M-Systems Flash Disk Pioneers Flash Memory manual Efficient Bad Block Handling

Implementing MLC NAND Flash for Cost-Effective, High-Capacity Memory

Efficient Bad Block Handling

x2 technology handles bad blocks, which can be randomly present in flash media, by enabling unaligned block access to two planes. Bad blocks are mapped individually on each plane, as shown in Figure 4. Good units can therefore be aligned or unaligned, minimizing the effects of bad blocks on the media. Without this capability, a bad block in one plane would cause a good block in the second plane to be tagged as a bad block, making it unusable. This customized method of bad block handling for two planes enhances data reliability without adversely affecting performance.

	Internal Bus
16-bit Data Bus	16-bit Data Bus

Good Unit		Aligned Unit							Good Unit
Good Unit		Aligned Unit							Good Unit
Good Unit		Aligned Unit							Good Unit
Bad Unit							d		Good Unit
					e
					n
				ig			t
			l
			a			i
		n			n
~	~	U		U				~	~
Good Unit									Bad Unit
Good Unit		Aligned Unit							Good Unit
Good Unit		Aligned Unit							Good Unit
Flash Plane 1									Flash Plane 2

Figure 4: Unaligned Multiplane Bad Block Access

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