SRAM and DRAM

 

SRAM

Static RAM is a random access memory type that retains information as long as power is provided to the SRAM. It does not have to be periodically refreshed.

But why termed as ‘Static’? This is because the data is held statically without any need of refreshing, i.e. the information in the memory is retained in the memory as long as power is supplied.

The SRAM gives quicker access to data and is more expensive than the next type of RAM we will be discussing, the DRAM.

                                                              SRAM with 6 MOSFETs

When we want to look for a piece of data in any memory, it takes a while for the information to be accessed from memory and to return the status of its availability, and the data (if present). The time taken for this action is called Access Time. The SRAM has a small access time, lasting about ten nanoseconds.

The internal structure of an SRAM consists of six transistors. Two transistors, i.e. transistor 5 & 6, are pass transistors which are connected to the bit lines. They are used during the read-write operations to manage the availability of a memory cell.  The remaining four transistors (i.e. Transistors 1, 2, 3  and 4) form two cross-coupled inverters. Thus, transistors 1 and 2 forms one CMOS inverter pair, and the remaining two transistors, 3 and 4, form the other CMOS inverter pair. Due to the complex architecture of the SRAM, it costs more for manufacturing this memory. We shall now see the simplified circuit.

                                                        

                                               Simplified SRAM Structure

Thus, the SRAM stores a memory bit of information on between these two cross-coupled inverters. The two stable states of the inverters characterize 0 and 1. If we give the input to any transistor as 1, its output is zero, thus acting as the input for the next inverter, whose output is 1, and this is how the system remains running as long as power is present.

We can also have another system configuration with only four transistors in the architecture.

                                                             SRAM with 4 MOSFETs

The only difference in this circuit is that the PMOS is replaced with high impedance resistors. Thus, this reduces the number of transistors being used. The only drawback of this circuit is that there is continuous power dissipation across the resistors, which results in heating of the system, and thus might degrade the performance and reduce the life of the SRAM.

Due to the intricate architecture and the increased manufacturing cost, the RAM on most computer motherboards is the DRAM.

Applications of the SRAM

Due to the high speed of operation, SRAM is used for cache memory and as part of the digital-to-analog converter on video cards. It is also found in CDs, printers, routers, DVDs and digital cameras.

DRAM

The Dynamic RAM, also called DRAM, is the most common type of RAM in the computer. It is termed as ‘Dynamic’ because the system needs to be activated frequently, or made ‘dynamic’ so that it doesn’t lose its information. But why would it lose information? Let us learn about it in the next section. DRAM chips have an access time ranging between 50 to 150 nanoseconds, which is a bit more when compared with the SRAM.

                                                   DRAM Structure

A DRAM memory cell consists of a transistor and a capacitor within an integrated system. A data bit is stored in the capacitor. As we use the DRAM cell, the transistor has a small amount of leakage. The capacitors thus slowly discharge over time, and the information contained in it might drain out. Hence, DRAM has to be periodically refreshed to maintain the data it is holding. A DRAM integrated circuit chip consists of dozens to billions of DRAM memory cells.

Applications of a DRAM

The DRAM is the main memory in computers and graphics cards. It is also used in many portable devices and video game consoles.

Types of DRAM?

• Synchronous DRAM (SDRAM) – The SDRAM “synchronizes” the speed of the memory along with the CPU clock speed. By doing so, the memory controller (which is a digital circuitry managing the flow of data from and to the main memory) is aware of the exact clock cycle by which the demanded data will be ready. Thus, the CPU’s efficiency is improved, and it can do many more instructions at a given time. A typical SDRAM works at speeds of up to 133 MHz.

• Rambus DRAM (RDRAM) – The Rambus DRAM is named after the company that introduced it, Rambus. It was mainly used for video game devices and on-computer graphics cards, having transfer speeds running up to 1 GHz.

• Double Data Rate SDRAM (DDR SDRAM) – This memory has nearly double the bandwidth of a single data rate (SDR) SDRAM. It works on the principle of “double pumping” – this permits data to be transferred on both the rising & falling edges of the clock. This type of memory has been succeeded by the DDR2, DDR3, DDR4 and most recently, the DDR5 SDRAM.

MEMORIES IN DIGITAL ELECTRONICS

In computers, memory is the most essential component for the normal functioning of any system – to store data, to perform calculations, to do complex operations, etc. We know that almost all our information and data is stored in the Hard Disk within the CPU. The Hard Disk/Hard Drive/Hard Disk Drive has the most extensive memory in the computer system. But a lot of important data of a computing device is stored in what we will study in this article known as primary memory.

Ask yourself, is it sensible to access the hard disk for every small action? If we need to find a piece of simple information, is it worth to have the computer browse through Gigabytes of memory for just one minor information? Doesn’t this compromise the efficiency of the system? Shouldn’t there be separate memory systems that deal with the data critical for the computer’s operation?

