Atmel AT91SAM9, Примечание к применению

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3
6255B–ATARM–26-Jun-09
Application Note
NAND Flash typically contains blocks that contain errors and cannot be used. A check must be
done by software to list and maintain a table of bad blocks. Data integrity is achieved by using
hardware or software techniques, such as ECC, that check and correct bad data.
2.2.3 Performances
Further differences between NOR and NAND Flash can be found in read/write performances.
Table 2-1 shows random access time for NOR Flash specified at 0.09 µs, whereas NAND ran-
dom access is significantly slower — 25 µs — for the first byte. Once the initial access has been
made, however, the remaining 2,111 bytes are shifted out of NAND at only 30 ns per byte. This
results in a bandwidth of more than 23 Mb/s for 8-bit I/Os or 37 Mb/s for 16-bit I/Os.
The real benefits for NAND Flash can be found in the faster program and erase times, since
NAND provides over five megabytes per second of sustained write performance. The block
erase times are an impressive 2 ms for NAND versus 200 ms for NOR.
2.2.4 Conclusion
Table 2-2 summarizes NAND/NOR advantages and disadvantages.
Clearly, NAND Flash has several significant positive attributes. The one negative attribute is that
it is not well-suited for direct random access.
Table 2-1. Differences in Performance
Characteristics
NAND Flash
K9F2G08U0M NOR Flash
AT49BV16x4-90
Random access read
25 µs (first byte)
30 ns each for remaining 2111
bytes
0.09 µs
Sustained read speed
(sector basis)
37 Mbytes/s 11 Mbytes/s
Random write speed 300 µs/2,112 bytes 20 µs / bytes
Sustained write speed (sector
basis)
5 Mbytes/s 0.05 Mbytes/s
Erase block size 128 Kbytes 64 Kbytes
Erase cycles 100,000 to 1,000,000 10,000 to 100,000
Erase time per block 2 ms 200 ms
Table 2-2. NAND/NOR Comparison
NAND NOR
Advantages
Fast writes
Fast erases
Lower bit cost
Higher density
Random access
Byte writes possible
Disadvantages
Slow random access
Byte writes difficult
Bad blocks management and ECC
required
Slow writes
Slow erase
Applications
File (disk) applications
Voice, data, video recorder
Any large sequential data
Execute directly from non volatile
memory