2.6.4. State Machine HDL Guidelines
Synthesis tools that can recognize a piece of code as a state machine can perform optimizations that improve the design area and performance. For example, the tool can recode the state variables to improve the quality of results, or optimize other parts of the design through known properties of state machines.
To achieve the best results, synthesis tools often use one-hot encoding for FPGA devices and minimal-bit encoding for CPLD devices, although the choice of implementation can vary for different state machines and different devices. Refer to the synthesis tool documentation for techniques to control the encoding of state machines.
To ensure proper recognition and inference of state machines and to improve the quality of results, observe the following guidelines for both Verilog HDL and VHDL:
- Assign default values to outputs derived from the state machine so that synthesis does not generate unwanted latches.
- Separate state machine logic from all arithmetic functions and datapaths, including assigning output values.
- For designs in which more than one state perform the same operation, define the operation outside the state machine, and direct the output logic of the state machine to use this value.
- Ensure a defined power-up state with a simple asynchronous or synchronous reset. In designs where the state machine contains more elaborate reset logic, such as both an asynchronous reset and an asynchronous load, the Intel® Quartus® Prime software infers regular logic rather than a state machine.
If a state machine enters an illegal state due to a problem with the device, the design likely ceases to function correctly until the next reset of the state machine. Synthesis tools do not provide for this situation by default. The same issue applies to any other registers if there is some fault in the system. A default or when others clause does not affect this operation, assuming that the design never deliberately enters this state. Synthesis tools remove any logic generated by a default state if it is not reachable by normal state machine operation.
Many synthesis tools (including Intel® Quartus® Prime integrated synthesis) have an option to implement a safe state machine. The Intel® Quartus® Prime software inserts extra logic to detect illegal states and force the state machine’s transition to the reset state. Safe state machines are useful when the state machine can enter an illegal state, for example, when a state machine has control inputs that originate in another clock domain, such as the control logic for a dual-clock FIFO.
This option protects state machines by forcing them into the reset state. All other registers in the design are not protected this way. As a best practice for designs with asynchronous inputs, use a synchronization register chain instead of relying on the safe state machine option.
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