Introduction to the DDR RAM Including Its History and Specs

There are different types of RAM, such as DRAM memory and SRAM memory. And this post focuses on DDR SDRAM. If you want to get more information about other types of RAM, then it is recommended to visit the MiniTool website.

Definition of DDR RAM

What is DDR SDRAM? It is short for Double Data Rate Synchronous Dynamic Random-Access Memory, which can be used in computers. And it is also called DDR1 SDRAM, which has been replaced by DDR2 SDRAM, DDR3 SDRAM, and DDR4 SDRAM.

The successors of DDR memory are not forward or backward compatible with it, so DDR2, DDR, and DDR4 memory modules cannot work on motherboards equipped with DDR1, and vice versa.

Compared with the single data rate (SDR) SDRAM, the DDR SDRAM interface can increase the transmission rate by more strictly controlling the timing of electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to achieve the required timing accuracy.

The interface adopts double pumping (transmitting data on both the rising and falling edges of the clock signal) to double the data bus bandwidth without increasing the clock frequency accordingly. One of the advantages of reducing the clock frequency is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller.

The name “double data rate” refers to the fact that due to this dual pumping function, the bandwidth of a DDR SDRAM with a specific clock frequency is almost twice the bandwidth of an SDR SDRAM operating at the same clock frequency.

By transferring 64-bit data at a time, the transfer rate (in bytes/second) of DDR RAM is (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Therefore, with a bus frequency of 100 MHz, the maximum transfer rate of DDR RAM is 1600 MB/s.

History of DDR RAM

• In 1997, Samsung released the first DDR RAM prototype.
• In June 1998, Samsung introduced the first commercial DDR SDRAM chip (64 Mb), followed by Hyundai Electronics (now SK Hynix) the same year.
• In June 2000, JEDEC finalized specifications of DDR RAM (JESD79). JEDEC has set the standard for the data rate of DDR RAM, divided into two parts. The first specification is for memory chips, and the second specification is for memory modules.
• In August 2000, the first retail PC motherboard using DDR SDRAM was released.

Specs of DDR RAM

Modules

In order to increase memory capacity and bandwidth, the chips are combined on a module. For example, a 64-bit data bus for DIMMs requires eight 8-bit chips addressed in parallel. Several chips with the common address lines are called a memory rank.

The purpose of introducing this term was to avoid confusion with chip internal rows and banks. A memory module can carry more than one rank.

Capacity

For non-ECC modules, the number of chips is a multiple of 8, and for ECC modules, the number of chips is a multiple of 9. The chip can occupy one side (single sided) or both sides (dual sided) of the module. For ECC, the maximum number of chips per DDR module is 36 (9 × 4), for non-ECC, the maximum is 32 (8×4).

Chip Characteristics

The size of the chip is measured in megabits. If most motherboards contain 64M × 8 chips (low density), only 1 GB modules can be recognized. If you use 128M × 4 (high density) 1 GB modules, they most likely will not work. The JEDEC standard allows 128M × 4 to be applied only to slower buffered/registered modules designed for some servers, but some general manufacturers do not comply.

High-Density RAM

In the context of the 1 GB non-ECC PC3200 SDRAM module, the visual effect of distinguishing low-density RAM from high-density RAM is hardly visible. Like low-density DDR RAM modules, high-density DDR RAM modules are usually double-sided, with eight 512 Mbit chips on each side. The difference is that each chip is not organized as 64M × 8, but as 2²⁷ 4-bit words or 128M × 4.

High-density memory modules are assembled using chips from multiple manufacturers. These chips are packaged in a common 22 × 10 mm (approximately) TSOP2 and a smaller squarer 12 × 9 mm (approximately) FBGA. High-density chips can be identified by the number on each chip.

High-density RAM devices are designed to be used in registered memory modules of servers. The JEDEC standard does not apply to high-density DDR RAM in desktop implementations.

Bottom Line

All in all, this post gives you some information about Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM) such as its definition and history.