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This article mainly discusses the use of combinational logic or sequential logic for output signals in FPGA
Eliminating Glitches:
The output of combinational logic changes immediately with input changes. If the input signal changes along different paths (resulting in race conditions), or if there are inherent delays in the logic gates, the output may experience brief, unexpected pulses (glitches).
These glitches can be catastrophic for downstream circuits (especially synchronous circuits), potentially leading to erroneous operations, state machine missteps, incorrect counting in counters, etc.
The output of sequential logic (registers) is only updated on the active edge of the clock (rising or falling edge). Between clock edges, regardless of how the internal combinational logic changes (including generating glitches), the output of the register remains stable. This fundamentally eliminates glitches on the output signal, ensuring a “clean” output signal.
Improving Timing Performance and Predictability:
Setup/Hold time requirements: After registering the output signal, the timing of output changes is strictly limited to the clock edge. This makes it easier for downstream circuits (if they are also synchronous) to meet their input setup and hold time requirements. The effective window of the output signal is very clear (stable for a period after the clock edge).
Simplifying timing analysis: After registering the output, the path from the FPGA internal register to the output pin becomes a standard register-to-output path. This simplifies the work of static timing analysis tools, making it easier to ensure that this path meets timing constraints at the target clock frequency. Combinational logic outputs require additional consideration of the delays introduced by the combinational logic itself on output timing, making the analysis more complex.
Reducing the impact of output path delays on internal logic: Output pins often need to drive external loads, with longer traces, and their load capacitance and impedance may be significant, leading to relatively large and difficult-to-control delays from the output buffer to the actual pin signal. If this delay is included in the combinational logic path, it can significantly increase the total delay of that path, potentially making it a critical path that is hard to meet timing. Register outputs isolate the delay of internal combinational logic calculations from the high-load output driving. Internal logic only needs to stabilize the results at the register input before the clock edge, while the delay from the register to the pin is a relatively fixed and independently analyzed path.
Enhancing Design Reliability and Robustness:
Stable, glitch-free output signals greatly enhance the reliability of the entire system (FPGA + external circuits).
The behavior of sequential logic outputs is more predictable, and the design intent is clearer.
Cross-clock domain processing is easier: If the output signal needs to be passed to another clock domain, using register outputs at the source is a prerequisite for reliable synchronization (such as dual-flip-flop synchronization). Direct combinational logic outputs for cross-clock domain synchronization are extremely dangerous and unreliable.
Complying with Synchronous Design Principles:
Modern digital system design, especially FPGA design, strongly advocates synchronous design methodologies. The core idea is that all signal changes occur on the active edges of the clock. Registering all module inputs and outputs is one of the key practices of synchronous design, minimizing the uncertainty brought by asynchronous behavior.
In simple terms: To achieve stable, reliable, and predictable outputs, place the output signal in a register and let it update on the clock edge.
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