Signaling Overview
PCIe uses differential signaling for noise immunity and high-speed data transmission.
Signaling Methods by Generation
- Gen1-5 (2.5-32 GT/s): NRZ (Non-Return to Zero) - 2 voltage levels
- Gen6-7 (64-128 GT/s): PAM4 - 4 voltage levels
NRZ Signaling (Gen1-5)
| Parameter | Gen1-2 | Gen3-5 |
|---|---|---|
| Differential Swing (Vp-p) | 800-1200 mV | 800-1300 mV |
| Common Mode Voltage | 0 V (AC coupled) | 0 V (AC coupled) |
| Encoding | 8b/10b | 128b/130b |
| Unit Interval (UI) | 400ps / 200ps | 125ps → 31.25ps |
PAM4 Signaling (Gen6-7)
PAM4 transmits 2 bits per symbol using 4 amplitude levels:
| Parameter | 64 GT/s (Gen6) | 128 GT/s (Gen7) |
|---|---|---|
| Symbol Rate | 32 GBaud | 64 GBaud |
| Bits per Symbol | 2 | 2 |
| Unit Interval | 31.25 ps | 15.625 ps |
| FEC | Required | Required |
| Gray Coding | Yes | Yes |
PAM4 Challenges
With 3 eyes instead of 1, PAM4 has ~9.5 dB less SNR margin compared to NRZ. This requires sophisticated FEC (Forward Error Correction) and equalization.
Impedance Requirements
- Differential Impedance: 85Ω ± 15% (traces) / 85Ω ± 10% (connectors)
- Single-ended: 42.5Ω nominal
- AC coupling capacitors required
Link Equalization
High-speed PCIe requires equalization to compensate for channel loss:
Transmitter Equalization (Tx EQ)
- 3-tap FIR filter: Pre-cursor (C-1), Cursor (C0), Post-cursor (C+1)
- 10 presets defined for Gen3+
- Coefficient negotiation during link training
Receiver Equalization
- CTLE (Continuous Time Linear Equalizer)
- DFE (Decision Feedback Equalizer)
- Adaptive algorithms for optimal performance
Connectors and Form Factors
| Form Factor | Lanes | Use Case |
|---|---|---|
| CEM (Add-in Card) | x1, x4, x8, x16 | Desktop, Server |
| M.2 | x1, x2, x4 | SSDs, WiFi |
| U.2 / U.3 | x4 | Enterprise SSDs |
| OCuLink | x4, x8 | External PCIe |
| EDSFF | x4, x8 | Enterprise SSDs |
Retimers
Retimers regenerate signals for extended reach:
- Full signal regeneration (not just amplification)
- Up to 2 retimers per link
- Essential for long traces and cables
- Participate in link training