Mixed-signal design engineers face increasing difficulties in design and verification of complex mixed-signal SOCs. In a survey of mixed-signal design engineers during the 2011 Mixed-Signal Tech on Tour, a worldwide series presented by Cadence Design Systems Inc., the 561 respondents identified mixed-signal verification as a top customer challenge.

The performance of Spice simulation is prominent in the difficulties being reported (figure 1). Analog Spice and Fast-Spice simulators are orders of magnitude slower than digital simulators and are slower still when compared with emulators and hardware accelerators. A June 2011 Design Automation Conference panel discussed the need for analog design and verification to become more like digital—that is, to become more structured and more top-down (reference 1). Verification planning tools are required, and debug methodologies such as ABV (assertion-based verification), MDV (metric-driven verification), and UVM (universal verification methodology)-like self-checking test benches must be created for analog/mixed-signal.

To tackle simulation-throughput issues, designers are turning to behavioral-modeling techniques, which can increase simulation speed. Such techniques include event-driven simulation based on Verilog-A, Verilog-AMS, and RNM (real-number modeling).

Analog behavioral models are typically written in Verilog-AMS, Verilog-A, VHDL-AMS, or SystemVerilog.

Verilog-A is a pure-analog subset of Verilog-AMS and is mainly used for detailed analog models for performance verification. The language is quite simple, but it is challenging to write a good behavioral model with Verilog-A that provides significant performance gains while retaining the right level of accuracy. The advantage of Verilog-A is the ability to use models in pure-analog simulations as well as in the mixed-signal environment. The models are too low-level, however, to enable efficient SOC-level verification of mixed-signal designs.

The RNM technique models electrical signals by representing them as real values. Provided that the modules are at a sufficiently high level of abstraction, the interfaces can be described by passing real numbers between blocks to represent the voltage, or current, signal being transferred. This is a powerful way to simulate complex systems rapidly. Traditionally, iterating to a solution involving feedback would require an analog solver.

RNM is available in the Verilog-AMS, SystemVerilog, and VHDL-AMS languages. A commonly used RNM approach is the wreal data type in Verilog-AMS. RNM uses a discrete event solver—without an analog solver—and can be used to simulate mixed-signal systems at incredible speeds. It is primarily limited to modeling at a high enough level of abstraction that bidirectional analog interactions between blocks are not significant. In other words, typical RNM defines blocks in terms of input/output transfer characteristics, with no strong direct feedback present among the blocks. Logic can be modeled naturally in these languages, so RNM is also a good choice for systems with only a small amount of analog content.

Top-down or bottom-up
Designers use two principal methodologies based on the creation of behavioral models for mixed-signal design. In a top-down methodology, models are developed before the circuits are designed. The behavioral models can be simpler ones that are sufficient for functional verification at the system level. In a bottom-up methodology, the models are written to match an already implemented block for performance verification and usually result in a more accurate but slower-running model.

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