Differences between NAND and NOR flash

Most flash devices supported by the eCos Flash library are categorised as NOR flash. These are fundamentally different from NAND flash devices, both in terms of the storage cells deep within the chip, and how they are addressed and used by applications.

Attribute NOR NAND
Addressing of dataBy byte address within the device. Usually expressed as memory-mapped addresses.By row (page) number. Pages are a power of two; commonly 512 or 2048 bytes. Optimised for reading and writing a page at a time. Sometimes supports column (byte) addressing, but this library does not expose such functionality.
Are direct reads and writes possible? [a]UsuallyNot in general, though a few special-case exceptions exist such as OneNAND devices.
Erase block sizeMay vary across the chipA fixed number of pages, typically 64
Out-of-band data Not supported A small number of bytes per page - typically 16 "spare" per 512 "data" bytes - are usable by the application. They are read, written and erased at the same time as the "real" page data.
May factory-bad regions [b] exist on the chip?No Typically up to 20 eraseblocks are marked as factory-bad in their OOB area. The OS is expected to scan these to create a Bad Block Table. [c]
May data be rewritten without being erased first?Usually (but only by resetting 1-bits to 0) Usually, on SLC NAND chips; not on MLC chips.
Error detection and correctionNot presentUsually automatic. Typically this involves an Error Correcting Code, automatically calculated and stored in the OOB area, then checked on read.
a. In other words, can the application read flash directly as if it was RAM, or does it have to invoke the driver to copy data in and out?
b. Regions which were found during manufacture to be bad and marked in some way - usually by placing a special code in the Out Of Band area.
c. Once a BBT exists it can then be used to keep track of any blocks which fail through wear during the lifetime of the device.

Since a NAND chip can in general only be read indirectly, its contents must be copied to RAM before they can be executed. This means that the caveats in the eCos FLASH library about disabling interrupts whilst programming do not apply here, except in special cases such as OneNAND devices.

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