PCIe is not operating

Shun

Hello,

PCIe is not operating in my UP Core.
PCIe reference clock is not outputted to 100-pin expansion.
Correctly, the clock is outputted immediately after supplying DC power to UP Core.
(I can see a 100-MHz signal with an oscilloscope.)
However, the clock is stopped just before BIOS starts.

My UP Core is 4GB + 64GB eMMC memory ver.
(RE-UPC-CHT01-A10-0464)

BIOS ver. is UCR1BM10.
All the setup of BIOS is initial values.

The clock request (CLKREQ#) is asserted.
(Voltage is Low level.)

Is it necessary to change the setup of BIOS in order to solve?
Or is software, such as a driver, required?

Thank you.