This may seem bad as we must not let the combinational logic to be too fast but we must ensure this due to non-idealities in the circuit. If the combinational logic is too fast, we may get a Thold violation in R2 because then the transition in Q1 is quickly reflected at D2 within lesser time than Thold for R2. This means that there is both a minimum and maximum delay between the two flip-flops R1 and R2. To be specific, the input D2 must be stable for at least Tsetup time before the clock edge and at least until Thold time after the triggering clock edge as discussed under the input timing constraints. Observe that the clock edge itself takes a finite time to raise which we have discussed in the previous article.īasically, we need to ensure correct input timing on R2. Figure 2 illustrates the above discussed timing constraints for the input of a delay flip-flop. So the aperture time is simply the time around the clock edge for which the data must be stable (Ta = Tsetup + Thold). Hence the input needs to be stable for some time before and after the triggering clock edge until it gets latched correctly. Hold Time (Thold): It's simply the amount of time after the clock edge for which the data (input 'D') must be stable. Setup Time (Tsetup): It's simply the amount of time before the clock edge for which the data (input 'D') must be stable (i.e. Setup time and hold time are defined as follows: It is similar to the aperture of a camera where one needs to keep the aperture long enough to capture a photo nicely. These constraints are together called as the Aperture time (Ta). To be able to sample it correctly, the circuit needs to satisfy two timing constraints called as Setup time and Hold time. The input 'D' must be stable when sampled by the rising or falling edge of the clock. Assuming that you know about the working of a D (Delay) flip-flop, its input 'D', output 'Q' and clock have timing requirements. Now that we have understood the significance of Sequential circuits, let's dive into the timing constraints of Sequential circuits. 'Y' is an internal state output and 'Z' is the final external output.įigure 1: Components of a Sequential Circuit (Abstract view) Input Timing Constraints Figure 1 illustrates the basic idea of a sequential logic circuit containing both the combinational and sequential logic elements. Sequential logic circuit elements sample and store an output from the combinational logic and this output is fed back to the combinational circuit in the next clock cycle to keep the state machine going from one state to another until it reaches a desired state and we get the required output. Basically, all the circuits in digital systems are a blend of Combinational and Sequential logic circuits. These circuits are used to construct Finite State Machines (FSMs) which are the basic building blocks of all the digital circuitry. In this article, we're gonna look at timing in more real circuits i.e. In the previous article ( ) of this series, "A Primer on Timing and Verification in Digital Circuits", we dealt with timing in Combinational circuits and glitches.
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