The Cortex-M0 DesignStart core is a low-power ARM Cortex-M0 CPU that is optimized for implementation in FPGAs. When targeting an FPGA, it is important to understand how efficiently the Cortex-M0 utilizes the available FPGA resources. This determines how much logic and memory remains available for the rest of the system design.
Logic Utilization
The Cortex-M0 core is very lightweight and requires minimal logic resources. In a typical Artix-7 FPGA using the default configuration, it requires around 300 LUTs and 150 FFs. This corresponds to just 1-2% of the total resources in a mid-size Artix-7 device. Even smaller FPGAs like the Spartan-6 are able to easily accommodate a Cortex-M0 core with room to spare.
The specific LUT and FF utilization can be reduced even further through configuration options. For example, disabling the MPU reduces logic usage by around 50 LUTs. The trace debugging capability adds around 20 LUTs and 80 FFs. So the core can be trimmed down if logic resources are very tight in the target FPGA.
In terms of DSP blocks, the Cortex-M0 does not use any DSP resources. And its modest block RAM usage is covered next.
Memory Utilization
The on-chip memory utilization of the Cortex-M0 DesignStart core includes:
- 32 KB instruction RAM
- 8 KB tightly-coupled data memory (DTCM RAM)
- Up to 64 KB additional data RAM (ATCM)
The instruction and DTCM memories consume block RAM resources within the FPGA fabric. Each RAM block is typically 4-9 kb in size, depending on the specific FPGA. So the 32 KB instruction RAM uses 4-8 block RAMs, while the 8 KB DTCM uses 1 block RAM.
The ATCM memory is implemented using logic cells, so it does not consume RAM blocks. Up to 64 KB ATCM can be added for frequently accessed data without using any additional block RAM.
In total, the on-chip memory utilizes around 5-9 block RAMs. In a mid-size Artix-7 FPGA with 265 block RAMs, this corresponds to just 2-3% of the total RAM resources.
Clock Utilization
The Cortex-M0 DesignStart core requires a single clock input for the system clock. This clock drives the core logic as well as the instruction and data RAMs. A typical clock frequency is 50-100 MHz.
No phase-locked loops (PLLs) are consumed since the core can run directly on a global FPGA clock. The one required clock represents minimal utilization of the clocking resources inside most FPGAs.
FPGA Resource Utilization Examples
Here are some examples of resource utilization for the Cortex-M0 in specific FPGA families:
- Xilinx Artix-7 100T FPGA:
- Logic cells – 300 LUTs, 150 FFs (1% of total)
- Block RAM – 5 (2% of total)
- Intel Cyclone V SE 5CSEBA6U23I7:
- Logic elements – 470 LEs (2% of total)
- RAM blocks – 8 M9Ks (3% of total)
- Lattice ECP5 LIFCL4096-7TN144C:
- LUTs – 463 (2% of total)
- Block RAM – 8 (3% of total)
For each FPGA, only a very small fraction of the total logic, memory, and clocking resources are consumed by the Cortex-M0 core. This leaves ample room for additional custom logic and peripherals.
Resource Utilization Summary
In summary, here are the key points on Cortex-M0 resource utilization in FPGAs:
- Requires only 300-500 LUTs (1-2% of typical FPGA logic)
- Requires 5-9 block RAMs (2-3% of typical FPGA RAM)
- Does not utilize any DSP blocks
- Requires just a single clock source, no PLLs
Due to its optimized, lightweight microarchitecture, the Cortex-M0 DesignStart FPGA logic footprint is very small. This makes it an ideal CPU choice when you need to add a processor to an FPGA that is already densely populated with user logic, IP cores, and other system components.
The low resource utilization leaves ample FPGA resources available for custom peripherals, accelerators, memory interfaces, and any other additional logic in your embedded system design.
Optimizing Resource Utilization
If you need to squeeze the Cortex-M0 into an even smaller FPGA, there are a few techniques to further optimize its resource utilization:
- Minimize or disable trace logic which consumes extra LUTs and FFs
- Reduce DTCM RAM size from 8 KB down to 4 KB or 2 KB
- Lower system clock frequency to reduce routing congestion
- Use FPGA-specific SDC constraints to help with packing and placement
By tuning the core configuration and optimizing the FPGA implementation, you can often reduce logic utilization by 15-25% versus the default Cortex-M0 configuration. This helps when targeting small, cost-sensitive FPGAs.
Utilization Comparison with Other Cores
Compared to other CPU cores, the Cortex-M0 DesignStart has one of the smallest logic footprints. For example:
- Cortex-M1 is around 50% larger than Cortex-M0 in LUTs and FFs
- Cortex-R5 is 5-10X larger than Cortex-M0
- MicroBlaze has 50-100% higher logic utilization
- Nios II economy core is 60-90% higher logic usage
So when evaluating processor options strictly on the basis of minimal FPGA resource utilization, the Cortex-M0 stands out as the most compact ARM CPU core.
Utilization in ASIC Implementations
The efficiency advantages of the Cortex-M0 also extend to ASIC implementations. Compared to an ASIC optimized custom CPU core, the Cortex-M0 consumes around 20-30% more logic gates.
However, it also brings advanced features and extensive ARM toolchain support which would take many engineering years to develop with a custom design.
For low-cost ASIC applications, the Cortex-M0 strikes a good balance between minimal logic utilization and design time/cost savings versus designing an ASIC CPU core from scratch.
Leveraging Small Resource Utilization
The modest resource requirements of the Cortex-M0 core enable several useful applications:
- Adding processing capabilities to existing FPGA designs
- Implementing multiple CPU cores in parallel for higher performance
- Prototyping larger Cortex-M3 or Cortex-M4 designs
- Cost-sensitive designs targeting small FPGAs
You can take advantage of the low logic, RAM, and clock utilization to craft innovative FPGA-based designs with capabilities that previously required an external processor or higher-cost FPGA.
Conclusion
The Cortex-M0 DesignStart core provides a ARM Cortex-M0 CPU with minimal utilization of FPGA resources. It requires only around 300 LUTs, 150 FFs, and 5 block RAMs in a typical implementation. Such a small footprint makes it feasible to add high-performance microcontroller capabilities to FPGA designs without consuming many valuable logic, memory, and clock resources.
Given its optimized design for low-cost FPGAs, the Cortex-M0 FPGA logic utilization is very competitive even against custom logic implementations. For designers looking to add an ARM CPU to their system on a budget, the Cortex-M0 is an excellent choice to consider.
