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PCB layout becomes more and more important for computer systems because of the trend to faster, higher integrated, smaller form factors,and lower power systems using the PCIe® interface. The higher the switching frequencies are, the more radiation occurs on a PCB. With good layout, many EMI problems like reflection or crosstalk can be minimized to meet the required specifications.
Many things can affect PCB transmission lines, like impedance mismatch or intra-pair skew and inter-pair skew of trace, EMI problems can occur. To reduce these problems, good PCB design is important, and with some simple design rules,the PCB designer can minimize these problems.
There are many differences in the various high-speed standards that need to be taken into account when designing the layout of a system. These differences include parameters like data-rates/frequency, AC coupling capacitors, inter-pair skew, intra-pair skew and trace impedance. Table 2-1 lists the standard values for PCIe standard.
| Parameter | Value |
|---|---|
| Frequency | PCIe® Gen 1: 1.25 GHz (2.5 Gbps) |
| PCIe® Gen 2: 2.5 GHz ( 5 Gbps) | |
| PCIe® Gen 3: 4 GHz (8 Gbps) | |
| PCIe® Gen 4: 8 GHz (16 Gbps) | |
| AC Coupling Capacitors | AC capacitors required:
75 nF–220 nF |
| Polarity Reversal | Allowed |
| Max Intra-Pair Skew | 5 mils |
| Max Inter-Pair Skew | No Inter-pair specification |
| Trace Impedance | PCIe® Gen 1 and 2:100 Ω ±5% differential; 50 Ω ±5% single ended |
| PCIe® Gen 3 and 4: 85 Ω ±5% differential; 42.5 Ω ±5% single ended |