Embedded Real-Time Systems Design

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Embedded Real-Time Systems Design

• Selecting memory

2
Memory for embedded systems

• Embedded systems have different memory
  requirements
• System frequently dont have hard disk
• No ability to swap
• They run unattended
• May required unattended boot
• Wont do normal system logging
• Power down reliability

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Memory options
Type Positive Negative
ROM Read Only Memory High Density Low cost Cant write to
EPROM Electrically Programmable ROM High Density Low Cost Difficult to write to
EEPROM Electrically Erasable PROM Easier to erase than EPROM High cost Low density
SRAM Static Random Access Memory Highest Speed Low density Volatile
DRAM Dynamic Random Access Memory High Speed Low cost Volatile
FLASH High Density High Speed Wears out!
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Why Flash?
EPROM E2PROM
Updateable
Nonvolatile
FLASH
DRAM
ROM
High Density
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Why Flash Memory?

• Flash has characteristics of both ROM and RAM
• It is non-volatile
• Can be written to
• Has high read access speeds
• It is reliable

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What is Flash memory?

• Flash covers a wide range of different memory
  technologies and Architectures
• DOC (Disc On Chip) system also known as NAND
  flash
• CFI (Common Flash Interface) also known as NOR
  flash
• JEDEC an older standard, still used but
  becoming obsolete by CFI
• Non-DOC Nand
• Old non-CFI Flash

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Flash memory

• Flash memory, both nand and nor, work differently
  from other memory devices
• In many ways they operate as block devices such
  as disk drives.
• The devices are started as a whole series of 1
  (empty)
• Writing just means resetting the 1s to 0s where
  necessary

8
Flash memory

• Writing and erasure must be done in blocks
• Blocks can vary in size
• Nor flash can set individual bits in a block
• Nand can have pages in a block to change
• Whole blocks have to be erased before they can be
  written to
• Blocks can be locked and unlocked

9
Common Flash Interface

• CFI or the Common Flash Interface was developed
  by Intel and AMD
• The parameters for the flash are stored on chip
  in a common protocol
• Software can query the devices
• A number of levels of compliance

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Flash Memory Maps

• As flash is block orientated it can be laid out
  in differing ways
• Boot flash
• With small (normally 8k byte) boot blocks at
  either top or bottom
• Then 64k erase blocks
• Flash memory
• Arranged in erase blocks of 64 or 128k

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Flash limitations

• Writing of data can only be done in blocks 9or
  pages on Nand flash)
• Erasing and re-writing is slow
• For DOC Nand pages can only be written a limited
  number of times (10) before block must be erased
  and re-written
• There is a overall limitation on the number of
  writes around 100,000 times

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Flash file systems

• Flash devices have to have separate file systems
  and devices
• On Linux this is the MTD (Memory Technology
  Device) interface
• The file systems need special characteristics
• Some devices require a translation layer for a
  file system to run FTL (Flash Translation
  Layer) or NFTL (Nand FTL)

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JFFS JFFS2

• The Journaling Flash File System is a popular
  choice for flash
• It maintains a log structure of the file system
  in Flash
• Loads the file system in to ram a boot time
• Does wear levelling avoids problems of
  over-writing
• Cant do XIP (eXecute In Place) because of
  compression/decompression
• Not for real-time garbage collection

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CRAMFS

• CRAMFS (Compressed RAM File System)
• Developed by Linux Torvalds
• Compressed version of file system in flash run in
  ram
• Cant be written to only remade off-line
• Use tmpfs for volatile data