Spectre Tech Tips: How to Migrate to Spectre X?

Moving to Spectre X is an important decision to increase your productivity in times when the demand for more simulations and the number of verification tasks are increasing dramatically. Nowadays, due to an increase in the complexity of advanced node devices and the functionality required per chip, the typical size of a testbench is 10 times what was 5 years ago.

The increase in productivity means you need a simulator that can run faster than before and can be used for bigger testbenches without increasing the simulation time. On top of that, you need a simulator that can run out of the box without the need for tweaking or fine-tuning to get the best performance. There is no time now to experiment with different options while the verification task is becoming more and more complex. However, migration to a new simulator will have the risk of compatibility issues and mapping of options between the new and the old netlists.

Here is the good news. If you are migrating from Spectre APS to Spectre X, you can reuse your testbench with all its options without fearing the risk of compatibility issues.

Spectre APS and Spectre X use the same interface. Therefore, there will be no need to change your netlist at all to adapt to Spectre X. But what about the fine-tuned options you used in the netlist for Spectre APS? Will that affect Spectre X performance and accuracy? We will answer these questions later in the post, but first, let's see how Spectre APS errpreset modes are mapped to Spectre X preset modes. 

Spectre APS has three errpreset accuracy modes: conservative, moderate, and liberal. Spectre X has five preset modes: CX, AX, MX, LX, and VX. So, how to map your current Spectre APS errpreset mode to a Spectre X preset mode?

Table 1: Mapping of Spectre APS modes to Spectre X modes

If you are using Spectre APS moderate, then the go-to Spectre X preset mode is MX. You may try LX mode as it gives sufficient accuracy when compared to moderate, with much better performance for designs like PMIC and high-voltage circuits.

If your testbench is using Spectre APS conservative mode, you may start with Spectre X AX mode, as many conservative runs do not really need that high level of accuracy. AX mode was introduced to fill the gap in accuracy and performance between moderate and conservative. However, if you need more accuracy than what is provided by AX mode, you can always switch to CX mode.

If you are using the liberal mode of Spectre APS for your custom IC verification task, here is the good news for you. Spectre X introduced the VX mode especially for that. VX mode gives much better performance than liberal mode and will scale well for large testbenches that are common for custom IC verification tasks.

Figure 1: Performance and accuracy comparison of Spectre X and Spectre APS modes

Keep in mind that the accuracy required from a simulation depends on the measurements and it is not directly related to the testbench itself. That means the same testbench can be simulated with CX preset mode if you are measuring sensitive signals (like jitter or highly linear THD), or you can simulate the testbench with VX mode if you are checking the functionality and the connections among the subsystems inside your testbench.

Back to the question about the fine-tuned options you already used for Spectre APS in the netlist. Will they affect Spectre X's performance and accuracy? We already thought about this, and by default, Spectre X will ignore all such options that were used in the netlist to fine-tune Spectre APS. It will report these options in a log file for your convenience. This way, Spectre X will allow you to use any legacy testbench without worrying about these options. It also gives you the out-of-the-box performance as promised. 

Does this mean that these options are no longer used by Spectre X? The answer is no. These options can still be used by Spectre X. However, to honor them in the netlist, you need to add the following command-line argument:

-preset_override=<names of the netlist options>

For example, by default, Spectre X ignores the option maxstep. However, if you want to use it for a specific reason in your testbench, you need to add the following argument in your Spectre command line:

-preset_override=maxstep

What about postlayout testbenches? Here is another good news. Spectre X, by default, recognizes the postlayout circuits and applies parasitics optimization algorithms based on the preset mode in use. So, every Spectre X preset mode has certain parasitics optimization aggressiveness from very conservative with CX to very liberal with VX preset mode. Still, if you want to control the parasitics optimization level manually, you can use the command-line argument +postlpreset=<mode> with one of these values: off, cx, ax, mx, lx, and vx.

The simple use model of Spectre X allows you to migrate risk-free from Spectre APS and focus more on your testbenches and your verification tasks while Spectre X provides you with the best performance out of the box.

Moustafa Mohamed

Related Resources

• Spectre Classic Simulator, Spectre Accelerated Parallel Simulator (APS), and Spectre Extensive Partitioning Simulator (XPS) User Guide

You may also contact your Cadence support AE for guidance.

For more information on Cadence products and services, visit www.cadence.com.

About Spectre Tech Tips

Spectre Tech Tips is a blog series aimed at exploring the capabilities and potential of Spectre^® circuit simulator. In addition to providing insight into the useful features and enhancements in Spectre, this series broadcasts the voice of different bloggers and experts, who share their knowledge and experience on all things related to Spectre. Enter your email address in the Subscriptions box and select SUBSCRIBE to receive notifications about our latest blog posts.