Agilex™ 7 Configuration User Guide

ID 683673
Date 4/01/2024
Document Table of Contents

4.3. Gating the PLL Reset Signal

In older FPGA device families, designs frequently used the PLL lock signal to hold the custom FPGA logic in reset until the PLL locked. In newer Intel device families the lock time of PLLs can be less than the initialization time. In some cases the PLL may lock before the device completes initialization. Consequently, if you use the locked output of the PLL to control resets in the Agilex™ 7 device, you should gate the PLL reset input with nINIT_DONE as shown the figure.

The nINIT_DONE signal, whether driven directly from the Reset Release IP or from an external input through FPGA core logic cells, is unstable during FPGA configuration. Any logic that uses nINIT_DONE to control the reset should be designed to tolerate re-assertion during the FPGA configuration.

Figure 64. Using nINIT_DONE to Gate the PLL_Reset Signal

Another alternative if you are using PLL_Lock in your reset sequence is to gate the PLL_Lock output with the nINIT_DONE signal, (PLL_Lock && !nINIT_DONE).