Different Memory used in Embedded Systems

SRAM, DRAM, EEPROM, FLASH, Cache Memory, Virtual Memory 

Static RAM (SRAM)

  • The word static indicates that the memory retains its contents as long as power is being supplied. However, data is lost when the power gets down due to volatile nature.
  • SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis and require less power.
  • Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. So SRAM is used as cache memory and has very fast access.

Dynamic RAM (DRAM)

  • DRAM must be continually refreshed in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second.
  • DRAM is used for most system memory because it is cheap and small. All DRAMs are made up of memory cells which are composed of one capacitor and one transistor.

Electrically erasable programmable ROM (EEPROM)

  • EEPROM is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, for example, calibration tables or device configuration.
  • Modern EEPROMs allow multi-byte page read/write operations as well as data read/write lock.Limitation of EEPROM is less number of data program cycles

FLASH

  • Flash memory is non-volatile advanced type of EEPROM mostly used in Microcontrollers and other electronics device to store the firmware. There are two main types of flash memory
  1. NAND Flash – NAND type flash memory may be written and read in blocks (or pages). The NAND type is primarily used in main memory, memory cards, USB flash drives, solid-state drives, and similar products, for general storage and transfer of data.
  2. NOR Flash – NOR type flash allows a single machine word (byte) to be written to an erased location or read independently. NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROM
  • The main feature of flash memory is that when packaged in a “memory card,” it is extremely durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water.
  • Flash memory has better kinetic shock resistance than hard disks.

CACHE MEMORY

  • Cache memory, also called CPU memory used to reduce the average time to access data from the main memory. The cache is a smaller, faster memory which stores copies of the data from frequently used main memory locations.
  • It is in-built in CPU.

VIRTUAL MEMORY

  • Virtual memory is a feature of an operating system (OS) that allows a computer to compensate for shortages of physical memory by temporarily transferring pages of data from random access memory (RAM) to disk storage. Virtual Memory is not included as it is not actual memory.

 

 

SRAM

DRAM

EEPROM

FLASH

CACHE MEMORY

           

Volatile

Yes

Yes

No

No

Can be change According to OS

           

Erase Size

Byte

Byte

Byte

Sector

NA

           

Cost (per Byte)

Expensive

Moderate

Expensive

Moderate

Very Expensive

           

Speed

Fast

Moderate

Fast to read, slow to erase/write

Fast to read, slow to erase/write

Very faster

           

Max Erase Cycles

Unlimited

Unlimited

Limited (consult datasheet)

Limited (consult datasheet)

Unlimited

           

Size(footprint)

Large

Small

Large

Large

Smaller

           

Power Consumption

Moderate

High(refresh on time interval)

Less

Less

High

           

 

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