An Under-the-Hood View of PCIe 3.0 Link Training (Part I)

Figure 1: An overview of the elements of PCIe 3.0
dynamic link equalization

Now that we've looked at the basics of PCIe 3.0 dynamic link equalization and at some of the particulars of de-emphasis and preshoot, it's time to dive a little deeper into what actually happens in the link training process. It all happens in the blink of an eye but there's enough going on to warrant some dissection.

PCIe 3.0 Dynamic Link EQ: De-Emphasis, Preshoot, Cursors, and Presets

Figure 1: De-emphasis, a key transmit-side equalization
technique for PCIe 3.0, boosts high-frequency content

In an earlier post, we looked at some of the basics of dynamic link equalization for PCIe 3.0, and in particular the reasons why it's not only necessary but mandated by the PCI-SIG for compliance testing. Essentially, the boost in data rates from 5 GT/s in PCIe 2.0 to 8 GT/s in PCIe 3.0 wreaked havoc in terms of signal integrity in the channel. The solution is found in equalization both before (TxEQ) and after (RxEQ) the channel.

The Hows and Whys of PCIe 3.0 Dynamic Link Equalization

Figure 1: SI problems are the root cause for
dynamic link equalization in PCIe 3.0

If you're designing a computer peripheral these days, chances are that you'll use the Peripheral Component Interconnect Express (PCIe) protocol for communication between the device and the host system. With the emergence of PCIe, a bunch of older bus standards were kicked to the curb. PCIe itself became the basis for more specialized standards, most notably ExpressCard for laptop expansion cards and SATA Express for storage interfaces.