Demystifying DDR Memory

Introduction

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. 

Background

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:

• Volatile memory refers to memory that requires a continuous power supply to retain its data.

• When power is removed, volatile memory loses its stored information.

• Examples of volatile memory include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).

• Volatile memory is commonly used as the main memory in computer systems due to its fast read and write speeds.

Non-volatile Memory:

• Non-volatile memory retains its stored data even when power is removed.

• Non-volatile memory is essential for storing data that needs to persist across power cycles.

• Examples of non-volatile memory include Read-Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), and Flash memory.

Random Access Memory (RAM):

• RAM is a type of volatile memory used in modern computer systems.

• It serves as the main working memory for storing data and executing programs.

• RAM allows for fast random access to any memory location, enabling efficient read and write operations.

Read-Only Memory (ROM):

• ROM is a non-volatile memory that contains pre-programmed data and instructions.

• It is often used in computer systems and microcontrollers to store firmware or boot code necessary for system initialization.

• ROM cannot be modified by end-users or software.

Two Types of RAM:

• Dynamic Random Access Memory (DRAM):

  • DRAM consists of a single transistor and a capacitor to store each memory bit.

  • It offers higher memory density per die size, making it cost-effective.

  • DRAM is widely used as the primary volatile memory in computer systems.

• Static Random Access Memory (SRAM):

  • SRAM is constructed using six transistors for each memory cell, making it more complex and expensive compared to DRAM.

  • SRAM offers faster access times and lower power consumption than DRAM.

  • It is commonly employed as cache memory or in applications requiring high-speed, low-latency access.

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:

• DDR SDRAMs contain memory cells organized in a matrix-like structure.

• Each memory cell stores a single bit of data as electrical charge within a capacitor.

Banks and Rows:

• DDR SDRAMs are divided into multiple banks, typically consisting of thousands of rows and columns.

• Each bank contains rows, and each row can store multiple bits of data (e.g., 8 bits).

• The number of rows and columns can vary depending on the specific DDR SDRAM device.

DRAM Controller:

• The DRAM controller is responsible for managing the access and operation of DDR SDRAM.

• It sends control signals and addresses to the DDR SDRAM, enabling read and write operations.

• The DRAM controller communicates with the memory cells through the memory array interface.

Addressing Scheme

• DDR SDRAM uses a specific addressing scheme to identify memory cells within a bank.

• The address bus (ADDR) carries the row address and column address signals to select the desired memory location.

• Row addresses are used to activate a specific row, while column addresses are used to access a particular column within the activated row.

Bank Group:

• DDR SDRAM may have multiple bank groups, allowing for parallel access to different banks simultaneously.

• Bank groups enable higher memory throughput and performance by distributing access requests across multiple banks.

Data Strobe (DQS):

• DDR SDRAM utilizes a Data Strobe (DQS) signal for proper data timing during read and write operations.

• DQS serves as an output for read data and an input for write data, ensuring accurate data transfer and synchronization.

Memory Access Timing:

• The memory access timing, including parameters such as Row Access Strobe (RAS) and Column Access Strobe (CAS), determines the performance and latency of DDR SDRAM.

• RAS and CAS signals indicate when the respective row or column addresses on the address bus are valid for data access.

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:

• Selecting a Bank Group:

  • DDR3 and DDR4 memories are organized into bank groups. The memory controller selects a specific bank group for accessing data.

• Selecting a Bank:

  • Within the selected bank group, the memory controller chooses a specific bank for data access. The bank is identified by a bank address.

• Activating a Row:

  • The memory controller activates a specific row within the selected bank, indicating that the following address signals correspond to the row address.

  • When the Row Access Strobe (RAS) signal is asserted, the address on the address bus (ADDR) represents the row address.

• Activating a Column:

  • After activating the row, the memory controller proceeds to activate a specific column within the row.

  • The Column Access Strobe (CAS) signal is asserted, indicating that the address on the address bus (ADDR) now represents the column address.

• Data Output to DQ (Data Input/Output Bus):

  • Once the row and column are activated, the requested data is read from or written to the memory cells.

  • The data is then output to the Data Input/Output bus (DQ) for further processing or storage.

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

• Column Access Strobe Latency (CL):

  • Column Access Strobe (CAS) latency represents the time delay between issuing a read command and the availability of data from the memory. It is typically specified in clock cycles.

  • Lower CAS latency values indicate faster access times and improved memory performance.

  • CAS latency plays a crucial role in determining the memory's throughput and affects overall system responsiveness.

• DDR Bandwidth:

  • DDR3 SDRAM:

    • DDR3 SDRAM supports different data transfer rates, including 1866 Mb/s, 2133 Mb/s, and 2400 Mb/s.

    • These data rates provide higher bandwidth capabilities compared to earlier DDR2 memory, enabling faster data transfer between the memory and the processor.

  • DDR4 SDRAM:

    • DDR4 SDRAM further advances data transfer rates, with options such as 2400 Mb/s and 3200 Mb/s.

    • These higher data rates offer increased memory bandwidth, resulting in improved system performance and responsiveness.

• Voltage Level:

  • DDR3/DDR3L:

    • DDR3 operates at a typical voltage of 1.5V, while DDR3L (Low Voltage) operates at a reduced voltage of 1.35V or 1.5V.

    • DDR3L is commonly used in applications where power consumption needs to be minimized.

  • DDR4:

    • DDR4 operates at a lower voltage level of 1.2V, providing improved power efficiency compared to DDR3.

    • The reduced voltage level contributes to lower power consumption and helps enhance overall energy efficiency in electronic systems.

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

• Selecting DDR4 Memory:

  • Consider required capacity, bus width, and CAS latency.

  • Choose DDR4 parts with appropriate density and bus width (e.g., 8Gb with a 16-bit bus).

  • Compare CAS Latency among vendors and select the lowest value.

• Memory Cascade Connection:

  • Connect two DDR4 modules in cascade to fulfill a 32-bit SoC memory bus width.

  • Ensure proper synchronization and timing between the modules.

• Vendor Options:

  • Evaluate DDR4 memory options from reputable vendors such as Samsung, SK Hynix, and Micron.

  • Consider factors such as performance, reliability, and cost when selecting a vendor.

Performance Analysis:

• • Analyze memory performance metrics and requirements in collaboration with the software team.

  • Assess key parameters like memory throughput, latency, and access times.

  • Optimize the memory subsystem for improved system performance and responsiveness.

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

• CAS latency (CL) plays a crucial role in memory performance, impacting throughput and access times. 

• Volatile memory, such as DRAM and SRAM, requires continuous power to retain data.

• Non-volatile memory, including ROM, EEPROM, and Flash memory, retains data even without power.

• RAM serves as the main working memory in computer systems, providing fast random access for data storage and program execution.

• ROM contains pre-programmed data and firmware used for system initialization.

• DRAM offers higher density and cost-effectiveness, while SRAM provides faster access times and lower power consumption.

• DDR (Double Data Rate) memory offers increased data transfer rates 

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.