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PCIe 5.0 Equalization Modes: Reducing Link Bring-Up Time

Billions of internet-connected devices and data-intensive real-time applications are expected to appear on the market in the near future and 100 Gigabit Ethernet (GE) speeds, common in data centers today, will just not be fast enough to handle the bandwidth. Therefore, we’re already anticipating the need for data center operators to migrate their networks from 100 GE to 400 GE, creating demand for faster memory and faster serial bus communications.

PCIe is a high-speed, differential, serial standard for point-to-point communications. Each new generation of the PCIe standard offers additional features and faster data transfer rates than the previous generation. The latest generation, PCIe 5.0, will double the throughput rate of PCIe 4.0. The transfer rate of PCIe 5.0 is 32 gigatransfers per second (GT/s) versus the 16 GT/s supported by PCIe 4.0. With 64 gigabytes per second (GB/s) of unidirectional transfer bandwidth, PCIe 5.0 provides data throughout at 128 GB/s of bidirectional traffic.

In addition to doubling the bandwidth, PCIe 5.0 delivers other new features such as:
• Equalization Bypass Modes for faster link initialization
• Allows alternate protocols to negotiate through link training
• Precoding support to help avoid burst errors
• Loopback enhancements allowing to mimic crosstalk behavior

In this blog, we will cover the PHY logical changes at 32 GT/s and walk through the major changes that take place in the equalization procedure.
But, before diving into the new equalization modes, let’s talk about why we need new modes in the first place…

An increase in data rate also means signal attenuation increases, which can affect higher frequency components the most. This ultimately results in distortion which needs to be compensated for through an Equalization Procedure. Like PCIe 3.0 and 4.0, Equalization is a recommended process for a device operating at 32GT/s to adjust the transmitter and receiver setup to improve the signal quality. The equalization phases (phase 0,1,2,3) for PCIe 5.0 remain the same as the previous generations.

Let’s look at the steps involved to bring-up link to 32 GT/s. The link must initially train to L0 at 2.5 GT/s followed by equalization at 8.0 GT/s, 16 GT/s and 32 GT/s sequentially. This is known as the conventional ‘Full Equalization’ Mode.

As data rates get higher and higher, this approach becomes more time consuming and increases latency of link bring-up. As per some vendors, single speed transition with equalization takes about ~100ms, meaning it would take ~300ms to bring-up link to 32GT/s. The need to optimize link bring-up led to two new optional modes being introduced:

To facilitate these optional equalization modes, Symbol5 (Training Control) of TS OS have been modified. Link equalization mode is decided by mutually supported EQ mode advertised in Configuration States

Below are some snippets from Synopsys’ in-house simulations showing link up time to 32GT/s with different equalization modes

Snippet showing 32G link up with ‘full equalization mode’. (takes 111,045 ns simulation time)


Snippet showing 32G link up with ‘Equalization bypass to highest rate’ (takes 41,162ns simulation time)

Snippet showing 32G link up with ‘No equalization required’ mode (takes 30,368 ns simulation time)

 

The graph below shows how link bring-up time to 32GT/s is drastically reduced through new equalization modes:

Synopsys PCIe 5.0 VIP is fully compliant with latest PCIe 5.0 specifications. Our VIP helps to solve the main verification challenge for link equalization to ensure that the LTSSM is transitioning correctly for all three of the equalization modes listed in the figure above.  Selecting the optional equalization features support, allowing to cover the full range of LTSSM state, and speed transition possibilities during link up are all fully configurable. Users can also configure presets, preset hints or coefficients, etc. to ensure the DUT can correctly perform Link Equalization.

Synopsys VIP and Test Suites provide a complete solution with a range of tests to verify various types of DUT (EP DUT, RC DUT, PHY DUT and Retimer DUT) for the PCIe 5.0 equalization feature.