Implementing the Cortex-M1 CPU on Artix-7 and Spartan-7 FPGAs can be challenging, but following some key tips and best practices can make the process smoother. Here are some recommendations for successfully integrating Cortex-M1 into your Artix-7 or Spartan-7 design:
Selecting the Right IP Core
Xilinx offers Cortex-M1 IP cores optimized for 7-series FPGAs like Artix-7 and Spartan-7. Choose the right core for your design needs – there are options for different memory interfaces, peripherals, and optimization features. Pay close attention to the IP core datasheet specifications.
Clocking the Design
Use the Xilinx clocking wizard to properly generate the clocks needed for Cortex-M1 and the FPGA fabric. Cortex-M1 requires a fast clock (typically 166MHz or higher) while the FPGA fabric may run at a slower speed. Managing clock domains is key. Use asynchronous FIFOs when crossing between clock domains.
Cortex-M1 supports various memory types like SRAM, SDRAM, and Flash. Select the right IP core for your memory interface and pay close attention to timing closure. Use FPGA memory controllers and PHYs as needed. Size memory appropriately for your application – Cortex-M1 has a 4GB address space.
Attach appropriate peripherals to the Cortex-M1 AHB/APB buses to implement your desired functionality. UART, SPI, I2C, Ethernet, USB are commonly used. Use IP cores from Xilinx or third-parties. Write custom logic as necessary. Follow ARM AMBA specifications for interfacing.
Interrupts and Exceptions
Properly handle interrupts and exceptions from peripherals and errors. Map interrupt sources to the NVIC controller. Write interrupt handlers in C/assembly. Set appropriate priority levels. Use interrupt coalescing to avoid overload. Mask/unmask interrupts appropriately.
Build in debugging capability early via JTAG, CoreSight, or other ARM standard debug components. Use Xilinx debug IP cores. Include trace buffers for runtime debugging. Leave debug ports accessible and reserve FPGA pins. Use ARM Development Studio, DS-5, or other advanced debug tools.
Perform extensive functional simulation of your Cortex-M1 system before hardware implementation. Model entire system in C/C++ or SystemC at the behavioral level. Simulate peripherals, interrupts, memory traffic, and application code. Verify key interfaces and functionality via testbenches.
Board and Pin Planning
Plan out your PCB and FPGA pin connections early. Allocate pins for core IP interfaces, peripherals, memory buses, debug ports, and more. Follow Xilinx pinout recommendations. Design board for timing closure – use proper termination, trace lengths, layer stacking.
Timing closure is critical and difficult with Cortex-M1 on Artix-7/Spartan-7. Follow Xilinx guidelines and tools like PlanAhead. Apply timing constraints carefully. Use FPGA primitives where helpful. Pipeline slow interfaces. Reduce logic levels. Upsize critical paths. Iterate until design meets timing.
Analyze power early using Xilinx Power Estimator. Budget power for core, memory, and peripherals. Use low-power techniques like clock gating, FPGA primitives, disabling blocks. Provide adequate board-level power supplies and regulation. Follow Xilinx guidelines for power-optimized design.
Safety and Security
Incorporate safety and security features as dictated by the application. Use lockstep cores for reliability. Encrypt sensitive data paths. Authenticate firmware updates. Disable JTAG or limit access. Follow best practices for safe and secure Cortex-M1 deployment.
Leverage a robust design flow from modeling to deployment. Use Xilinx SDSoC or Vitis for high-level synthesis. Validate with simulation. Use Xilinx toolchain for place and route. Optimize in hardware and software. Use revision control. Perform code reviews. Follow structured design practices.
Develop firmware in C and/or assembly targeting Cortex-M1. Use drivers for IP cores and peripherals. Implement task scheduling and RTOS if needed. Validate code via simulation and debugging. Profile software performance. Follow embedded software best practices for robustness, maintainability and reusability.
Optimize for maximum performance. Identify hot spots using profiling. Optimize slow software routines. Pipeline interfaces and add FIFO buffers. Tune compiler settings to balance size and speed. Use DSP48s and BRAMs judiciously. Take advantage of custom instructions. Iteratively improve though analysis.
Employ multiple verification strategies. Validate IP integration via testbenches. Perform HDL simulation of full system. Develop test cases to cover corner cases. Stress test design with random stimuli. Prototype on development board. Follow rigorous acceptance testing procedures before sign off.