Can the KSZ family of devices (particularly KSZ8041) operate with a reference clock slower than 25 MHz?

No, the devices cannot run with a slower reference clock.

The KSZ8041 and all 10/100 Mbps Ethernet PHYs generate line side signaling and timing, as defined under the IEEE 802.3 Standard, based on the local reference clock (usually 25 MHz, but can be 50 MHz for PHYs with RMII). It is not possible to slow down the reference clock and achieve proper operation.

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