SD Card Access while in USB MSC mode

[holeygaurav]

Posted on June 18, 2014 at 15:01

Dear All,

I am implementing a solution where I have to read file when device is in usb mass storage mode and after reading I have to delete the file. I have implemented USB Mass Storage device based on STM32F105 and I am using emfile - emb OS from segger. When device in MSC, I am stuck at SD_Readbyte flag. My doubts are : 1. Is USB blocking access to SD Card/SPI lock? 2. At what point I should be able to access the SD Card?     - After completion of USB MSC Initialization? Please revert ASAP. Thanking you in anticipation. Best Regards, hgaurav

[Tesla DeLorean]

Posted on June 18, 2014 at 17:06

You have to disconnect the USB, or have it present that the media is ejected, changed, not present, and/or not ready. If you don't do this the contention/coherency issues are practically impossible to overcome, as you don't control enough of the software in the system.

[holeygaurav]

Posted on June 24, 2014 at 09:35 Dear Clive, How it is handled in USB based Card Reader....How is the implementation? Accessing the files through USB... I am bit confused regarding this? Please revert. Thanking you in anticipation. Best Regards, Gaurav Holey

[Tesla DeLorean]

Posted on June 24, 2014 at 16:10

The card reader will basically take the read, write, control functions passed as MSC/SCSI commands, and turn them into card access commands. It does this at a sector/block level.

If you have code on the STM32 that you want to access the card and files on it, you need an access layer/driver, and a file system, like FatFs. What is very difficult to do is allow a PC and STM32 access the card at the same time, you are likely to corrupt the content of the card as you have different views on what is current.