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will127534 / CM4-Nvme-NAS

A simple board for Raspberry Pi Compute Module 4 plus 4x NVMe SSD using ASM1184e
https://will127534.github.io/CM4-Nvme-NAS/
MIT License
74 stars 9 forks source link

+/- of PCIe RX and TX differential pairs flipped? #5

Closed dumtux closed 1 month ago

dumtux commented 1 month ago

Looking at your schematics, I found interesting flipped differential pair connection. TX and RX are flipped to M.2 M connector. But CLK is not flipped. Could you explain why? (It's same for all four M.2 connectors.)

Flipped TX and RX

ASM1184 M.2 M conn 1
PCIe_TX_N_1 (49) PETp0
PCIe_TX_P_1 (47) PETn0
PCIe_RX_N_1 (43) PERp0
PCIe_RX_P_1 (41) PERn0

Not-flipped CLK

ASM1184 M.2 M conn 1
PCIe_CLK_P_1 (55) REFCLKp
PCIe_CLK_N_1 (53) REFCLKn
dumtux commented 1 month ago

I found that PCIe is diff pair polarity agnostic. Further reading: https://www.teledynelecroy.com/doc/understanding-lane-reversal-and-polarity