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memory chips selection guide     memory chips selection guide     memory chips selection guide

Image credit: Newark/element14 | Microchip Technology | Digi-Key

 

Memory chips are semiconductor devices used as internal storage areas within a computer.

 

Understanding Memory

 

Primary vs. Secondary and Volatility

Computer memory refers to the storage of programs or data on a temporary or permanent basis. Memory devices can be classified into two distinct groups: primary and secondary. Memory chips may be grouped into either of these categories depending on the type as described below.

 

  • Primary memory is directly accessible to the central processing unit (CPU) and includes any data actively or continuously operated upon. Random access memory (RAM) is a good example of a primary memory type. Primary memory allows for fast data access but is limited to smaller storage space, typically 256-1024 MB.
  • Secondary memory is not directly accessible by the CPU and is used for longer-term data storage. It is much slower to access compared with primary memory, but it allows for much greater capacity (typically 20-120 GB of data). Secondary memory devices include hard disks and optical drives.

 

Memory devices may also be grouped as volatile or non-volatile, depending on the device's behavior when power is interrupted. Volatile memory (more frequently known as temporary memory) retains information as long as it is supplied with power, but experiences the total loss of stored memory when power is cut off. Conversely, non-volatile memory retains stored data even when not powered. Because of this fact, non-volatile devices are often used for long-term (secondary) storage. As with the distinction between primary and secondary types, memory chips may be volatile or non-volatile depending on the device's technology and application.

 

Composition and Operation

As semiconductor devices, memory chips are composed of transistors and capacitors: the capacitors serve to store two binary logic bits (0 or 1), while transistors allow reading and writing of data to the capacitors. A memory chip is comprised of thousands of these tiny circuits, known as memory cells. Data stored in the cells can be nondestructively accessed (or "read") by means of binary memory addresses assigned to each cell. While early chips could hold only a few bits of information, the largest semiconductor memory chips can now store several gigabits, with higher capacities constantly being developed and introduced.

 

Chip Types

 

Memory chips are primarily identified and specified based on their technology type, which includes their methods for reading, writing, and storing data. Before exploring each specific type, it is helpful to examine two broader classes of memory chips distinguished by their data storage qualities.

 

Random access memory (RAM) is used as a generic term for any memory chip that can be written to and read from which is not a read-only (ROM) chip, as described below. Most RAM is classified as volatile; special types of non-volatile RAM (NVRAM), such as ferroelectric RAM (FRAM), are non-volatile. RAM types include:

 

  • Dynamic RAM (DRAM)
  • Static RAM (SRAM)
  • Non-volatile RAM (NVRAM)
  • Ferroelectric RAM (FRAM)
  • Non-volatile static RAM (NVSRAM)

 

Read-only memory (ROM) is more difficult to modify than RAM and is normally used to read permanent data. ROM is typically non-volatile and is used to store frequently-read (but not written/modified) system software. ROM types include:

 

  • Programmable ROM (PROM)
  • Erasable programmable ROM (EPROM)
  • Electrically erasable programmable ROM (EEPROM)
  • Flash
  • Mask ROM

 

The image below shows the function of both memory types in the hierarchy of a typical computer. Note that ROM is distinct from the computer's hard disk, another type of permanent storage.

 

memory chips selection guide

Hierarchy of computer memory. Image credit: Ateno de Manila University

 

The table below describes specific types of memory chips and their common parameters. The attributes listed in the subtype column are described below the table.

 

Type

Volatility

Subtype

Applications/description

Time of introduction

DRAM

Volatile

Byte-addressable

Main memory

1969

SRAM

Volatile

Byte-addressable

High speed buffers/caches

1970s

EPROM

Non-volatile

Rewritable-when-removed

Microcontroller programming

Early 1970s

EEPROM

Non-volatile

Byte-addressable (symmetric)

Small data storage

Early 1980s

Flash

Non-volatile

Block write (asymmetric)

Hard drive replacement / serial data access

1988

PROM

Non-volatile

Non-rewritable (permanent)

Mobile phones, RFID, implantable medical devices, video games

Mid-1960s

Mask ROM

Non-volatile

Non-rewritable (permanent)

Microprocessors / microcontrollers

 

NVRAM (includes NVSRAM & FRAM)

Non-volatile

Byte-addressable

Effectively backed-up RAM chips

1980s-present

 

  • Byte-addressable: data can be read and written one byte at a time.
  • Rewritable-when-removed: chips must be removed from the circuit board and reprogrammed externally.
  • Symmetric byte-addressable: data can be read and written one byte at a time; reading and writing speeds are equal or nearly equal.
  • Asymmetric block write: data is read at byte level but written at block level; writing speed is slower than reading speed.
  • Non-rewritable: data on the chip can never be changed.

 

Specifications

 

Numerical Specs

When selecting memory chips, the buyer should consider the important specifications listed below, if the manufacturer provides them. Most specifications are related to a chip's speed and efficiency.

 

  • Data rate is expressed in hertz (Hz) and represents the number of bits which can be transferred within the chip each second.
  • Access time is another important spec related to a memory chip's speed. This figure represents the elapsed time (in nanoseconds) from the moment the CPU calls for data to the moment it receives the request back.
  • Cycle time represents the time necessary to complete a single read/write process and to reset the chip for another cycle.
  • Bandwidth refers to the amount of data a memory chip can move or process within a set time frame. It can be expressed in bits per second (bps or b/s), bytes per second (Bps or B/s), or hertz (Hz or cycles/second).

 

Standards

Standards related to memory chips are typically developed for specific memory types, such as EEPROM and SRAM. Some general memory standards include:

 

 

References

 

Ateno de Manila / Carlos Oppus - Memory Storage, RAM, ROM

 


Related Products & Services

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    Dynamic random access memory (DRAM) chips are single-transistor memory cells that use small capacitors to store each bit of memory in an addressable format that consists of rows and columns. Because capacitors are unable to hold a charge indefinitely, DRAM memory chips require a near-constant pulse of current to retain stored information.

  • EEPROM

    Electrically erasable programmable read-only memory (EEPROM) chips are similar to PROM devices, but require only electricity to be erased.

  • EPROM

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    First-in, first-out (FIFO) memory chips are used in buffering applications between devices that operate at different speeds or in applications where data must be stored temporarily for further processing.

  • FLASH Memory Chips

    FLASH memory chips offer extremely fast access times, low power consumption, and relative immunity to severe shock or vibration. They do not need a constant power supply to retain their data. 

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