Intel® Agilex™ Configuration User Guide

ID 683673
Date 7/05/2022

A newer version of this document is available. Customers should click here to go to the newest version.

Document Table of Contents

4.5.3. Protecting State Machine Logic

To guarantee correct operation of state machines, your reset logic must hold the FPGA fabric in reset until the entire fabric enters user mode.

The following example shows how an inadequate reset strategy might result in an illegal state in a one-hot state machine. In this example, the design does not reset any of the state machine registers. The state machine design depends on registers entering an initial state. Without an adequate reset, this state machine begins operating when part of the device is active. Nearby logic included in the state machine remains frozen, before INIT_DONE asserts.

Figure 60. Partially Initialized Design - INIT_DONE = 0

Register B in the active section is operational and takes on the value of Register A in the next clock cycle. Register A is still in the freeze register state and does not respond to the clock edge. Register A remains in the current state.

Figure 61. Advance One Clock Cycle, Device Completely In User Mode - INIT_DONE = 1

The entire fabric is now in user mode. The state machine enters an illegal or unknown state with two ones in a one-hot state machine. To prevent this illegal state, use the Reset Release IP to hold the circuit in reset until INIT_DONE asserts indicating that the entire fabric has entered user mode.