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.
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?
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?
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
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.
Related
Can get size of disk sector via the Linux API/ABI? It's about the quantum of I/O disk, normally it's equal 512 bytes, but others values can be too (usually multiple 512 bytes).
Also it should not confuse to size of logical block or to size of sector of a file system.
A block device is reflected as file in a file system of an UNIX (/dev/sda, /dev/sr etc.) It means, can open that file and make some manipulations to its content like with content of the corresponded block device.
So specifically the work to a true block device similar the work to a virtual hard disk (the .vhd format for instance).
But i don't know how to get size of sector in general case.
At moment i've single solution: get the maximal CHS address and size of hard drive, both action via BIOS. But i think, it's bad idea, because portability lost
I have been asked to help out on an embedded firmware project where they are trying to mount a file system on an SPI flash chip (Cypress S25FL512S) with a 256KB (Kilo Byte) Erase sector size.
My past experience with file systems is that the file system has a block size upto 4Kbytes which is mapped onto erase sectors of 512bytes to 4Kbytes
The embedded controller is a small NXP device running at 180MHz with 512KBytes of RAM so I cannot even cache an erase sector. I note that the chip family does have pin compatible devices with smaller erase sectors.
My general question is how do you mount a file system with a block/cluster size that is smaller than the flash erase sector size? I've not been able to find any articles addressing this.
You can't do this in any sensible way. Your specification needs to be modified.
Possible solutions are:
Pick a flash/eeprom circuit with smaller erase size.
Pick a flash/eeprom with more memory and multiple segments, so that you can back-up the data in one segment while programming another.
Add a second flash circuit which mirrors the first one, erase one at a time and overwrite with contents of the other.
Pick a MCU with more RAM.
Backup the flash inside MCU flash (very slow and likely defeats the purpose of having external flash to begin with).
I have a project where I have to update the data on disk very frequently in case of power loss. When overwriting exactly 512b (1 sector of my drive) in the file with data of equal length does the file-system mark the sectors that have been changed and update them on disk when ready to flush? Or does it write the whole file every time it flushes a change? I am mainly concerned with ext4 but I am curious if it is the same with every file-system.
If the standard is not to track changes but to overwrite the entire file is there a way to change this? Some write options?
In general with Linux files are cached in the page cache, and whether or not a page is dirty is tracked at the page level. On Intel x86 platforms, the page size is 4k, so if you dirty a 4k page, it is the 4k page which gets written back.
If you want to only overwrite a single 512 byte sector, and you have a HDD that has 512 byte sectors, you can open the file with the O_DIRECT flag and if you issue a 512 byte write, on a file offset which is a multiple of 512 bytes, and where the memory buffer from where you source the write is also 512 byte aligned, then you can bypass the page cache, and the write will go directly to the disk (hence O_DIRECT).
Note that a number of modern disks are really using a 4k physical sector, but they are emulating 512 byte sectors for backwards compatibility reasons. These disks are sometimes called 512e sectors (e for emulated). On these drives, if you do a 512 byte sector write, the disk will do a read-modify-write cycle, since the drive internally can only write 4k at a time. This will be visible to you as a performance hit, but from a functional perspective, it will otherwise look the same as a traditional, old-fashioned 512 sectored HDD.
I am using m25p40 flash memory with jn5148 MCU.In datasheet of this flash , it is written that:
Erase capability:
Sector erase: 512Kb in 0.6 s (TYP)
Bulk erase: 4Mb in 4.5 s (TYP)
I am facing problem in overwriting data stored in one page of sector. So , how can I erase one page, and write new data in that page? Is there any solution to erase one page of sector, without erasing other pages of same sector?
According to the datasheet:
The memory can be programmed 1 to 256 bytes at a time using the PAGE
PROGRAM command. It is organized as 8 sectors, each containing 256
pages. Each page is 256 bytes wide.
Although I don't know if it actually works, and I cannot test it, I also found that someone already did this with an avr µC, which should give you an example function write(address, word) if you don't want to read the page program sequence (datasheet p.27) and write your own.
Here is a sector erase procedure quoted from the documentation on m24p40
The SECTOR ERASE command sets to 1 (FFh) all bits inside the chosen
sector. Before the SECTOR ERASE command can be accepted, a WRITE
ENABLE command must have been executed previously. After the WRITE
ENABLE command has been decoded, the device sets the write enable
latch (WEL) bit. The SECTOR ERASE command is entered by driving chip
select (S#) LOW, followed by the command code, and three address bytes
on serial data input (DQ0). Any address in- side the sector is a valid
address for the SECTOR ERASE command. S# must be driven LOW for the
entire duration of the sequence. S# must be driven HIGH after the
eighth bit of the last address byte has been latched in. Otherwise the
SECTOR ERASE command is not executed. As soon as S# is driven HIGH,
the self-timed SECTOR ERASE cycle is initiated; the cycle's duration
is t SE . While the SECTOR ERASE cycle is in progress, the status
register may be read to check the value of the write in progress (WIP)
bit. The WIP bit is 1 during the self-timed SECTOR ERASE cycle, and is
0 when the cycle is completed. At some unspecified time before the
cycle is completed, the WEL bit is reset. A SECTOR ERASE command is
not executed if it applies to a sector that is hardware or software
protected.
You cannot rewrite one page. You must rewrite one sector at least.
So if you want change a.k.a rewrite at least one byte in any page in choosen sector you can do following:
Read ALL sector to RAM.
Erase this sector.
Change needed data in RAM.
Write back changed data to flash's sector.
YOU MUST READ THIS ARTICLE: Five things you never knew about flash drives
i know the cluster size of a USB flash drive can be changed , can we change the sector size too ??
Sector size isn't a configurable parameter in ATA/SATA/SCSI/etc devices and, from my experience, USB flash drives implement one of these protocols. The sector size is reported by the device itself but, even if you could set it to something other than 512, you would likely run into a latent bug somewhere in a driver or file system package that assumed a sector size of 512.
There are real reasons for using a sector size like 512, for example, addressing of larger sectors can be done more quickly and efficiently (not just in time but in size/space as well). Throughput to these devices is also better with something like 512. Consider that, if you could set sector size to something like 16-bytes, you might have less wasted space with 16-byte sectors compared to having a number of half-full 512-byte sectors, but your throughput to the device would probably be worse. In fact, writing a single 16-byte sector would only be slightly faster than writing a single 512-byte sector. On the other hand, writing 32 16-byte sectors (to write a total of 512 bytes) would likely take longer than writing a single 512 byte sector simply due to the overhead associated with transferring multiple sectors.
I would suggest you buy a larger USB flash drive if you are worried about wasting space with 512-byte sectors.