Demystifying DDR Memory

Created: May 2020Last Updated: 07/10/2023


Have you ever wondered how your smart electronics load, process, and store pictures quickly? The main memory in embedded system design plays a critical role in meeting fast read and write requirements for low latency user experiences. In this article, we explore the background, architecture, performance parameters, and practical design analysis of DDR4 SDRAM. 


In the realm of computer memory, it is essential to understand the distinction between volatile and non-volatile memory, as well as the significance of various memory types. Here are additional technical details related to the background of computer memory:

Volatile Memory:

Non-volatile Memory:

Random Access Memory (RAM):

Read-Only Memory (ROM):

Two Types of RAM:

Understanding the distinction between volatile and non-volatile memory, as well as the characteristics of different RAM types, aids in selecting the appropriate memory technology for specific applications and optimizing overall system performance.

DDR DRAM Architecture

DDR (Double Data Rate) SDRAM consists of a DRAM controller and memory cells organized into banks. Here are more technical details regarding the architecture of DDR SDRAM:

Memory Cells:

Banks and Rows:

DRAM Controller:

Addressing Scheme

Bank Group:

Data Strobe (DQS):

Memory Access Timing:

By understanding the architecture of DDR SDRAM, including the organization of memory cells, banks, and the role of the DRAM controller, engineers can effectively design memory systems and optimize their performance in electronic devices.

Memory Accessing flow

The memory accessing flow in DDR3 and DDR4 SDRAM involves several steps that ensure data is correctly accessed and transferred. Here are the technical details of the memory accessing flow:

The above steps represent a simplified overview of the memory accessing flow in DDR3 and DDR4 SDRAM. The memory controller coordinates the timing and signaling required for successful data access and transfer. Understanding this flow is essential for optimizing memory performance and ensuring efficient operation in electronic systems.

Performance Parameters

These performance parameters directly impact the memory's speed, throughput, and efficiency, making them critical considerations when selecting and optimizing DDR3 or DDR4 SDRAM for a given application. Understanding these parameters enables engineers to make informed decisions and achieve the desired performance objectives for their electronic systems.

Practical Design Analysis

Performance Analysis:

Q: What determines memory performance in DDR4 SDRAM?

A: The CAS Latency (CL) is a key factor that affects memory throughput and performance. Lower CAS Latency values result in faster data access.

Q: How is the actual data rate or bandwidth of DDR4 memory calculated?

A:The data rate is twice the I/O bus clock rate for DDR memory due to its double data rate nature. The total data rate is obtained by multiplying the memory clock rate by the I/O bus width. For example, if the memory clock rate is 200 MHz and the I/O bus width is 16 bits, the total data rate would be 400 MT/s (Mega Transfers per second) with a bandwidth of 3200 Mb/s (Megabits per second).

Q: What prevents all devices from using DDR4 memories?

A: While DDR4 offers benefits such as lower power consumption at higher data rates, it is generally more expensive compared to DDR3 memory. The cost of DDR4 memory is expected to decrease as the market volume increases and transition from DDR3 to DDR4 becomes more widespread.

Q: How does DRAM contribute to the main system memory in smart devices?

A: DRAM, such as DDR3/DDR4 SDRAM, serves as the main system memory that enables the main processor to operate with low memory access latency. Selecting the appropriate DRAM based on factors like bandwidth, density, I/O bus width, and latency is crucial for overall product performance. Collaboration between electronic system engineers and software teams is essential for determining memory requirements and achieving optimal performance.

Q: What is the difference between ranks and channels in DDR4 memory?

A: Ranks: DDR4 memory modules can have one or multiple ranks. Each rank consists of a separate set of memory chips and operates independently. Having multiple ranks allows for better memory utilization and increased capacity. Channels: DDR4 memory controllers support multiple channels, which are independent data paths between the memory controller and the memory modules. Each channel can have one or more memory modules. Utilizing multiple channels can enhance memory bandwidth and improve system performance.

Q: How do ranks and channels impact memory performance?

A: Multiple ranks provide improved memory utilization and increased capacity but may introduce additional latency due to the need for command and data coordination between ranks. Multiple channels offer increased memory bandwidth, allowing for faster data transfer between the memory modules and the memory controller. Utilizing multiple channels can significantly enhance memory performance in applications that benefit from higher data throughput.

Summary & Conclusion

Understanding the differences between volatile and non-volatile memory and the characteristics of various RAM types is essential for optimizing memory selection in computer systems. By considering specific system requirements, engineers can make informed decisions to achieve efficient and reliable memory performance in electronic devices.