Introduction to Storage System

    The MaixPy storage system is roughly as shown in the figure below:

    As can be seen from the above figure, the storage medium in MaixPy is mainly composed of Flash and SD cards, which are divided into three areas, namely MaixPy.bin firmware area, xxx.kmodel model area, and file system area: Flash is spiffs (SPI Flash File System), SD card is Fatfs (FAT file system).

    MaixPy.bin firmware area

    It is used to store MaixPy.bin firmware, starting at 0x000000, because K210 will start to run the program from 0x000000.

    xxx.kmodel model area

    It usually starts at 0x300000. The reason why the model file is not burned in the file system of Flash (the file system will be explained later) is due to the following reasons:

    1. The memory of the file system in Flash is not large enough to fit in a large model, and a larger model can be placed in an SD card.
    2. Reading model files directly is faster than reading through the file system.

    There is no file system management in this area. You need to operate files based on the starting address during programming. For example, the reading method when the model is programmed at 0x500000:

    KpuTask = kpu.load(0x500000)
    

    File system area

    Usually starting from 0xD00000, this area is managed by the file system. We reserve the space 3MiB at the end of Flash, which will be managed by spiffs, and Also supports FAT32 (Fatfs) SD card. These file systems provide interfaces so that we can read and write files through file name instead of using file start address as in the model area. At the same time, it can also help us effectively manage storage media. For example, wear leveling (Flash has a wear life, please search for relevant knowledge) can give full play to the life of Flash.

    Use of MaixPy File System

    Because the file systems of Flash and SD cards are different and the interfaces are inconsistent, different file systems need to call different interfaces. At this time, the virtual file system (VFS) in MaixPy is used to solve this problem, VFSYou can mount multiple file systems of different types, and provide a unified interface for users to operate these file systems, and users can ignore the differences between different file systems when using these interfaces. These interfaces are implemented in the os module. Examples of usage are as follows:

    import uos
    
    print("files:", uos.listdir("/flash"))
    
    with open("/flash/test.txt", "w") as f:
        f.write("hello text")
    
    print("files:", uos.listdir("/flash"))
    
    with open("/flash/test.txt", "r") as f:
        content = f.read()
    
    print("read:", content)
    

    In the above example, the spiffs file system is automatically mounted to the /flash directory when booting. The user only needs to pass in the "/flash" directory name as a parameter when using the os interface to access the file system .

    Interpretation:

    • Import the uos module
    • List all files in the /flash directory
    • Write a file named test.txt to the /flash directory with the content of hello text,
    • List all files in the /flash directory, you will find the existence of test.txt
    • Read the content of the file into the content variable
    • Print the content variable and output hello text, which is the content of the file just written
    • This content is powered on after the development board is powered off, the correct content can still be read (the content will not be lost after the Flash is powered off)

    Of course, SD card is also supported. If you need to use it, SD card needs to meet the following points:

    • Support SPI mode, most genuine cards on the market support
    • The partition is MBR (msdos)
    • Format as FAT32
    • The size tested is the largest 128GiB available

    sAfter inserting the SD after power off, the SD card will be mounted on /sd if it is powered on. If there are multiple partitions, the second partition name is /sd2

    Note that / (root directory) cannot write data, only write data to /flash or /sd
    If there is a SD card at boot, it will automatically switch the current directory to /sd, if not, it will automatically switch to /flash