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  • EEdesign Exclusive

    Automatic Memory IP Characterization

    By Charles Longway, Conexant Systems Inc., You-Pang Wei, Legend Design Technology Inc.
    (12/06/00 13:18 p.m. EST)

    Designers of systems-on-a-chip (SoC) must integrate a wide variety of intellectual property (IP). This includes all kinds of memory, logic and control functions, with many memory sizes and types now being placed on chip. The upshot is that IC design, once logic dominant, has become memory dominant. In fact, it is not unusual to see more than 100 different instances of SRAM, ROM and other specialty memories on a single SoC.

    But to use these embedded memories in IC designs, accurate timing and power models are essential. And to generate such models, characterization is required. This often implies many simulations with updated Spice models.

    At the same time, technology is moving rapidly from 0.25 micron to 0.18, 0.15 and 0.13 micron. Along the way, generic CMOS technology has diversified into variants such as high speed, low power and high density. Now, each process variant must have its own characterization. With high-volume products, it is increasingly common to manufacture a part at several foundries to meet customer needs. Since each process variant at each foundry is slightly different, it is necessary to recharacterize memory for the specific process and foundry to be used. Thatís why a tool that automatically characterizes embedded memory has become indispensable.

    Memory Characterization

    Memory characterization is a very time-consuming and error-prone process. Due to the complexities and varieties of the timing parameters involved, it has become a key part in the memory compiler development flow as shown in Figure 1.

    Figure 1: Memory Compiler Development Flow

    Typically, the characterization process builds timing models, which can be applied to all memory instances in the compiler. Variations on memory instances could be, among others, word count, bit count or aspect ratio. Memory is usually characterized in the following steps:
    1. Select corner instances. Typical selections are based upon criteria such as small, large, narrow and wide.
    2. Characterize the selected instances.
    3. Build the lookup table or equations by fitting curves.
    4. Generate timing model.

    The conventional way of characterizing memory is a two-step method. First, manually build a parameterized composite Spice netlist of the tileable blocks. The major memory blocks, such as column, row and memory array are parameterized as functions of number of words, bits per word and multiplexing. Second, iteratively run simulations over the netlist and obtain timing information over the range of parameters.

    The conventional method separately builds the netlists of major blocks for simulation instead of treating the memory instance as an entity from layout extraction. To extend the model to different instance variations, compiler designers need to develop a parameterized method for timing estimation, which is sometimes called a black box model.

    Moreover, the conventional method is always based on the following simplified assumptions:
    1. A memory can be segmentedóthe timing of a memory instance is just the summation of the timing of its major blocks. This assumption can lead to ignoring coupling and distributed effects, which become more pronounced for more advanced technology.
    2. A memory can be parameterized--the Spice netlist of major blocks can be made as a function of words, bits per word and multiplexing to accurately reflect the actual layout over a broad range of multidimensional variations. The actual memory circuit is often nonlinear, and such things as special power and spacer cells add to this nonlinearity. If a mistake has been made in parameterization it will likely not be detected.

    To produce high-quality models, the conventional method needs numerous correlations between hand-built critical-path netlists and the real ones extracted from the layout. For the present technology, the task of detailed correlation is getting more difficult because the function is multidimensional and multiordered.

    To compensate for inaccuracy and inconsistency, the conventional-style memory compiler adds large margins into its timing models. We have found that these margins often exceed 20 percent and could make it difficult to meet design specifications when the performance requirement is critical.

    New Needs

    In todayís technology, with chips having a heavy memory content built with different processes and going to multiple foundries, the conventional approach to memory characterization needs to be revisited. For memory compilers, a change in process models requires resimulation of the full models. Now simulation of the various device parameters is necessary for porting, debugging, performance optimization and overall yield improvement.

    During technology porting, foundries characterize their compiler-generated memories. However, other kinds of embedded memories (such as three-port SRAM) still require resimulation. When porting from a slow technology to a fast one, setup time is normally not a problem, but hold time may cause functional failures. Therefore, memories require careful simulation on an instance basis.

    Memory resimulation can be an overwhelming task for both engineering and simulation. A memory compiler data sheet usually contains 20 to 30 timing parameters, most of which require an optimization such as bisectional analysis for accuracy and speed in characterization. For each technology corner, there will be a huge number of circuit parameters to simulate. And for each parameter, the simulation stimuli and controls, measurement statements and optimization setups require special arrangements. Therefore, if not automated, memory characterization requires an enormous amount of human effort.

