FATFS porting on STM32F103 SPI Flash

1
votes

I have ported FATFS for Free RTOS on STM32F103 SPI Flash of 32 Mbit. In a demo Application I have successfully created a file, written a file, and read back from the file. My requirement is like I have to store multiple files (images) in SPI flash and read it back when required.

I have the following conditions/queries.

  1. I have set the Sector Size to 512 bytes, and block erase size for SPI flash is 4K. As in the SPI Flash, block needs to be erased before written. Do I need to keep track on whether a particular block is erased or not or its the file System who is managing this?

  2. How can I verify that the sector, which I am writing in erased or not? What I am currently doing is, Erase the Complete Block for the sector, which I am going to write?

  3. How can I make sure, The Block for SPI flash I am going to erase will not affect any Sector containing useful data?

Thanking in an Anticipation,

Regards,

AK

1
filesystemsstm32spifreertos

1 Answers

0
votes

The simplest solution is to define the "cluster" size to 4K, the same as the page size of your flash. That mean each file, even if only 1 byte, takes 4K, which is 8 consecutive sectors of 512 bytes each. As soon as you need to reserve one more cluster, when the file grow above 4096 bytes, you pick a free cluster, chain it to the FAT, and write the next byte.

For performance reason and to increase the durability of the flash, you should avoid to erase of flash sector when not needed. It is many order of magnitude faster to read then erasing. So, as you select a free cluster, you can start a loop to read each of the 8 sectors. As soon as you find even a single byte not equal to 0xFF, then you abort the loop and call the flash erase for that sector.

A further optimization is possible if the flash controller is able to perform the blank test directly. Such test could be done in a few microsecond while reading 8 sectors and looping to check each of the 4096 bytes is probably slower.