Cyclone® V Device Handbook: Volume 1: Device Interfaces and Integration

ID 683375
Date 10/18/2023
Document Table of Contents External Feedback Mode

In 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.

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.

When using EFB mode, you must use the same I/O standard on the input clock, feedback input, and clock outputs.

This mode is supported only on the corner fractional PLLs. For Cyclone® V E A2 and A4 devices, and Cyclone® V GX C3 device, EFB mode is supported only on the left corner fractional PLLs.

Figure 65. EFB Mode in Cyclone® V Devices

Figure 66. Example of Phase Relationship Between the PLL Clocks in EFB Mode