The Cortex-M1 processor from ARM is a popular choice for FPGA implementations due to its small footprint and low power consumption. However, when implementing the Cortex-M1 in an FPGA, there are some limitations that need to be considered regarding the file system support.
Limited Internal Memory
One of the biggest limitations of the Cortex-M1 is its small internal memory size. The Cortex-M1 only contains 16KB of internal SRAM which must be used for both code and data storage. This leaves very little room for implementing an internal file system for storage. While it is possible to add external SRAM, this increases the cost and footprint of the design.
No Memory Management Unit
The Cortex-M1 does not contain an MMU (memory management unit). This means that all code executes in a single flat address space with no separation between user and privileged modes. An MMU provides vital support for multi-tasking operating systems which allow file system implementations through abstraction of memory spaces.
Limited DMA Support
Direct memory access (DMA) controllers allow for fast data transfers between memory and peripheral devices without involving the CPU. Efficient DMA is important for interfacing with external storage devices to implement a hardware file system. However, the Cortex-M1 only contains a single DMA channel which limits concurrency and throughput for storage operations.
No Cache or Write Buffer
The Cortex-M1 does not contain instruction or data caches nor write buffers. These components are very useful for optimizing file system performance by reducing accesses to slower external storage. All storage requests on the Cortex-M1 go directly out to the bus which adds significant latency for file transactions.
Single-Core Only
The Cortex-M1 is a single core processor with no support for symmetric multiprocessing (SMP). File system requests can monopolize the single CPU core which may stall time critical processes. SMP allows load balancing of storage and application tasks across multiple cores.
No Memory Protection Unit
A memory protection unit (MPU) provides access control mechanisms for memory regions. This prevents tasks from overwriting each other’s data which is important for file system integrity. The lack of an MPU on Cortex-M1 leaves the file system vulnerable to malfunctioning software.
Minimal Peripheral Support
The Cortex-M1 has a basic peripheral set missing many ports useful for storage such as SDIO, QuadSPI, and USB OTG. This limits the ability to connect to external devices or flash memory cards needed for adding file system storage.
Reliance on External Flash
Due to the very limited internal memory, the Cortex-M1 requires external flash memory chips/cards to implement non-volatile file system storage. External flash has higher latencies, uses more power, and increases the PCB footprint compared to internal flash.
Software File Systems Only
Hardware file systems provide better performance by offloading transactions from the main CPU. However, the limited internal memory of the Cortex-M1 does not allow implementing a hardware file system. Only simple software-based file systems can be used which tax the CPU.
No Native File System
The Cortex-M1 does not include any dedicated hardware or OS support for an internal file system. Any file system must be custom designed and ported to the processor which is a complex software undertaking.
Challenges for FAT File System
The FAT (File Allocation Table) file system is commonly used on external flash drives due to its simple design. However, FAT has some inherent challenges when used with the Cortex-M1:
- FAT is not designed for embedded systems with limited resources.
- The FAT table itself consumes a considerable amount of RAM.
- No built-in wear leveling which reduces external flash lifetime.
- File system integrity easily compromised if power lost during writes.
- Slow random read/write performance due to simple lookup tables.
Mitigating File System Limitations
While the Cortex-M1 presents many challenges for building a file system, there are methods to help mitigate the limitations:
- Add external RAM to support larger software file system code and data structures.
- Use an optimized flash file system tailored for microcontrollers such as LittleFS.
- Increase number of DMA channels through a DMA controller peripheral.
- Connect external flash chip/card designed for performance and endurance.
- Reduce number of files and optimize file access patterns to limit fragmentation.
- Use a minimal real-time OS to add basic multi-tasking support.
- Implement software caching techniques to reduce external storage reads.
- Compress file system data to reduce overall storage requirements.
Potential Solutions
Here are some potential solutions to help overcome the file system limitations of the Cortex-M1 FPGA implementation:
SD/MMC Storage Card
An SD/MMC card slot peripheral provides removable storage media for building a FAT32 file system. Cards are inexpensive and widely available. An optimized FAT32 implementation such as Petit Fat FS can reduce RAM usage and improve performance.
External SPI Flash Chip
Fast SPI flash chips offer higher performance and endurance compared to SD cards. A flash translation layer improves wear leveling while providing a basic block device for a file system.
FTL with LittleFS
A flash translation layer manages external SPI flash while presenting a block device to LittleFS, an optimized flash file system designed for microcontrollers. LittleFS consumes less RAM and provides better power-fail resilience than FAT32.
External RAM Disk
Add a parallel RAM chip and implement a software RAM disk for temporary storage. The RAM disk bypasses the need for slow external flash access while benefiting from a standard file system interface.
MTP Responder
Media Transfer Protocol (MTP) allows accessing the Cortex-M1 file system from a host computer. This avoids needing to implement a full USB mass storage stack on the limited Cortex-M1 resources.
RTOS File System
A real-time OS like FreeRTOS adds task scheduling and synchronization. This support enables multi-threaded access to a shared file system without corruption.
Conclusion
While the Cortex-M1 has limited memory and peripheral resources, with careful design and optimization it is still possible to implement basic file system support on an FPGA. Using solutions like external storage media, flash file systems tailored for microcontrollers, and real-time OS support allows building file management capabilities despite the constraints of the Cortex-M1 architecture.
