three step process. First the proposed block functionality is verified by including an idealized model of the block in system-level simulations. Then, the functionality of the block as implemented is verified by replacing the idealized model with the netlist of the block. This also allows the effect of the block’s imperfections on the system performance to be observed. Finally, the netlist of the block is replaced by an extracted model. By comparing the results achieved from simulations that involved the netlist and extracted models, the functionality and accuracy of the extracted model can be verified. From then on, mixed- level simulations of other blocks are made more representative by using the extracted model of the block just verified rather than the idealized model.
Bottom-up verification should not delayed until the end of the design process, but should rather be done continuously during the entire design process. Once a block has been implemented to the degree that a more representative model can be extracted, that model should replace the idealized top-level model as long as it does not evaluate sub- stantially slower. Doing so tends to improve the effectiveness of mixed-level simulation and the accuracy of the extracted models. Thus, as a side benefit, the models that would be needed if the block were to be made into a shared IP block are already available and tested at the end of the project. If the model development for bottom-up verification were postponed to the end of the design process, the natural pressure to meet schedule targets as designs near tape-out often result in some of the verification, and perhaps all of the modeling, being skipped. This increases the chance of error and decreases the opportunity for reuse.
When done properly, bottom-up verification allows the detailed verification of very large systems. The behavioral simulation runs quickly because the details of the imple- mentation are discarded while keeping the details of the behavior. Because the details of the implementation are discarded, the detailed behavioral models generated in a bottom- up verification process are useful for third-party IP evaluation and reuse.
In a top-down design process, SPICE-level simulation is used judiciously in order to get its benefits without incurring its costs. All blocks are simulated at the transistor level in the context of the system (mixed-level simulation) in order to verify their functionality
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