NAND flash driver

emFile comes with two drivers for NAND flash devices:

The NAND drivers are fail-safe. The drivers make only atomic actions and they assume responsibility for the validity of the data managed by the file system. In case of a power loss or a power reset during a write operation, it is always assured that only valid data is stored in the flash. If the power loss interrupts the write operation, the old data is kept and the block not corrupted.

Both NAND drivers come with support for active and passive wear leveling. The purpose of wear leveling is to extend the life of the NAND flash device by making sure that the NAND blocks are evenly erased. This is done by keeping track of the lowest erase count and of the index of the NAND block last erased. When an erased block is required the NAND driver searches for the next available NAND block in the row and it checks the difference between the erase count of the found NAND block and the lowest erase count. If the difference is larger than a specified maximum then the NAND block with the lowest erase count is used. This procedure guarantees that the NAND blocks which store constant data are also erased.

The quality of data stored on a NAND drive fades over time. Degradation generally occurs over the course of a few years so there is plenty of time to fix the problem.

To ensure data integrity, the Universal NAND flash driver performs Data Scrubbing with each read operation.

In the scrubbing process, data is read and checked for errors. Errors, if any, are corrected and then the accurate, “refreshed”, data is written back to the same memory location. Error correction is done using Error Correcting Code software such as SEGGER’s emLib ECC.

The precise rate of data degradation is difficult to predict, so determining how often to scrub data is an issue. Scrubbing too often puts wear on the storage device. Not scrubbing often enough puts the data at risk because there is a point after which the data can no longer be corrected.

The SEGGER NAND drivers allows for configuration of the threshold that determines when data is scrubbed. The threshold can be set at runtime and thus adjusted over time as desired.

Blocks can become defective which makes necessary to identify them as such. Bad-block management helps the NAND drivers identify and mark blocks that are defective. Blocks marked as defective at manufacturing time or by the NAND driver are not used for data storage. A block is recognized as defective when the first byte in the spare area of the first or second page of that block is different from 0xFF. The driver marks a block as defective in the following cases:

• When the NAND flash device reports an error after a write operation.
• When the NAND flash device reports an error after an erase operation.
• When an uncorrectable bit error is detected via ECC on the data read from NAND flash device.

The NAND drivers perform garbage collection automatically during write operations. If no empty blocks are available to store the data, new empty ones are created by erasing blocks that contain invalid data. The garbage collection operation can potentially reduce the write throughput of an application since the block erase operation takes a relatively long time to complete. For applications which require maximum write throughput, the NAND drivers can do garbage collection when the file system is idle.

Block grouping is a feature that can be used to reduce the RAM memory usage for block management and thus enable the sue of large capacity NAND flash devices with smaller microcontrollers. With this feature enabled, the NAND drivers treat multiple consecutive blocks as a single block to considerably reduce the memory used for administrating the NAND flash memory. The size of a block group is scalable to the requirements of the application.

A NAND flash is a serial-type memory device which utilizes the I/O pins for both address and data input/output as well as for command inputs. NAND flash devices consist of a number of blocks. Every block contains a number of pages, typically 64. The pages can be written to individually, one at a time. When writing to a page, bits can only be changed from 1 to 0. Only entire blocks (all pages in the block) can be erased at a time. Erasing means bringing all memory bits in all the pages of the block to logical 1.

Small NAND flash devices (up to 256 MB) have a page size of 528 bytes - 512 bytes for user data and 16 spare bytes for storing management information (ECC, block index, etc.) related to the user data in the page. Large NAND devices (256 MB or more) typically have a page size of 2112 bytes - 2048 bytes for user data and 64 bytes for management information.

The NAND drivers can handle any common page and block size, as well as logical sector sizes smaller than page size. This allows using a file system with a logical sector size of either 512, 1024 or 2048 bytes per sector on a NAND flash with 2 KB pages.

For example, a typical NAND flash with a size of 256 MB has 2048 blocks of 64 pages of 2112 bytes (2048 bytes for data + 64 bytes spare area).

DataFlash:

DataFlash devices work in a similar way to NAND flash devices but they have capacities of only a few mega bytes.

General Information

• Data transfer width is 8 bit.
• Chip Select (CS) sets the card active at low-level and inactive at high level.
• Clock signal must be generated by the target system. The serial flash chips are always in slave mode.
• Bit order requires most significant bit (MSB) to be sent out first.

DataFlash devices are commonly used when low pin count and easy data transfer are required.

Pin Description of DataFlash