Volatile memory

Data storage and preservation device

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RAM The full name of Random Access Memory is Random Access Memory, which is equivalent to the Removable Storage , used to store and save data.

Chinese name: Volatile memory
Foreign name: Random Access Memory

Full Name: random access Memory
Role: Used to store and save data

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brief introduction

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It can be read and written at any time. RAM is usually used as a temporary part of the operating system or other running programs storage medium (can be called system memory ）。

However, RAM cannot retain data when the power is turned off. If data needs to be saved, they must be written to a long-term storage Medium (such as hard disk). Because of this, RAM is sometimes called "variable memory". RAM memory It can be further divided into static RAM（ SRAM ）And Dynamic memory （ DRAM ）Two categories. DRAM is widely used as the main memory of the system because of its low price per unit capacity.

Application flexibility

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Different applications in different Capacitive load Different operating frequencies are required under, which is closely related to the performance of chipsets and the layout and complexity of circuit boards. For example, the high-frequency working environment usually requires strict optimization of electrical performance. Design engineers need to consider the electrical noise on the entire circuit board to reduce the Parasitic capacitance 。 In this case, reduce the storage output Driver The strength of is more popular. In addition, it is necessary to working frequency Optimize instruction execution speed. Sometimes, it is necessary to obtain appropriate and efficient throughput , empty space must be reduced Clock cycle Times.

Final use

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In application circuit board Test phase , in order to stimulate correctly storage , view the memory response, micro controller It requires a full set of commands and functions. This operational flexibility test is usually used to detect all System components To ensure that the product life cycle Function within. Instead, the standard customer end application uses only one streamlined Instruction set 。 For example, when using SPI flash memory, the final application usually uses read instructions (normal, fast, and/or 4-bit I/O I/O), download the startup code to RAM memory.

Designers should optimize nonvolatile memory to reduce code read and download time during system power on. On new and advanced platforms, such as automotive electronics, computer CD-ROM drive or Bluetooth module SPI flash memory may be used to directly read some system firmware from non-volatile memory to shorten system firmware download to high-speed volatile memory storage Process. Of course, the latest applications that appear at present have more stringent requirements on the flexibility of memory, which will be explained later in this article.

Adaptive mode

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SPI The advantage of flash memory is that the number of pins is small and fixed (8 or 16). This feature of serial flash memory can simplify the layout of the circuit board, and the firmware can be upgraded without changing the hardware, thus reducing the system development The total cost of.

Due to the strong advantages in simplicity and cost, PC and consumption Electronic market Appears Parallel port Flash memory is changed to SPI flash memory Development trend 。 As long as the performance requirements are met, the device manufacturer will give priority to serial flash memory. Computer CD-ROM drive Automotive Electronics 、 Bluetooth module , set-top box, and Modem The market is introducing the ability to read codes directly to non-volatile storage SPI flash memory in.

XiP (On chip execution) Applications require serial port memory to provide a "random access" simulation function, that is, memory content can be accessed without sending instructions, and the maximum throughput Access memory. Because the traditional purpose is to store and download code, SPI memory is a synchronous device, XIP Functions force designers to develop flexible memory that can Chipset Flexible configuration of features Serial Flash 。 For example, after the system is powered on, devices with XIP function need to be based on command, address and data JEDEC Protocol, so some logic devices are not allowed to manage serial flash memory like XIP devices.

In addition, some logic devices only A line XIP mode is supported on or at most two lines. Because of the inherent hardware limitations, it is impossible to enable 4-bit I/O Input/output mode.

Finally, because of the tradition of implementing a hybrid protocol and accepting commands storage With the difficulty in designing nonvolatile memory that does not accept commands, chipset manufacturers prefer to retain the original SPI instruction structure, that is, commands, addresses, and data. In these cases, the scheme of combining high-speed protocol with parallel command, address and data is more popular in the market.

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