The principle of Flash programming is that only 0s can be written from 1s, but 1s cannot be rewritten as 0s. Therefore, before writing to a Flash memory, the corresponding block must first be erased. This erasing process sets all bits in the block to 1, resulting in all bytes being 0xFF. As a result, programming involves setting specific bits to 0, while erasing involves resetting them to 1—these two operations are essentially opposites.
(1) The basic units for reading and writing in Flash memory differ between NorFlash and NandFlash. NorFlash operates on "words" as its basic unit, often divided into logical blocks of 128 KB or 64 KB. When accessing data, both the block number and the offset within the block must be specified. In contrast, NandFlash is organized in "blocks," typically 8 KB in size, with each block further divided into pages (usually 512 bytes). To modify a single byte in NandFlash, the entire block must be rewritten, making it less flexible for small updates.
(2) NorFlash is ideal for applications with small data volumes due to its random access capability, while NandFlash is better suited for large-scale storage due to its sequential nature.
(3) NorFlash has separate address and data lines, allowing it to be connected like SDRAM. This makes it easy to execute code directly from the chip (XIP), which is why it's commonly used as a boot device in embedded systems. NandFlash, however, shares address and data lines and requires additional control signals, making it more complex to use as a boot device without a controller.
(4) NandFlash does not support byte- or block-level erasure due to shared buses. Instead, fixed-size areas must be erased. NorFlash allows word-level erasure, making it faster for small I/O operations. For example, a NorFlash may take 10 µs per word, while NandFlash takes about 234 µs for 512 bytes, including search and erase times.
(5) NandFlash offers larger capacities, up to 8 GB, and uses a two-level block/page structure. Each page includes an out-of-band (OOB) area for error correction. Because of this, NandFlash is generally faster than NorFlash when handling large data transfers.
(6) Reliability differs significantly between the two. NorFlash has a simpler interface, fewer bit swaps, and fewer bad blocks, making it more reliable for critical applications. NandFlash, with its complex interface and higher likelihood of errors, requires error detection and correction algorithms (EDC/ECC) to maintain data integrity.
(7) Since NandFlash shares address and data lines, it can impact read/write speed. NorFlash, with separate lines, generally performs faster in read/write operations.
Commonality Between NAND and NOR Flash
Both types require erasing before writing, though NorFlash erases one word at a time, while NandFlash erases entire blocks. Both have limited write cycles, and near the end of their lifespan, writes may fail. NAND typically has a higher number of rewrites than NOR, but because a single failed page invalidates an entire block, its overall lifespan may be shorter.
Additionally, flash memory operations are not just physical; they require algorithms to manage data. These are usually implemented in the MTD or FTL layers of the system. NAND is better suited for complex file systems, though it demands more sophisticated management.
Interface Comparison
NorFlash uses a standard SRAM interface, making it easy to connect to a CPU’s address and data bus. It supports XIP, allowing code to run directly from the flash. NandFlash, on the other hand, uses a serial I/O interface and requires a controller. Without special CPU support, running code directly from NandFlash is difficult, often requiring a small NorFlash for initial boot.
Capacity and Cost
NorFlash has smaller capacities, typically 1–16 MB, and is more expensive. NandFlash offers much larger capacities, up to 8 GB, and is cheaper due to a simpler manufacturing process. For example, a 4 MB NorFlash might cost around $20, while a 128 MB NandFlash could cost around $30.
Bad Blocks
NandFlash devices have randomly distributed bad blocks, requiring initial scanning and marking. NorFlash, on the other hand, rarely has bad blocks.
Bit Flip Issues
NandFlash is more prone to bit flips, which can be critical for important files. EDC/ECC algorithms are recommended to mitigate this risk.
Upgrade Flexibility
Upgrading NorFlash can be challenging due to varying address line requirements, whereas NandFlash upgrades are easier due to standardized interfaces.
Read and Write Performance
NandFlash erases faster than NorFlash, with erase times as low as 4 ms compared to 5 seconds for NorFlash. Read speeds are slightly faster on NorFlash, but NandFlash excels in bulk data transfers.
File System Support
Linux uses MTD to manage Flash chips. Cramfs is read-only, while JFFS and YAFFS are used for read/write operations. YAFFS2 supports large pages in NandFlash, making it suitable for modern devices.
In summary, NorFlash is ideal for fast, reliable execution and small data, while NandFlash is better for large storage and cost-effective solutions.
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