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CMOS setup time is the minimum interval that data must be stable at a receiving sequential element’s input before its active clock edge. It is a requirement of a specific flip-flop or other sequential cell—not a universal value shared by all CMOS devices.
What does CMOS setup time mean?
In a synchronous circuit, a receiving register samples data when its clock makes the active transition. The data must arrive and settle early enough for the register to capture it reliably. The required lead time before that transition is the cell’s setup time.
Intel defines clock setup time as the minimum interval between the assertion of a data-input signal and the low-to-high transition on the clock input. The relevant edge and timing condition depend on the specific device and its specification. Intel’s timing-analysis documentation describes the term.
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For a rising-edge-triggered register, data must be stable for at least the setup interval leading up to the rising capture edge:
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Data: ---- old value ----| new value stable ---------------->
<------ setup time ------>
Clock: _________________________________/‾‾‾‾‾‾‾‾‾‾
↑ active edgeThe data transition shown must occur early enough that the new value remains stable through the setup interval before the edge. The diagram is conceptual; actual timing specifications may define requirements for particular signal transitions and operating conditions.
How setup time differs from a setup check
Setup time is a characteristic of the receiving sequential cell. A setup check is a design-level timing analysis: it tests whether data traveling along a path reaches that cell soon enough, given the clock relationship and modeled delays. Microchip describes a setup check as specifying when data must be present at a sequential component’s input for the clock to capture it effectively. Microchip’s SmartTime documentation explains the check.
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A static timing analyzer evaluates constrained paths without needing input vectors. Its result depends on valid clock definitions, path constraints, clock relationships, modeled delays and uncertainty. False paths must be identified and constrained appropriately; a report cannot establish a meaningful setup result for a path that was modeled incorrectly or omitted from analysis. Microchip’s timing-analysis guidance discusses static timing analysis and constraints.
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How setup slack is calculated
For a setup relationship, AMD defines slack as required time minus arrival time. Required time is the latest time the data may arrive and still satisfy the check; arrival time is when the data reaches the destination. AMD’s slack-calculation guide gives the equations:
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- Data required time (setup) = capture edge time + destination clock path delay − clock uncertainty − setup time
- Data arrival time (setup) = launch edge time + source clock path delay + datapath delay
- Setup slack = data required time − data arrival time
Positive setup slack means the data arrived before the required time under the analyzed assumptions. Negative slack means the path missed the setup requirement in that analysis. Slack is calculated timing margin, not by itself evidence of a measured hardware failure or a universal limit.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How setup relates to hold time
Setup and hold are paired constraints around a capture edge. Setup limits how late data may arrive before the edge; hold limits how soon data may change after it. Passing one check does not guarantee passing the other. AMD states that a transfer between source and destination flip-flops occurs safely when both setup and hold slacks are positive. AMD’s methodology guide explains the relationship.
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How to find the setup-time value for a device
There is no single setup-time number for “CMOS.” The applicable value belongs to a specific sequential cell or device under stated operating conditions. For chip or FPGA work, use the timing table in the exact part’s datasheet, matching the part, speed grade and conditions relevant to the design. Do not transfer a figure from another device or cell. Microchip advises checking the device PDF documentation for device-specific data. Microchip’s SmartTime documentation provides the setup-check context.
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