184.108.40.206. External Feedback Mode
In external feedback (EFB) mode, the output of the M counter (fbout) feeds back to the PLL fbin input (using a trace on the board) and becomes part of the feedback loop. EFB mode is only supported for I/O bank I/O PLL, not supported for fabric-feeding I/O PLL.
One of the dual-purpose external clock outputs becomes the fbin input pin in this mode. The external feedback input pin, fbin is phase-aligned with the clock input pin. Aligning these clocks allows you to remove clock delay and skew between devices.
In EFB mode, you must use the same I/O standard on the input clock, feedback input, and clock outputs.
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