The i.MX 8M Mini and i.MX 8M Plus have one PCIE interface. There is a pair of pins with the name of PCIE _CLK_P/N. These pins are bi-directional which can either be used to feed 100 MHz reference clock to the PHY from external clock source, or to output an internal generated 100 MHz reference clock to PCIE connector or PCIE device. Note that the internal clock exhibits larger jitter than that from PCIE clock generator.

NXP recommends to use an appropriate external PCIe reference clock generator. The NXP EVK board design uses an IDT 9FGV0241device. However, NXP does not recommend one supplier over another, and does not suggest that this is the only clock generator supplier. The device used should support all the specs (jitter, accuracy, etc.)

Route PCIe TX/RX and reference clock signal pairs with 85 Ω differential impedance.

Pin Signal Requirements
75A PCIE_TXN_P It is recommended to use a 0.1 μF cap on both the PCIE_TXP and PCIE_TXN outputs.
100A PCIE_RXN_P PCIe specification compliance requires AC coupling at each transmitter. The receiver must be DC coupled.

PCIe Design Recommendations

Parameter MIN TYP MAX
Operating Speed     4 GHz
Signal Trace Length     12 cm
Differential Pair Skew     1 ps
Differential Trace Impedance 73 Ω 85 Ω 97 Ω
Single-ended Trace Impedance 38 Ω 45 Ω 52 Ω

QS Standard Contact Group Index

Pins Function Pins Function Pins Function Pins Function
1-4 SPI 5-10 I2C 11-14 CAN 15-18 SAI
19-35 Ethernet 36-42 SD Interface 43-48 USB 49-52 Power Supply, Reset and Boot mode
53-76 18-bit RGB Display Interface 77-78 Backlight and Display Control 53-72 Camera 79-88 MIPI DSI Display
89-96 UART 97-100 SPI 1A,75A,76A,100A PCIe 25A,26A,50A,51A USB3 Super Speed