Clock relationship between reset synchronizer and fanout flip-flops

As we know, all flip-flops which are required to be "out of reset" at the same time are placed in fanout of a single reset synchronizer. In this post, we will discuss if there is any relationship required between clock frequency of reset synchronizer and the clock frequency of the flip-flops in fanout. For now, let us assume that all the flip-flops in the fanout of reset synchronizer work on a single clock "CLK". <dsfdsf> discussed the case when the flip-flops are working on multiple clocks.

Let us first assume that reset synchronizer's clock period is N*CLK_PERIOD; i.e. reset synchronizer gets a DIVIDE_BY_N clock of the flip-flops' clock. Figure 1 below shows the setup check from a clock with period N*CLK_PERIOD to a clock with period CLK_PERIOD. Since, all the flip-flops have same setup check being formed, all will get out of reset at the same edge; thus, fulfilling the requirement.

Similarly, there is a definite setup check from a clock with period CLK_PERIOD/N to a clock with period CLK_PERIOD as shown in figure 2 below. Thus, if reset synchronizer works on clock with frequency "N" times the flip-flops in fanout, we get all the flip-flops out of reset at same time, thereby, fulfilling the requirement again.

Thus, we see that if all the flip-flops in fanout of reset synchronizer work on a single clock, there is no relationship required between frequency of reset synchronizer and frequency of fanout flip-flops as long as we meet the setup and hold requierements.   However, this is not true when flip-flops work on multiple clocks as discussed in <SDFDSF>.

Design problem: Reset synchronizer clock for multi-frequency flip-flops in fanout

Design problem: A set of flip-flops, some working on 100 MHz clock and others working on 200 MHz clock are required to come out of reset together. What should be the clock of reset synchronizer

Solution: Since all the flip-flops are required to come out of reset in the same cycle, all these must get reset from a single reset synchronizer. Now, as the question states that the flip-flops in the fanout of reset synchronizer are working on two clocks. We need to find the correct-by-design clock that reset synchronizer should be working on. Let us assume that the correct clock to be connected to reset synchronizer is one of the two frequencies given.

Figure 1: Reset synchronizer

First, let us check by assuming that reset synchronizer works on positive edge of 100 MHz clock. Figure 2 shows the setup checks for 100 MHz -> 100 MHz and 100 MHz -> 200 MHz paths. Let us say, reset deassertion propagates to R1/Q at edge (1). Going by figure 2, all flip-flops working on 200 MHz clock will be out of reset at edge (3) and all flop-flops working on 100 MHz will be out of reset at edge (5). Thus, reset synchronizer working on positive edge of 100 MHz clock does not solve our purpose.

Figure 2: Reset synchronizer works on positive edge of 100 MHz clock

Now, let us check the same when reset synchronizer works on positive edge of 200 MHz clock. In this case, reset can deassert either on edge (1) or edge (3). If reset deasserts on edge 3, then, we have both the categories of flops coming out of reset at same time edge (5). But if reset deasserts on edge (1), both categories of flops get out of reset at different times. Thus, we can get the reset synchronizer working on 200 MHz clock, but we have to ensure by design that reset gets deasserted on the edge of 200 MHz clock that coincides with negative edge of 100 MHz clock. Figure 3 and figure 4 discuss these scenarios.

Figure 3: Reset synchronizer works on positive edge of 200 MHz clock coinciding with positive edge of 100 MHz clock

Figure 4: Reset synchronizer works on positive edge of 200 MHz clock coinciding with negative edge of 100 MHz clock

Same scenarios are expected as figure 3 & 4 when we make reset synchronizer work on negative edge of 200 MHz clock.

Now, let us explore the last option; i.e., reset synchronizer working on negative edge of 100 MHz clock. In this case, as shown in figure 5, both 100 MHz and 200 MHz flip-flops come out of reset on same edge. Thus, this case works perfectly. Figure 5 illustrates this.

Figure 5: Reset synchronizer works on negative edge of 100 MHz clock

Can you provide any other solution that is possible and better than ones discussed here.