    Not only does memory characterization needs to be automatic, it also must be universal, meaning that the same setup can be applied to all instances of a memory compiler. With a universal approach, automatic characterization can be rerun any time a Spice model changes. This is particularly useful when a process gets new models or when you need to analyze loss of yield on a particular set of wafers.

    On-Chip Memory IP Characterization

    On large chip designs there are often 100 or even more blocks of SRAM, ROM and other specialty memories. It is common for embedded memory to be in the critical path of the design, as shown in Figure 2.

    Figure 2: Embedded Memory on 'Critical-Path' of IC Design

    Critical path simulation and verification are very important, especially for high-performance IC designs. Usually, the cells and gates along the path can be characterized easily and accurately. However, memory IP blocks on the critical path may still be treated as a black boxes with timing models interpolated or extrapolated from the corner instances of memory compilers. If so, it is very difficult to optimize the critical paths.

    To enhance the performance of high-speed designs, it is necessary to have the so-called white box model from actual simulations. Since this characterization is based on extraction and simulation of the layout, it is more accurate than the models from lookup tables. Thus, the dependency on technology and layout can be thoroughly measured, especially for verification and debugging.

    Legend Design Technology Inc. has developed an EDA tool called MemChar to address the need to characterize memory IP automatically. MemChar takes, as an input, specifications for each parameter found in the memory data sheet. The other input needed is the extracted netlist of the memory circuit, which can be produced by tools such as StarRC and Arcadia. The circuit data file in Spice format normally includes MOSFETs, resistors and capacitors. If the memory instance is too large for extraction, the GDSII layout of the memory array must be reduced to a ring shape for extraction. This can be done either by a layout editor or by Legendís GDSCut program. Legendís SpiceCut has been designed to handle ring-shaped memory arrays and can emulate them to full-size arrays in building critical-path netlists for characterization.

    Based upon the parameter specifications from the data sheets, MemChar can automatically generate the simulation stimulus and controls. The controls for optimization, such as bisection models, are very critical for the setup time, hold time, minimum clock width, etc. Since characterization could take hundreds of simulations, the CPU time of each one does have an impact on the performance of the whole process. To enhance the performance, SpiceCut is used to build the critical-path netlist for circuit reduction and RC reduction. A number of Spice netlists are generated with the necessary stimulus and measurement statements. Circuit Simulation Manager is then called to run simulations automatically in sweep loops or optimization loops. Users can specify the preferred circuit simulator, after which timing data is obtained from the simulation results and organized as the timing database for the models. Figure 3 shows MemCharís process and data flow.

    Figure 3: Process and Data Flow of MemCharTM Program

    MemChar has been developed to automate all processes in the flow, including simulation and optimization. Although a memory compiler can generate several tens of thousands of instances, only one instance is needed to set up the automatic characterization flow of MemChar. For all other configurations of the memory compiler, the same setup can be used.

    The bottleneck of memory characterization always resides in the circuit reduction. This is the process of building critical-path circuits for simulation. The patented SpiceCut-Memory tool has been used for circuit reduction in many designs. To further enhance the performance, an Asymptotic Waveform Evaluation (AWE)-based RC reduction has been built into SpiceCut. The critical coupling effects that are necessary for memory simulation are always taken into account. To get optimized setup times, hold times, minimum pulse width, etc., SpiceCut-Memory can automatically generate the stimulus and controls of the bisection models used for optimization.

    Typical is the way Conexant Systems uses MemChar for both characterization and debugging. It has developed all setup files for characterizing our 0.18-micron memory compilers and also verified the timing for the memories on several products.

    Recently, Conexant had a memory failure on a new part. Before MemChar, when a product failed there were two choices: use a relaxed circuit simulator and simulate the whole circuit without the full analog resolution of spice, or spend several days setting up a test bench to run spice on a handcrafted circuit. Neither approach is desirable or fast.

    With MemChar, the companyís engineers can resimulate the failing memory with parasitic in one day, at Spice-level accuracy. The accurate timing models obtained from this characterization have helped in debugging the new part. In addition to reducing the simulation CPU time, the greatest benefit of using MemChar is reducing engineering time.

    For high-performance networking and communication designs, MemChar is especially useful for its unique capability of performing on-chip embedded memory characterization. Since the input is the layout-extracted data of the exact configuration, the characterization results directly reflect the on-chip models, not interpolated values. Therefore, the margins can be well controlled and system performance can be accurately simulated.

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