Nand Flash, which is widely used in digital devices such as mobile phones and tablets, cannot avoid the existence of bad blocks due to process reasons, but we can use the high-tech to turn waste into treasure and effectively use the "bad block" to meet our application. Demand, let the bad block not "bad".
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If you want to turn waste into treasure, use bad blocks effectively. We must first understand what a "bad block" is, and know how to know ourselves and what we can use. A bad block is characterized by the inability to pull certain bits high when programming or erasing this block, causing errors in programming and block erase operations that can be reflected by the value of the status register. These invalid blocks cannot determine the state at the time of programming, which is what people often call "bad blocks." So how did these "bad blocks" come about?
Figure 1 Schematic diagram of the bad block
Due to technical problems, Nand Flash will inevitably have factory bad blocks (2% of bad blocks are allowed at the factory), and new bad blocks will also be generated during the use of Nand Flash. The maximum number of erasures per block of Nand Flash is about 1 million. If it exceeds this number, it will become a bad block. In addition to the above, there are many reasons for bad blocks, such as memory unit errors, address line errors, etc...
So the point is that since we can't completely eliminate the "bad block", then we can choose to use it to achieve some special and necessary functions. Why do you say that bad blocks can be turned into waste? In addition to the types of bad blocks described above, we need to artificially mark certain blocks of the Nand Flash chip as bad blocks in some special cases. For example, if a block is very unstable and exceeds the error correction range of the system ECC, this block needs to be marked as a bad block at this time; or if we want to verify that the bad block processing part of the programming scheme is correct, we also need to make some bad blocks. For test verification; in order to meet the characteristics of a certain type of system, you need to make the specified bad blocks, such as the UBI file system we mentioned at the beginning. These special circumstances require us to purposely set the specified bad blocks to meet the corresponding needs.
It is clear what is the bad block and how the bad block is produced. Then let’s take a look at how to artificially set the bad block and let the bad block be used for us to achieve the goal of “turning waste into treasureâ€.
First of all, we need to know what is the "bad block mark" of the chip (BI for short). Bad block flags are not the same for different devices. Usually for a 512-byte/page NandFlash chip, the bad block flag is at the 5th byte of the spare area on page 0, 1 (or other page), and the chip bad block mark position is at 2K bytes/page. 0 bytes.
Figure 2 Bad block mark
Then, in principle, we only need to write a specific value in the BI position of a certain chip of the chip according to the specifications in the chip manual, and then mark the block as a bad block. Most chips have a bad block flag value of "not 0xff" which is considered a bad block. However, some chips are considered to be bad blocks when they are "0x00", and the page of the bad block flag is also changed, so for uniformity, we use A general and effective method: write the entire block to a value of 0x00. The 6000F-PLUS programmer developed by Zhiyuan Electronics and the accompanying SmartPRO III software can easily mark the bad blocks.
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