As 1549 does not have external flash(LPC1768 had 2MB external flash) and only had on-chip flash, which should be 256KB from the data sheet, why it showed 512KB in the driver on PC? does 1549 have external flash as well?

It might simply be a small error in the config data of the interface chip's code, which causes it to show up as too large drive. (There is nothing stopping you from showing your mbed as 1TB drive, windows will happily write to it, and actually show the data is their. Of course when you then replug it there is no actual data on it).

I wouldn't worry about it if it works fine.

Hi Erik, I did the test and I could copy 256k bytes file into the mBed disk successfully and failed to copy a 285k bytes files into mBed disk. So you are right.

Here I have a little confusion. Usually the internal flash is a nor flash to keep the image file. We can use JTAG or other interface to flash the image file into the internal flash, then reset the board and the new image will be executed on the nor flash directly. However with mBed disk, it seems the interfnal Nor flash is exposed as mBed disk totally. After we copy the bin file into mBed disk, it seems MCU load the bin into other place. Even we repower board the bin file still can be exectued. And the mBed disk is still there without the bin file.

From my understanding, there is only one internal 256k bytes flash, which has been exposed to mBed disk totally. Then where is the bin file loded by MCU?

Thanks.

What you see isn't what is on the flash of the target, it is what the interface chip wants you to see. For example the mbed.htm file which should be on it by default, also doesn't recide on the target, but on the interface chip. When you upload a new file it will check if it is a .bin file, and if that is the case it will simply program the target block per block: It receives for example 512 bytes from your PC, it programs that into the target, and then tells your PC he can send the next block.

A slightly similar example I made myself. This is based on the standard mbed USB-MSD device code within USBDevice:

int ConfigDisk::disk_write(const uint8_t * data, uint64_t block) {
    if (block == 4)
        parseFile(data);
    return 0;
}

int ConfigDisk::disk_read(uint8_t * data, uint64_t block) {
    if (block < 4)
        memcpy(data, &disk[block*512], 512);
    else if (block == 4)
        createFile(data);
    else 
        memset(data, 0, 512);
    return 0;
}

I made here a very simply file system of only 5 sectors. If the PC wants to write to any except the last sector, it will simply return 0: Which means the PC thinks it has succesfully written data to it, but infact it was completely ignored. If it is to the last sector it parses the data in that package.

Similar if the PC wants to read from the first sectors, it copies data from a read-only array, unless it is the last block, then it dynamically creates a file which is supposed to be in that sector. (Actually it thinks there are a bit more than 5 sectors, and it returns zero for all others).

It is very clear and I understand. Thanks you very much. :-)