Advance SDC Techniques for Efficient Timing Closure - Part 1

Introduction to Advanced SDC Techniques

SDC is an industry-standard format used to specify the timing, design, and operational constraints for ASIC and FPGA designs. It’s crucial for various EDA tools like synthesis, static timing analysis (STA), and place-and-route.

Note:- Basic Of SDC (Our Previous Blog)

Key Concepts:

• Clock Definition: Specifies the clock waveforms and timing characteristics.
• Input/Output Delays: Defines constraints for signal arrival and output times.
• Timing Exceptions: Includes paths that need special treatment (e.g., false paths, multi-cycle paths).
• Design Rules: Set up max fanout, transition times, and other physical rules.

Usage in Physical Design SDC files guides the timing closure process by helping tools optimize the layout and logic to meet the defined timing requirements. Accurate SDC constraints ensure designs meet performance specs and are manufacturable. 

This is all we have covered in the previous articles. Now, it's time to dive into advanced techniques for handling more complex designs and optimizing timing closure. There may be a few places, where you might be thinking that it's advanced and you ned more clarity of topics, in that case, drop me an email or comment - I will try to add those on priority

1. Multi-Mode Multi-Corner (MMMC) Constraints 

In real-world designs, chips operate in various modes (e.g., functional mode, test mode, high-performance mode and low-power modes etc) and under different conditions (process, voltage, temperature). SDC must account for these variables simultaneously, which is managed through MMMC constraints. We have to make sure that the design meets timing in all such modes. 

Tools like Synopsys PrimeTime allow for multiple scenarios to be loaded, helping analyze the timing across different modes and corners. 

Attention:- If you are not aware of "Functional and Test Mode" and what are these "different conditions - process, voltage, and temperature" - Wait for some time, and I will explain it to you in another article. 

Command Pointer (for Synopsys tools):- set_operating_conditions and set_mode commands can be used to switch between these modes. 

Note:- Detailing for this topic is in next article

2. Clock Grouping and Complex Clock Relationships 

Many designs have multiple clocks running at different frequencies or phases, which makes timing analysis complex. And as we know that clock is very crucial part of the design and register to register timing analysis depends on clock definition itself. Now, assume that one set of data is travelling from 1 flipflop (triggered by 1 clock) and captured by other flipflop (triggered by other clock), then how will you do the timing analysis between them. If need to handle the constraint between them properly.

Similary, when clocks are asynchronous, you can specify them as "logically exclusive," meaning no paths are analyzed between them, thus simplifying timing. 

SDC provides commands like set_clock_groups to define relationships between clocks. In multi-clock systems like SoCs, the set_false_path command can be used for paths between clocks that don't need timing analysis. Defining proper clock domains with create_clock and grouping them helps avoid unnecessary timing violations. 

Attention:- If you are not aware of "clock domain", "asynchronous clocks", "register to register timing analysis", "basic clock constraints" - read my previous articles or wait for upcoming articles.

Command Pointer:- set_clock_groups, set_false_path

Note:- Detailing for this topic is in next article

3. Handling Advanced Timing Exceptions 

Designers often apply multi-cycle paths or min/max delay constraints when some paths can take more than one clock cycle to complete. These exceptions help optimize timing and reduce pessimism in the analysis. as per your circuit requirement, a path between two low-speed components may have relaxed timing because data does not need to be transferred every clock cycle. In such scenario,  using set_multicycle_path, we specify how many cycles a path can take, and set_min_delay or set_max_delay helps control specific delays between launch and capture registers. 

Pointer:  May be you thinking what's the big deal if it's taking more than1 cycle and you are right but problem is how tool will come to know that it's taking more than 1 cycle. We as a designer have to inform the tool that it will take more than 1 cycle to give me correct data. Remember - correct data is important at this point because capture flipflop will receive some data at the end of the 1 cycle but as a timing analysis tool, it don't know what is correct and wrong data. For the tool it's a data but Designer only know what's correct and wrong. So, designer has to inform the tool when the correct data will reach - after 1 cycle or 2 cycle or more then that. And this information we provide as part of Timing Exception using Multicycle path. We will discuss more about this in other article.

Attention:- If you are not aware of "Multi-cycle paths", "min/max delay constraints" or "Timing Exceptions" - read my previous articles or wait for upcoming articles.

Command Pointer:- set_multicycle_path, set_min_delay, set_max_delay

Note:- Detailing for this topic is in next articles

4. Power-Aware SDC for Low-Power 

Designs Low-power constraints are essential in designs where energy efficiency is a priority. Power-aware SDC integrates power domains with timing constraints. You can define power gating, multi-voltage islands, and clock gating effects through SDC to define how signals interact between different power domains. 

Tools like Synopsys PrimeTime PX use these constraints to perform power-aware timing analysis. 

Attention:- If you are not aware of "Low Power", "power gating" or "Multivoltage islands", "clock gating" - read my previous articles or wait for upcoming articles.

Command Pointer:- set_isolation, set_level_shifter, set_power_domain

Note:- Detailing for this topic is in next articles

5. Automating SDC Generation and Validation 

Managing SDC for complex designs requires automation to reduce errors. Custom scripts can help generate SDC files for different blocks or modes automatically. Automation scripts can extract timing requirements from RTL or netlists and generate appropriate SDC constraints, saving time during design iterations.

Tools like Synopsys Design Compiler provide utilities to validate and automate SDC generation, ensuring that constraints are consistent and correct throughout the design flow. 

Note:- Detailing for this topic is in next articles

This article is setting a base for future articles related to SDC (Synopsys Design Constraints). We will discuss in upcoming articles the advanced part of SDC from the SOC point of view.

Stay Tune.