Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

ID 683323
Date 9/24/2018
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1.1.1. Implementing Synchronous Designs

In a synchronous design, the clock signal controls the activities of all inputs and outputs.

On every active edge of the clock (usually the rising edge), the data inputs of registers are sampled and transferred to outputs. Following an active clock edge, the outputs of combinational logic feeding the data inputs of registers change values. This change triggers a period of instability due to propagation delays through the logic as the signals go through several transitions and finally settle to new values. Changes that occur on data inputs of registers do not affect the values of their outputs until after the next active clock edge.

Because the internal circuitry of registers isolates data outputs from inputs, instability in the combinational logic does not affect the operation of the design if you meet the following timing requirements:

  • Before an active clock edge, you must ensure that the data input has been stable for at least the setup time of the register.
  • After an active clock edge, you must ensure that the data input remains stable for at least the hold time of the register.

    When you specify all your clock frequencies and other timing requirements, the Intel® Quartus® Prime Timing Analyzer reports actual hardware requirements for the setup times (tSU) and hold times (tH) for every pin in your design. By meeting these external pin requirements and following synchronous design techniques, you ensure that you satisfy the setup and hold times for all registers in your device.

    Tip: To meet setup and hold time requirements on all input pins, any inputs to combinational logic that feed a register should have a synchronous relationship with the clock of the register. If signals are asynchronous, you can register the signals at the inputs of the device to help prevent a violation of the required setup and hold times.

    When you violate the setup or hold time of a register, you might oscillate the output, or set the output to an intermediate voltage level between the high and low levels called a metastable state. In this unstable state, small perturbations such as noise in power rails can cause the register to assume either the high or low voltage level, resulting in an unpredictable valid state. Various undesirable effects can occur, including increased propagation delays and incorrect output states. In some cases, the output can even oscillate between the two valid states for a relatively long period of time.