Multicycle Paths - Perspectives and Intent

Multicycle paths are a common challenge in timing analysis and closure. They occur when a signal takes more than one clock cycle to propagate from a source register to a destination register. In this blog post, we will explore the different perspectives and intents of multicycle paths, and how they can be handled effectively.

Perspectives of multicycle paths

There are two main perspectives of multicycle paths: the designer's perspective and the tool's perspective. The designer's perspective is based on the functional behavior and intent of the circuit, while the tool's perspective is based on the physical implementation and timing constraints of the circuit.

The designer's perspective of multicycle paths is that they are intentional and valid. The designer knows that the signal does not need to arrive at the destination register within one clock cycle, and that it can be safely captured by a later clock edge. The designer may use multicycle paths to optimize the circuit for area, power, or performance, by reducing the number of logic levels or clock frequency. The designer may also use multicycle paths to implement different modes of operation, such as normal mode, low power mode, and ultra low power mode, by changing the clock frequency or gating the clock signal.

The tool's perspective of multicycle paths is that they are potential timing violations. The tool does not know the functional behavior and intent of the circuit, and it assumes that every signal needs to arrive at the destination register within one clock cycle. The tool may report multicycle paths as setup or hold violations, and try to fix them by inserting buffers, resizing gates, or changing placement or routing.

Intent of multicycle paths

The intent of multicycle paths is to communicate to the tool the functional behavior and design constraints of the circuit. By specifying multicycle paths, the designer can instruct the tool to ignore or relax certain timing checks, and to avoid unnecessary or counterproductive optimizations.

There are two main types of multicycle paths: false paths and true paths. False paths are multicycle paths that never occur in reality, due to some functional condition or logic dependency. For example, a false path may exist between two registers that are enabled by mutually exclusive signals, or between two registers that belong to different modes of operation. False paths can be ignored by the tool, as they do not affect the functionality or performance of the circuit.

True paths are multicycle paths that do occur in reality, but are functionally correct. For example, a true path may exist between two registers that are synchronized by a handshake protocol, or between two registers that operate at different clock domains. True paths can be relaxed by the tool, by specifying a larger setup or hold time window for the destination register. This allows the tool to optimize the circuit without violating the functional requirements.

Architectural and design intent of multicycle paths

Multicycle paths are not only a result of physical implementation, but also a reflection of architectural and design intent. The designer may choose to use multicycle paths to achieve certain goals, such as reducing power consumption, increasing performance, or simplifying design complexity. For example, the designer may use multicycle paths to implement a pipeline stage that has a long combinational logic path, or to implement a memory controller that has a variable latency depending on the memory type. By using multicycle paths, the designer can avoid splitting the logic into multiple stages or adding extra logic to handle different latencies.

However, using multicycle paths also comes with some challenges and trade-offs. The designer needs to ensure that the multicycle paths are functionally correct under all possible scenarios, such as changes in temperature, voltage, process variation, or input data patterns. The designer also needs to verify that the multicycle paths do not introduce any glitches or metastability issues in the circuit. Moreover, the designer needs to balance the benefits and costs of using multicycle paths, such as area overhead, power consumption, timing margin, or design complexity.