Introduction to PCIe 4.0 Electrical Compliance Test

Figure 1: PCI Express is now in its fourth generation
and poses daunting physical-layer test challenges

The Peripheral Component Interface Express standard (PCI Express, or PCIe) has been with us for some 14 years now, a pretty good run by computer-industry standards, and it shows no signs of fading away anytime soon. Now in its fourth generation, which sports data-transfer rates up to 16 Gb/s, PCIe presents daunting physical-layer test requirements (Figure 1).

We've covered electrical compliance test for PCIe 3.0 in some detail, but with the test specifications for PCIe 4.0 rounding into shape, it's time for a deep dive into electrical compliance test for this ubiquitous peripheral interface protocol. The good news is that today's PCIe electrical compliance test equipment is not only up to the task for PCIe 4.0, but it's locked and loaded for the upcoming PCIe 5.0 specification that could see rollout in 2019. And, of course, it's fully capable of handling PCIe 3.0 testing for those who haven't made the jump to higher bit rates.

First, let's look at some PCIe terminology and a little bit of its history. PCIe standards are maintained by the Peripheral Components Interface Special Interest Group (PCI-SIG). It's important to understand that there are actually three specifications for each generation of PCIe:

• The base specification defines device behavior at the chip level
• The Card ElectroMechanical (CEM; say "chem") specification defines device behavior at the card-edge connector
• The test specification describes precisely how to test a device for CEM-spec compliance

The CEM specification addresses the point at which a PCIe device connects to a PCIe motherboard, which is the real-world test point for interoperability compliance. Thus, the PCIe test specification derives directly from the CEM specification.

When a device is found to be CEM-compliant at a PCIe workshop, it then gets added to the "integrator's list" and is thenceforth an officially PCI-compliant device,

Each of the three specifications is updated to a new version for each new generation of the PCIe standard:

• 2005: PCIe 1.0, 2.5 Gb/s
• 2007: PCIe 2.0, 5 Gb/s
• 2010: PCIe 3.0, 8 Gb/s
• 2018: PCIe 4.0, 16 Gb/s

Figure 2: The timetable for PCIe 4.0 rollout remains in flux

As of now, we're in the process of PCIe 4.0 test development, which is expected to be completed this year. With PCIe workshops ongoing, changes are made to the test procedures themselves and to the PCIe 4.0 timetable (Figure 2).

PCIe workshops, at which devices are tested for electrical compliance, come in three flavors:

• Preliminary workshop: Primary purpose is test and specification development. Test-equipment vendors (such as Teledyne LeCroy) and PCIe-device vendors bring their respective wares and test procedures are run through for subsequent correlation testing. Test results at preliminary workshops are not required to be shared with device vendors.
• FYI workshop: PCIe-device vendors receive pass/fail results only; these results have no bearing on the PCIe 4.0 integrator's list of compliant devices. Typically, at least two FYI workshops take place before official compliance workshops commence.
• Compliance workshops: Currently, compliance workshops are being held for PCIe 3.0 devices. The 3.0 test specification is fully complete and approved. Devices that pass these tests are added to the PCIe 3.0 integrator's list.

It's currently expected that PCIe 4.0 compliance workshops will begin around mid-2018. So while there's lots of industry chatter about PCIe 4.0, there is as yet no official compliance testing taking place. Having said that, test-equipment vendors, Teledyne LeCroy included, are delivering PCIe 4.0 test solutions now so that developers can test their works in progress. In doing so, they boost their chances of a passing test result when official compliance workshops kick off.

In the next post in this series, we'll take a look at test equipment requirements for PCIe 4.0 electrical compliance testing.