Flash Translation Layer
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Flash Translation Layer

FTL is a standard flash translation layer which allows an array of NAND flash to be addressed as a set of standard 512Byte Logical sectors. NAND flash is a very cost effective flash storage media. However, it has certain characteristics which mean that it cannot be used as a simple continuous storage array.

NAND flash may contain bad blocks; bad bits may develop in previously good blocks; and heavily used blocks will “wear-out”. To allow a higher level system – such as a file system – to use NAND flash effectively it is necessary to use an FTL which carefully manages the flash while providing a simple logical sector interface to the host system.

Key features

  • Up to 4 Terra Bytes in single wear-levelled array
  • High Performance
  • Zero Copy Block read/write
  • Cache options
  • 528, 2112 and 4224 Byte page support
  • Safe From unexpected reset
  • Effective Wear-Levelling Algorithm
  • Bad Block management
  • Automatic Garbage Collection
  • Small Code Footprint
  • Small RAM usage
  • All standard NAND devices supported
  • Sample drivers and ECC algorithms
  • Typically >98% of blocks available
  • Fast Initialisation

 


Without Tabs >>>
  • Fail-Safe
  • Wear
  • Supported NAND
  • Atmel DataFlash
  • Verification
  • Upper Layer Interface
  • Physical Device Interface

Fail-Safe

FTL is designed and tested to cleanly handle unexpected system resets. If a write had been initiated when the system was reset then if it was not complete the old state of the logical sector being addressed will be presented on start-up. If the write operation was complete then the new state of the logical sector will be read. There are no in between states or complex recovery procedures required.

Wear

All flash devices suffer from wear – that is the repeated use of a particular area of flash increases the likelihood that it will fail. Distributing the usage of blocks across the whole flash array greatly increases the lifetime of NAND devices but effective wear levelling is a complex task. Seemingly “obvious” algorithms tend to fail in real world testing. HCC have developed sophisticated wear algorithms (both static and dynamic) which are tuneable to your systems needs. These ensure that all the blocks in a NAND flash array are written to evenly within a configurable maximum distance metric.

Supported NAND Devices

FTL has been designed and tested to work with most types of NAND flash currently available. NAND flash is constantly evolving and HCC work to ensure that the product works with the latest technology. The system is tested in-house on Samsung, ST Micro and Micron devices. It has been successfully ported to several other types including Toshiba and Hynix. The system has been extensively tested on both 528 byte page and 2112 byte page devices.

To verify if your target device is supported please send mail to sales@nohau.co.uk

Atmel DataFlash

HCC also provide a version of EFFS-FTL for use with Atmel DataFlash. EFFS-FTL-DF can be used to handle arrays of Atmel DataFlash devices eficiently while giving them excellent wear characteristics. Additonally ECC (error correction codes) can be used to correct bit errors.

EFFS-FTL-DF only works with Atmel DataFlash which has the "spare area" - i.e. those with 528 or 1056 byte page size. An additional feature of this system is that it can enhance performance of your system - one reason is that the FTL will use block erase rather than individual page erases which are considerably faster.

Verification

FTL is extensively tested both on real NAND flash systems and in simulation. Because wear analysis and fail safe safety in particular are both complex, HCC have developed a complete PC simulation environment to run millions of test cases to verify both the wear and fail safe characteristics FTL.

Upper Layer Interface

The FTL is designed to be easy to use by standard file systems – and provides a very simple logical sector based interface for the higher level application to use. The interface functions are:

hl_init Initialises the FTL
hl_format Performs an initial low-level format of the flash
hl_getmaxsector Returns the number of logical sectors in the system
hl_open Opens one or more sectors for a read/write operation
hl_close Closes an operation
hl_read Reads a number of logical sectors from the flash
hl_write Writes a number of logical sectors to the flash

This interface is suitable for use in conjunction with FAT, THIN and/or EUSB-MS. Integrated projects can be provided on request.

Physical Device Interface

FTL has carefully designed to work with most NAND flash types and creating the interface to a aprticular device is straightforward. The FTL requires some functions for accessing the NAND flash device to be provided – included with the package are sample drivers for several NAND flash types though the interface is designed to be easy to port to any NAND device. Also included are sample ECC algorithms – though where possible these are best done with hardware support. The interface functions - called by FTL - that the lower level interface must provide are:

ll_init() Initialises physical layer and gets config info
ll_erase() Erases a flash block
ll_writedouble() Writes a flash page from two buffer pointers
ll_read() Reads a flash page