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.