Primary and Secondary Computer Memory

Apart from the HDD, our computer has two other types of memory present. These memory types are used in storing small amounts of memory, or frequently used data which needs to be easily accessed. The two major types are:

• Primary memory (RAM and ROM) and
• Secondary memory (Hard Drive, CD, etc.).

The Primary Memory is also known as system memory, whereas the Secondary Memory is called the Storage. The Secondary memory is physically located within a distinct storage devices, such as an HDD or a solid-state drive (SSD).

1) Primary Memory (Main Memory)

It is also referred to as Main Memory. It is volatile. The reason behind is, Primary memory holds only those data and instructions on which the computer is currently working that is it does not store the data permanently.

• It also stores the operating system and data required to run the computer.
• It is a limited capacity memory and data or information is lost when power is switched off. Primary Memory is generally constructed with a semiconductor device.
• Registers are much faster than these memories but it is faster than secondary memory.
• It contains all the data and instructions that are required to be processed.

It is further divided into two subcategories RAM and ROM.

i) RAM (Random Access Memory)

It is Random Access Memory because of the random selection of memory locations. It performs both read and writes operations on memory. It stores data temporarily.

If power failures happen in the system during memory access then you will lose your data permanently. So, RAM is a volatile memory.

RAM categorized into following types:

1. DRAM
2. SRAM

a) SRAM (Static random access memory)

It holds data in a static form, that is, as long as the memory has the power as the dynamic RAM, it is not needed to refresh it again and again.

• Static RAM provides faster access to data and is more expensive than DRAM as each cell must contain multiple transistors.
• SRAM does not use capacitors.
• SRAM is also highly recommended for use in PCs, peripheral equipment, printers, LCD screens, hard disk buffers, router buffers and buffers in CDROM / CDRW drives.

b) Dynamic RAM (Dynamic random access memory)

It is a type of random-access memory used in computing devices. It is made up of capacitors and transistors.

• This type of memory uses separate capacitors or transistors to stores each bit of data and it has two states of value in one bit called 0 and 1.
• As compared with other RAM's it is less expensive.
• Data were written by DRAM at the byte-level.
• In DRAM, data is written at the byte-level and it reads data at the multiple-byte page level.
• DRAM requires less power than other RAMs.

ii) ROM (Read Only Memory)

ROM offers huge types of standards to save data as it is a permanent memory location. But it works with the read-only operation. whenever power failure occurs during the ROM memory work in computers then no data lose happens.

• It is Used where the programming requires no change and also in embedded systems or.
• It is Used in peripheral devices and calculators.

Types of Read Only Memory (ROM)

1. PROM (Programmable read-only memory)
2. EPROM (Erasable Programmable read only memory)
3. EEPROM (Electrically erasable programmable read only memory)

a) PROM (Programmable read-only memory)

Developers created a type of ROM known as programmable read-only memory (PROM) because Creating ROM chips from scratch are time-consuming and very expensive.

• It can be coded by the user. Once coded, the data and instructions in it cannot be changed.
• It is used to store permanent data in digital electronic devices.
• It can be bought at a low cost as compared to other RAMs.

b) EPROM (Erasable Programmable read only memory)

This is the type of memory that can be reprogrammed. We can erase data from it and reprogram it that is erase all the previous data by using high voltage Ultraviolet light.

• It is required to erase each cell in EPROM.

c) EEPROM (Electrically erasable programmable read only memory)

The data can be erased and reprogrammed by applying an electric charge. There is no need for ultraviolet light and we can erase only portions of the chip.

• It was a replacement for PROM and EPROM chips and later it is used for computer's BIOS.
• Configurations parameters are stored by using EEPROM. In modern computers, they replaced BIOS CMOS memory.
• It is required that data to be written or erased by EEPROM one byte at a time.

Intro of VLSI

 The method combining thousands of transistors into a single chip is called VLSI. VLSI stands for 'Very Large scale integration' and began in the 1970s. In that time complex semiconductors and communication technologies were in the early stages of development. The microprocessor is a VLSI device. Before VLSI, IC's were limited in the performance of their functions. An electronic circuit may contain a CPU, ROM, RAM, etc and VLSI enabled IC designers to add all of these into one chip. The growth of the electronic industry has been massive due to rapid growth in large scale integration technologies and system design applications. With VLSI, the number of applications of integrated circuits in high-performance computing, controls, telecommunications, image and video processing, and consumer electronics has grown at a tremendous pace. Current technology phenomenons such as high resolution, low bit rate video, and cellular communications provide users with portability, applications, processing power, etc. The trend is growing rapidly with very important implications on VLSI design and systems design.

VLSI Design Flow 

The various levels of design are numbered and show processes in the design flow. Specifications come first, they describe abstractly, the functionality, interface, and the architecture of the digital IC circuit to be designed. 

Design Specifications

Schematic Capture

Create a Symbol

Simulation

Layout

Design Rule Check

Extraction

Layout vs Schematic Check

Post Layout Sim