One of the guys asked me a question, "Why is hold always checked on the same edge?" Normally, it is taught in books/colleges that hold is frequency independent because it is checked on same edge. But, is it really true? It is true only for some of the many cases. Hold can be checked on the same edge, next edge or previous edge depending upon the scenario. In this post, we will discuss those cases one by one, and try to generalize if this statement holds true.
How to determine the edge on which hold check needs to be checked: For most of us, it seems quite confusing to arrive at the conclusion of how to determine the hold edge. Let us try to use a state machine perspective here. In state machine theory, we study that synchronous digital circuits can be considered as state machines moving from one state to another. This state transition happens on each clock edge as shown in figure 1 below.
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| Figure 1: Each clock edge corresponds to a design state |
If we look at each flip-flop, every positive edge-triggered flip-flop changes its state at positive clock edge and all negative edge-triggered flip-flops transition state at negative clock edge. Similarly, all negative edge-triggered flip-flops transition state at negative clock edge as shown in figure 2 below.
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| Figure 2: State transition for positive edge-triggered and negative edge-triggered flip-flops |
Or, we can represent the states of positive edge-triggered and negative edge-triggered flops as separate as shown in figure 3 below.
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| Figure 3: States of positive and negative edge-triggered flops represented symbolically |
Let us have a scenario of a timing path from a positive edge-triggered flop to a positive edge-triggered flop. In the figure 4 below, flip-flop "2" transitions to state (K+1) depending upon the value of flip-flop "1" at state (K).
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| Figure 4: A sample timing path from positive edge-triggered flip-flop to positive edge-triggered flip-flop |
Here, the data launched from ff1 should help ff2 transition to state "K+1", meaning, it should be captured at the corresponding clock edge. This represents setup check. Also, it should not disturb state "K" of ff2, meaning it should not get captured at this edge. This represents hold check. So, in this case hold check is on the same edge as the present state of start and end flops is the same edge.
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| Figure 5: Setup and hold checks for positive-to-positive edge-triggered timing paths |
Now, let us take a look on the scenario where-in hold check is not on the same edge. Let us take a timing path launching data from negative edge and capturing at positive edge. This scenario is shown in figure 6 below.
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| Figure 6: Timing path from negative-to-positive edge-triggered flop |
Here, positive edge-triggered flip-flop transitions states on positive edge and negative edge-triggered flop transitions on negative edges. So, the data launched from negative edge-triggered flop corresponding to state "X" should get captured on positive edge-triggered flop on state "Y+1", which corresponds to setup check. And it should not get captured on state "Y", which corresponds to hold check.
Thus, we have looked upon different cases of hold capturing edge being same or different than the launch edge. For all the possible cases of setup and hold checks, you can follow below posts: