FuturePlus Systems, FPS
Matt Simon
DDR memory is a widely used memory technology found in many computer systems ranging from desktop PCs to servers to mobile devices. Testing for signal integrity and protocol compliance is critical to reliable DDR memory operations. This blog post will therefore introduce the concepts behind signal integrity testing and protocol testing for DDR memory as well as test tools used for this purpose.
Signal integrity testing for DDR memory
Signal integrity testing analyzes the quality of electrical signals transmitted over the DDR memory bus. These signals include address, data, and control signals that must be transmitted with sufficient quality to ensure proper operation of the memory subsystem. The following key parameters are tested during signal integrity testing for DDR memory:
Test parameter #1: Voltage levels
Voltage levels of the DDR memory signals must lie within a certain range to ensure proper operation of the memory subsystem. In addition, voltage levels must also be consistent across all memory devices to ensure that the signal levels are not too high or too low.
Test parameter #2: Timing
Timing of the DDR memory signals needs to be precise to ensure that the data is transmitted and received correctly. Timing parameters such as setup and hold times, clock skew, and jitter must be measured and verified down to the picosecond for the best results.
Test parameter #3: Crosstalk
Crosstalk refers to unwanted coupling of signals between different conductors in the DDR memory bus. Crosstalk must therefore be kept to a minimum as it can cause signal degradation and interference, leading to errors in data transmission.
Test parameter #4: Reflections
Reflections occur whenever a signal encounters an impedance mismatch or a change in the transmission medium. As with crosstalks, reflections can cause signal degradation that leads to errors in data transmission and must therefore be minimized as much as possible.
Test tools for signal integrity testing
Signal integrity testing for DDR memory requires specialized test equipment to measure and analyze the DDR memory signals. The following are some of the test tools that are commonly used for signal integrity testing of DDR memory:
Test tool #1: Oscilloscope
An oscilloscope is a tool that measures the voltage and timing characteristics of DDR memory signals. This allows for the identification of issues such as timing violations, crosstalk, and reflections.
Test tool #2: Signal generator
A signal generator produces test signals that emulate DDR memory traffic. This can help verify the performance of the memory subsystem under various traffic patterns that can be adjusted according to test parameter settings.
Test tool #3: Time-domain reflector (TDR)
A TDR can be used to measure the impedance characteristics of the DDR memory bus. This can help identify issues such as impedance mismatches, stubs, and other discontinuities.
While oscilloscopes are mostly commonly used for in-system testing and device characterization, signal generators and TDRs are used for device characterization and characterizing the memory channel itself.
Protocol testing for DDR memory
Protocol testing is the process of testing the DDR memory protocol used to transfer data between memory controllers and DDR memory devices. Protocol testing specifications for DDR memory involves verification of memory subsystem compilations according to specifications by the Joint Electron Device Engineering Council (JEDEC).
Because DDR memory protocol defines complex rules for command, address, and data transfer, timing, and error handling, it is important for testers to understand the key parameters involved in ensuring that memory controllers and memory devices abide by the DDR memory protocol.
The following are some of the key parameters that are tested during protocol testing for DDR memory:
Protocol testing parameter #1: Command spacing
Command spacing refers to the verification of commands to ensure that they are neither too close nor too far apart. This is critical during the process of reading and writing data to and from memory devices. For example, reading or writing data when memory is undergoing a Refresh operation would be incorrect.
Protocol testing parameter #2: Data integrity
Data stored in memory devices can become corrupted if command spacing rules are violated. Furthermore, excessive activation can result in the corruption of charge leakage memory contents. These issues may or may not be detected during error code correction (ECC), and therefore must be identified via data integrity checks during protocol testing.
Protocol testing parameter #3: Clock-cycle timing
Clock-cycle timing is critical to DDR memory protocol testing since discrepancies in timing can result in data errors and system instability. The DDR memory protocol defines timing parameters such as tCK, tRCD, tRP, tRAS, tWR, tWTR, and tRRD which specify the timing requirements for various operations such as row activation, column access, and write recovery.
Test tools for protocol testing
Protocol testing for DDR memory requires specialized software and tools to generate and analyze DDR memory traffic respectively. The following are some of the test tools that are commonly used for DDR memory protocol testing:
Protocol test tool #1: Logic analyzer
A logic analyzer can be used to capture and analyze DDR memory bus signals while the system under test is running any software. This can help identify issues such as data errors, protocol violations, and bus contention.
Protocol test tool #2: Traffic generator
A software traffic generator is a diagnostic memory test that can be used to generate DDR memory traffic and simulate different traffic patterns. This can help verify the performance of the memory subsystem under different workloads, including conducting a stress-test to illicit protocol violations.
Protocol test tool #3: Protocol Analyzer
A protocol analyzer, such as the FuturePlus Systems DDRDetective®, can be used to capture and analyze DDR memory traffic in real-time. This can help identify issues such as protocol violations, performance issues, and potential row-hammer events.
The value of protocol testing – a hypothetical scenario
Signal integrity testing and protocol testing are critical components of DDR memory testing. Signal integrity testing ensures that transmitted DDR memory signals have sufficient quality to ensure reliable operation, while protocol testing ensures that the memory subsystem follows the DDR memory protocol correctly.
Specialized test tools such as oscilloscopes, logic analyzers, signal generators, and protocol analyzers, are used to perform these tests. By performing thorough signal integrity and protocol testing, system designers and engineers can ensure the reliable operation of DDR memory in a wide range of computer systems and embedded designs. On the flipside, failure to consider engaging in protocol testing can result in undesirable consequences. For example, consider the following hypothetical scenario that is based on real-world events:
To keep up with increasing demand for new products, TechCo, a company that manufacturers and sells computers to customers, decides to skip on DDR memory protocol testing to reduce production time and save cost. Sales grow initially as the company receives positive feedback for keeping up with demand.
However, TechCo soon starts receiving complaints about system crashes, data loss, and other memory module-related performance issues from computers that have been assembled and shipped without undergoing the appropriate DDR memory module test protocol.
TechCo's reputation takes a hit as more customers respond with negative reviews that drive away new customers from purchasing products manufactured by the company. TechCo is not only forced to recall non-compliant computers, but also has to spend millions of dollars and billable hours recovering their tarnished brand image and broken customer trust.
The recall process ends up being more time-consuming and complicated than anticipated, creating an environment where TechCo employees are constantly overwhelmed with documenting recalls, replacements, and retests.
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