NAND128-A, NAND256-A, NAND512-A, NAND01G-A
Figure 21. Garbage Collection
Old Area
New Area (After GC)
Valid
Page
Invalid
Page
Free
Page
(Erased)
AI07599B
Garbage Collection
Error Correction Code
When a data page needs to be modified, it is faster
to write to the first available page, and the previous
page is marked as invalid. After several updates it
is necessary to remove invalid pages to free some
memory space.
To free this memory space and allow further pro-
gram operations it is recommended to implement
a Garbage Collection algorithm. In a Garbage Col-
lection software the valid pages are copied into a
free area and the block containing the invalid pag-
es is erased (see Figure 21.).
An Error Correction Code (ECC) can be imple-
mented in the Nand Flash memories to identify
and correct errors in the data.
For every 2048 bits in the device it is recommend-
ed to implement 22 bits of ECC (16 bits for line par-
ity plus 6 bits for column parity).
An ECC model is available in VHDL or Verilog.
Contact the nearest ST Microelectronics sales of-
fice for more details.
Figure 22. Error Detection
Wear-leveling Algorithm
For write-intensive applications, it is recommend-
ed to implement a Wear-leveling Algorithm to
monitor and spread the number of write cycles per
block.
New ECC generated
during read
In memories that do not use a Wear-Leveling Algo-
rithm not all blocks get used at the same rate.
Blocks with long-lived data do not endure as many
write cycles as the blocks with frequently-changed
data.
The Wear-leveling Algorithm ensures that equal
use is made of all the available write cycles for
each block. There are two wear-leveling levels:
XOR previous ECC
with new ECC
NO
NO
>1 bit
All results
= zero?
= zero?
YES
YES
■
First Level Wear-leveling, new data is
programmed to the free blocks that have had
the fewest write cycles
Second Level Wear-leveling, long-lived data is
copied to another block so that the original
block can be used for more frequently-
changed data.
22 bit data = 0
11 bit data = 1
1 bit data = 1
ECC Error
■
Correctable
Error
No Error
ai08332
The Second Level Wear-leveling is triggered when
the difference between the maximum and the min-
imum number of write cycles per block reaches a
specific threshold.
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