Meeting timing closure requirements is essential for successfully implementing Cortex-M0 designs on low-density FPGAs. If timing constraints are not met, the design will fail to operate as intended. This article provides guidance on techniques and best practices for closing timing on Cortex-M0 implementations in low-density FPGA architectures.
Understanding the Challenges
There are several factors that make timing closure difficult with Cortex-M0 on small FPGAs:
- Limited routing resources – Low-density FPGAs have fewer interconnects between logic blocks. This makes routing congestion more likely.
- Lower performance logic blocks – The logic blocks in small FPGAs usually operate at lower speeds compared to higher density options.
- Complex processor logic – The Cortex-M0 has complex combinational logic that can be difficult to map efficiently.
- Tight timing margins – With small FPGAs, there is less margin between required and achieved timing.
Due to these constraints, special care must be taken to meet timing when targeting low-density architectures.
Design Methodology
Following good design practices from the start of the design process is critical for achieving timing closure. Here are some key methodology tips:
- Use synchronous design techniques – Adhere to recommended clocking, reset, and CDC methods.
- Apply timing constraints early – Develop accurate and complete timing constraints as the design progresses.
- Perform incremental synthesis – Resynthesize regularly to identify timing issues early.
- Simulate with SDF – Include SDF timing annotation in simulations to capture timing delays.
- Leave timing margin – Avoid defining constraints at the limit of the technology.
Setting up the design properly from the beginning establishes a solid foundation for closing timing.
FPGA Optimization Techniques
Once the design methodology is robust, there are FPGA-specific optimization techniques that can help achieve timing closure:
- Use FPGA IP cores – Leverage optimized IP like DSP blocks rather than coding custom logic.
- Floorplan with care – Plan FPGA resource usage to minimize routing delay.
- Optimize critical paths – Restructure or simplify logic on critical timing paths.
- Balance clock trees – Evenly distribute the clock network to minimize skew.
- Add pipeline stages – Break long combinational paths with additional registers.
- Duplicate logic – Replicate timing-critical functions to reduce fanout delay.
Applying architectural optimizations makes efficient use of the FPGA resources to improve achievable timing.
Cortex-M0 Specific Techniques
The Cortex-M0 processor itself can benefit from certain optimization techniques:
- Use the smallest, fastest M0 option – Start with Cortex-M0r0 if possible.
- Eliminate unneeded M0 features – Disable the MPU if not required.
- Use low-latency interrupt modes – Favor latencies like PendSV over SysTick.
- Reduce peripherals and interfaces – Minimize cores, buses, and I/O in use.
- Use M0 configurator tools – Tailor the system topology to meet timing.
- Limit unsupported instructions – Avoid complex instruction encodings.
Optimizing the M0 configuration and firmware code can provide extra timing margin.
Timing Analysis Techniques
Analyzing timing reports is key for identifying where closure issues exist. Useful techniques include:
- Review worst negative slack – Check paths with the largest timing violations.
- Scan critical paths – Determine which logic stages are problematic.
- Analyze clock domains – Check timing across asynchronous boundaries.
- Verify timing constraints – Confirm no input errors or missing constraints.
- Examine routing congestion – Look for highly utilized routing resources.
- Check enable timing – Ensure proper handling of block enables.
Detailed timing analysis reveals where to focus optimization efforts. Constraints and reports should also be double-checked for errors.
Iteration and Refinement
With tight timing margins, closure is often achieved by iterative refinement:
- Make changes incrementally – Modify the design in small steps.
- Frequently verify improvements – Resynthesize and re-implement to check timing.
- Attack the worst violations – Focus on the most critical slack first.
- Balance utilization – Watch for overoptimization of one area at the expense of others.
- Validate with simulation – Confirm changes operate correctly.
An iterative approach helps avoid overcorrection and ensures progress towards closure.
When All Else Fails
If timing closure is still unsatisfied after exhaustive efforts, there are still a few options left:
- Reduce clock speed – Limit frequency to improve slack if possible.
- Use a faster FPGA – Move to a higher speed grade or larger device.
- Simplify design – Remove excess logic that isn’t critical.
- Divide processing across FPGAs – Spread design over multiple devices.
- Move processing off FPGA – Shift processing to processor or ASIC.
While not always practical, changing major design parameters can help resolve extremely difficult timing closure issues.
Closure is Worth the Effort
Timing closure requires extra diligence and effort to achieve with Cortex-M0 in low-density FPGAs. However, with careful methodology, analysis, and optimization, it is possible to meet requirements and realize a successful implementation. The closure techniques outlined here encompass a proven approach to overcoming the timing challenges of smaller FPGA architectures. Applying this guidance with patience and persistence will ultimately reward the designer with a high quality, high performance Cortex-M0 system.
