What is PCIe (Peripheral Component Interconnect Express)?

PCI Express or PCIe (Peripheral Component Interconnect Express) is a high-speed interface technology, typically used in a computer system to connect the CPU / memory subsystem (root complex side) to its peripheral components (endpoint side), such as graphics cards, storage devices, network cards, etc.

How does a PCIe link work?

Depending on the link width, a PCIe link consists of 1, 2, 4, 8 or 16 data lanes, each lane providing a bi-directional signal path between the two PCIe devices. PCIe revisions are backward compatible, i.e., every new revision also supports the data rates from the previous PCIe revisions and must be tested accordingly.

With the continuous need for higher processing power, the data rates provided by PCIe have continuously increased with each generation.

Evolution of PCIe link speed

Why is PCIe compliance testing important?

Compliance testing ensures that different PCIe devices from various vendors can work together seamlessly to offer optimal performance and reliability. Without compliance validation, signal integrity issues, timing violations or protocol errors can lead to unstable systems, data loss and performance degradation.

Compliance testing is mandatory for any vendor seeking to bring a PCIe-certified product to market, with specific test requirements depending on the device type.

The PCIe standard

The PCIe standard, technical specifications and device certification are managed by the PCI Special Interest Group (PCI-SIG). These specifications help to ensure industry-wide interoperability of PCIe systems. In order to meet the evolving demands of modern applications, the PCI-SIG consistently updates these standards across each new PCIe generation.

Key challenges in PCIe testing

PCIe compliance testing and design validation involve multiple test disciplines, each with its own technical complexity. The following overview highlights the primary challenges across all PCIe test areas:

• Time-consuming test setup: Manually setting up compliance tests can be a lengthy process – whether for interface, cable or clock testing.
• Test fixtures and software requirements: Identifying the necessary hardware, software, test fixtures (e.g. Compliance Load Board (CLB) and Compliance Base Board (CBB)) and accessories for effective testing can be complex and confusing.
• DUT configuration: Properly configuring the PCIe System Board or Add-in Card for compliance testing can be challenging to manage.
• Root cause analysis for test failures: When compliance tests fail, conducting a thorough root cause analysis to identify and resolve issues can be a daunting task.
• Measurement path characterization and deembedding: Particularly for chip testing, the measurement path from the oscilloscope up to the pin level of the chip needs to be characterized so that it can be removed from the measurement results. Accurate test fixture characterization and deembedding is required to achieve correct results.

PCIe reference clock testing

The PCIe reference clock (RefClk) is a periodic signal that serves as the timing reference for the entire PCIe link. Both the transmitter and receiver in a PCIe system rely on this reference clock to synchronize the pace at which data is transmitted. Consequently, the quality of the reference clock directly dictates the PCIe link’s overall signal integrity: a noisy or jittery reference clock introduces phase noise and timing errors that spread through the entire system, ultimately risking data corruption or transmission failures.

Jitter requirements for PCIe reference clocks

The performance requirements for PCIe reference clocks are defined in the PCI Express Base Specification. Along with the increasing data rates and the shrinking budgets for the TX jitter, the jitter requirements for the RefClk are also getting tighter with each new PCIe revision.

• The RefClk jitter values in the PCI Express Base Specification describe the effect of the RefClk jitter on the PCIe system at the RX latch. This is modeled by a weighting with the behavioral TX PLL, RX PLL and CDR transfer functions. For each transfer rate, a specific set of transfer functions is defined, and the weighted jitter values for all combinations need to be calculated and compared to the limit.
• As PCIe systems are typically operated with Spread Spectrum Clocking (SSC), the RefClk jitter also needs to be measured in SSC mode.
• To measure the true performance of the PCIe RefClk, the jitter measurement floor of the test equipment needs to be significantly lower than the jitter performance of the measured RefClk.

PCIe cable and connector testing

PCIe cables and connectors are the critical links that enable high-speed data transfer and reliable power delivery between system components. As PCIe technology continues to evolve toward higher data rates, ensuring the signal and power integrity of these interconnects through rigorous testing is more crucial than ever.

Multi-port testing complexity: x4 and x8 cable configurations

PCIe x4 and x8 cable configurations require measurement setups with 32 and 64 ports and require a total of 64 and 256 4-port measurements. The calibration of this setup can be quite cumbersome and error-prone. For the crosstalk tests, the unused ports need to be terminated for correct measurement results. Without automation, the measurement procedure is extremely time consuming and prone to connection errors. Efficient calibration routines and automated port switching are therefore essential for productive PCIe cable testing.