One of the many new features included in the PCI Express^® (PCIe^®) specification will be PAM4 (Pulse Amplitude Modulation with 4 levels) signaling. The use of PAM4 reduces the channel loss because it runs at half the frequency with two bits per Unit Interval (UI), enabling the specification’s channel reach to be similar to what the PCIe 5.0 specification delivers. PAM4 will allow the PCIe 6.0 specification to offer enhanced performance and support future PCIe technology products in a variety of markets. This blog provides answers to questions about PAM4 that were asked during the recent PCIe 6.0 Specification webinar.

1. How can the PCI Express 6.0 specification, using PAM4, interoperate with the PCIe 5.0 specification, using Non-Return-to-Zero (NRZ)?

The PCIe 6.0 specification will be fully backwards compatible with the previous five generations of specifications and the prior five data rates, which all leverage NRZ. The circuits can operate in all these modes. Even though the underlying frequency is the same as the PCIe 5.0 specification at 32.0 GT/s NRZ, there will be extra circuitry and logic involved for the PAM4 mode to track three eyes along with the logic changes needed to operate in Flit mode. These will co-exist with the circuitry and logic in the NRZ mode of the prior five generations of specifications. This is similar to the circuitry for the different data rates and logic between 8b/10b encoding and 128b/130b encoding. The additional overhead is necessary for maintaining full backwards compatibility and it produces silicon and platform savings as a result of not needing to allocate dedicated pins and slots for the different modes.

2. How does lane margining work on PAM4 mode?

The upcoming PCIe 6.0 specification will leverage the same infrastructure as the PCIe 5.0 specification for lane margining both in terms of voltage and time margining. The only change is using three eyes instead of one eye. For example, all the three eyes can be margined simultaneously.

3. Will PAM4 still use the same voltage amplitude?

The overall amplitude is the same as the amplitude of the PCIe 5.0 specification, but since there are three eyes, each eye uses reduced voltage. In the NRZ mode, one eye contained the entire amplitude, but in PAM4 mode, the three eyes take parts of the voltage amplitude and each has a fraction of the voltage.

4. Does PAM4 place any new electrical requirements on the channel?

PCI-SIG^® expects the channels to continue improving (e.g., lower losses) over time. The channel type and reach of the PCI Express 6.0 specification are expected to be similar to the PCIe 5.0 specification, with modest improvements. 

Looking for More Information about the PCIe 6.0 Specification?

The recording of the PCIe 6.0 Specification webinar is available to watch anytime on the PCI-SIG YouTube channel. Additionally, this series of Q&A blogs will continue to provide detailed answers to the insightful questions asked during the live presentation. Follow PCI-SIG on Twitter on LinkedIn for updates about these blogs.