The Cortex-M1 processor from ARM is a powerful 32-bit chip well-suited for embedded applications. However, it lacks support for filesystems and OS capabilities out of the box. This presents challenges for implementing file I/O, as traditional approaches rely on having an OS and filesystem available. Despite this limitation, with careful planning it is possible to read and write files on a Cortex-M1 in a bare metal environment.
Overview of the Cortex-M1 Architecture
To implement file I/O on the Cortex-M1, it is important to first understand some key aspects of its architecture:
- 32-bit ARMv6-M architecture optimized for microcontroller applications
- Built-in memory protection unit for protecting code and data
- Advanced peripheral bus architecture for connecting to on-chip and external peripherals
- Fast interrupt handling and low-latency interrupts
- Debug capabilities like breakpoints, watchpoints, and Embedded Trace Macrocell (ETM)
There are some critical limitations to note as well:
- No support for virtual memory or memory management unit (MMU)
- No filesystem built into the chip
- Typically runs bare metal without an OS
Understanding these architectural details will inform some of the design decisions needed to implement file I/O.
Planning for File I/O Capabilities
With no filesystem available, we’ll need to implement the necessary support directly in our application. Here are some considerations for planning file I/O capabilities:
- Storage medium – Most likely an SD card or external flash memory connected via SPI or I2C.
- File formats – Simple proprietary formats may be easiest to parse and interpret directly.
- Buffering – Use RAM for temporary storage/caching to reduce slow external memory access.
- Directories – May need to manage file metadata manually without filesystem support.
- Error handling – Robust error checking is a must when working directly with raw storage.
- Concurrency – File accesses will need to be properly locked/synchronized.
By thinking through these elements, we can design custom drivers and interfaces optimized for our use case.
Implementing Low-Level Storage Access
The first step is to add basic support for reading and writing raw data to our external storage medium. This will form the foundation for implementing file I/O capabilities on top.
For example, with an SD card connected via SPI:
- Write an SPI driver to initialize the bus and card into its native 4-bit mode.
- Implement SD protocol commands like READ_SINGLE_BLOCK and WRITE_SINGLE_BLOCK.
- Manage block addressing based on card capacity.
- Include CRC generation/validation for robust error detection.
This provides low-level access for reading and writing 512 byte blocks of the SD card. We can build on this to support general file I/O operations.
Designing a File Interface
With raw storage access in place, we can now design a simple file interface for our application needs:
- Open – Initialize file metadata and prepare for read/write.
- Close – Flush any cached data and free metadata.
- Read – Buffer and return requested number of bytes.
- Write – Buffer provided data and write to storage medium.
- Seek – Update current position within file.
- Tell – Return current position within file.
This provides a basic set of POSIX-like file handling capabilities. Additional functions like rename, delete, directory listing, etc. can be added as needed.
For simplicity, we’ll store file metadata including name, size, location, etc. in RAM rather than on the storage medium itself. This avoids complex and failure-prone filesystem structures.
Managing File Storage
Without an underlying filesystem, we’ll need to manually manage how file data is physically arranged and stored on our external memory.
A simple approach is to:
- Reserve space at the beginning of the medium for file metadata.
- Append new file data sequentially, maintaining file offsets.
- Load metadata into RAM when a file is opened.
- Update metadata on close to persist current state.
More advanced techniques like a log-structured filesystem can further optimize write performance and fragmentation.
Caching and buffering file contents in RAM is also essential for good performance. The Cortex-M1 has fast single-cycle access to tightly-coupled embedded SRAM that we can take advantage of.
Error Handling and Robustness
When working directly at the raw block level, careful error handling is needed for robustness. Some techniques include:
- Use CRCs to detect and handle corrupted blocks.
- Implement retries and back off for intermittent errors.
- Gracefully handle removed/unplugged storage devices.
- Safely flush cached data on power loss or resets.
- Handle failed or inconsistent writes that may corrupt file metadata.
By thoroughly testing error conditions and adding redundancy, we can build confidence in our file I/O implementation.
Example Code
Here is some example C code showing a simple function to append data to a file:
#define BLOCK_SIZE 512
/* File metadata structure stored in RAM */
typedef struct {
char name[32];
int size;
int location;
} File;
/* Append data to file, returning bytes written */
int file_append(File* file, char* buffer, int numBytes) {
// Calculate number of blocks needed
int numBlocks = (numBytes + BLOCK_SIZE - 1) / BLOCK_SIZE;
// Seek to end of file
file->location = file->size / BLOCK_SIZE;
// Write each block
for (int i = 0; i < numBlocks; i++) {
sd_write_block(file->location++, buffer + i*BLOCK_SIZE);
}
// Update file size
file->size += numBytes;
return numBytes;
}
This shows how we can leverage the low-level sd_write_block() function to append new data, manage file metadata, and handle block-level access.
Conclusion
Implementing file I/O on the Cortex-M1 without the help of an OS and filesystem is certainly a challenge. However, by carefully handling low-level storage access, designing a simple file interface, managing file metadata manually, implementing robust error handling, and leveraging the Cortex-M1’s strengths like fast SRAM access, it is possible to build a solution that meets the needs of embedded applications.
The end result is a highly optimized file I/O implementation tailored specifically for the target use case, with flexibility not always possible when confined to a filesystem’s constraints. With some strategic planning and clever engineering, even the most resource constrained microcontrollers can provide general file handling capabilities.
