2.5 KiB
Executable File
NAND Flash Basic
Note: What follows is a high-level description of the generic behaviour of flash. There are thousands of different NAND chips available, each potentially with slightly different instruction sets, block/page sizes, performance characteristics etc.
- The package is the memory chip, i.e. the black rectangle with little electrical connectors sticking out of it. If you look at an SSD, a flash card or the internals of a flash array you will see many flash packages, each of which is produced by one of the big flash manufacturers: Toshiba, Samsung, Micron, Intel, SanDisk, SK Hynix. These are the only companies with the multi-billion dollar fabrication plants necessary to make NAND flash.
- Each package contains one or more dies (for example one, two, or four). The die is the smallest unit that can independently execute commands or report status.
- Each die contains one or more planes (usually one or two). Identical, concurrent operations can take place on each plane, although with some restrictions.
- Each plane contains a number of blocks, which are the smallest unit that can be erased. Remember that, it’s really important.
- Each block contains a number of pages, which are the smallest unit that can be programmed (i.e. written to).
The important bit here is that program operations (i.e. writes) take place to a page, which might typically be 8-16KB in size, while erase operations take place to a block, which might be 4-8MB in size. Since a block needs to be erased before it can be programmed again (*sort of, I’m generalising to make this easier), all of the pages in a block need to be candidates for erasure before this can happen.
MTD Overview
MTD subsystem (stands for Memory Technology Devices) provides an abstraction layer for raw flash devices. It makes it possible to use the same API when working with different flash types and technologies, e.g. NAND, OneNAND, NOR, AG-AND, ECC'd NOR, etc.
**(Note: **
- **MTD is neither a block nor a char device. **
- **Ext2 ,ext3 and FAT are file systems work with block device not for MTD devices, MTD device work with jffs2 , yaffs2 or squashFS . **
- **the /dev/mtdblockX device nodes are simple Flash Translation Layers (FTLs) over the MTD devices.) **
- MTD subsystem does not deal with block devices like MMC, eMMC, SD, CompactFlash, etc