>> Due to the multiplexing scheme used in high-density NAND flash devices,
>> even the non-programmed cells are exposed to a fraction of the programming
>> voltage and there are very low limits on the number of write cycles to
>> a page before it has to be erased again. Exceeding that can cause some
>> unwanted bits to change from 1 to 0. Typically, however, it is enough
>> to write each 512-byte portion of a page independently.
> Well, I'm not sure. The Toshiba and Samsung NANDs I've read manuals for
> seem to limit number of writes to a single page before block erase, --
> is 512-byte portion some implementation detail I'm not aware of?
No. I just meant that I generally see "you may program each 2K page a
maximum of 4 times before performing an erase cycle", and I assume the
spec came from 2048/512 = 4, so you can program each 512-byte sector
separately. I would assume if the page size were changed again, they'd
try to keep that property.
E.g. from the Samsung K9K8G08U1A/K9F4G08U0A data sheet (p. 34):
PAGE PROGRAM
The device is programmed basically on a page basis, but it does
allow multiple partial page programming of a word or consecutive
bytes up to 2,112, in a single page program cycle. The number
of consecutive partial page programming operations within the
same page without an intervening erase operation must not exceed
4 times for a single page. The addressing should be done in
sequential order in a block.
[...]
The internal write verify detects only errors for "1"s that are
not successfully programmed to "0"s."
>> Now, erasing is done in larger units called BLOCKs. This is more
>> variable, but a power of two multiple of the page size. 32 to 64 pages
>> (16 k for small page/32-page blocks to 128K for large page with 64-page
>> blocks) is a typical quantity. You can only erase a block at a time.
> Typical? Are you aware of a "large page" NAND FLASH with different
> number of pages per block? It's not just curiosity, it's indeed
> important for me to know if there are CF cards in the market with
> physical block size != 128K.
No, I'm not aware of any violations of that rule; I just hadn't looked
hard enough to verify that it was a rule, but I had seen the device ID
bits that allow a wide range to be specified.
The "more variable" statement is really based on NOR flash experience,
where it truly does vary all over the map.
> [... skip interesting and nicely put details of NAND technology ...]
Hopefully this makes descriptions like the start of the Samsung data
sheet more comprehensible:
Product Information
The K9F4G08U0A is a 4,224 Mbit (4,429,185,024 bit) memory
organized as 262,144 rows (pages) by 1,112x8 columns. Spare
64x8 columns are located from column address of 2,048-2,111.
A 2,112-byte data register is connected to memory cell arrays
accomodating data transfer between the I/O buffers and memory
during page read and page program operations. The memory array
is made up of 32 cells that are serially connected to form a
NAND structure. Each of the 32 cells resides in a different page.
A block consists of two NAND structured strings. A NAND structure
consists of 32 cells. Total 1,081,244 NAND cells reside in
a block. The program and read operations are executed on a page
basis, while the erase operation is executed on a block basis.
The memory array consists of 4,086 separately erasable 128K-byte
blocks. It indicates that the bit-by-bit erase operation is
prohibited on the K9F4G08U0A.
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