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The approach is characterized by design goals like “keeping the human in the loop and in control” and the proposal that “smart spaces make people smarter”. Therefore, we’d better put the “Comb.” block after the DFF3 flip-flop as shown in Figure 6.This paper presents different manifestations and problems of the ‘smart-everything’ paradigm, provides a critical reflection of its implications and proposes a human-centered design approach resulting in the provision of ‘people-oriented, empowering smartness’. The delay between the clk1 rising edge and the En_Out transition corresponds to the clock-to-Q delay ($$T_$$. Since the En_Out signal is generated by the A clock domain, its low-to-high transition can occur after a rising edge of clk1 as shown in the figure. Figure 1 shows the register generating the enable signal in A and the register receiving it in B.Īssume that the clock waveforms are as shown in Figure 2 and the system is rising edge-triggered. This connection corresponds to an active-high enable signal that initiates an algorithm in B after a particular operation is done by A. In our simple example, the B section has an input, En_In, which is connected to the En_Out output of the A section. The Metastability ProblemĪssume that we have two sections of logic, A and B, that operate at 50 MHz and 100 MHz, respectively. The next section discusses some of the problems that we may face when using a multiple-clock system. Having different clock domains can be beneficial but is not as easy as it seems to be. As you can see, there are many circumstances in which we need to employ different clock rates for different parts of the system.Ī section of the design in which all the synchronous elements, such as flip-flops and RAMs, use the same clock signal is referred to as a clock domain. Obviously, this is not reasonable because not only we’ve overdesigned a large portion of the system (parts that could be operated at 20 MHz) but also we’ve unnecessarily increased the system dynamic power consumption. If we decide to use one clock signal for the entire system, then we would have to operate the system at 100 MHz to accommodate the highest clock rate available in the system.
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For example, assume that the whole system can be operated at 20 MHz except for a subsystem which needs a 100-MHz clock. Sometimes, we may be able to choose the clock frequency for the different parts of the system but, even in this case, it may not be a good idea to operate the entire system at a given clock frequency. Note that the clock frequency of the interfaces can be predefined and we may not be able to choose it based on the clock utilized inside the FPGA. Here, we have to deal with three different clock frequencies. For example, consider an FPGA design operating at 20 MHz which is communicating with two external devices using interfaces operating at 100 MHz and 150 MHz. However, this methodology doesn’t always give the most efficient solution and sometimes it’s not even possible to have a single clock for the whole system. The general digital design methodology recommends using one clock signal for the entire system mainly because it simplifies both the design procedure and the system timing analysis. This article will discuss a well-known technique called “double flopping” to transfer a single-bit control signal between two clock domains.
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Since the clock signals of different clock domains are independent in general, transferring data between the different clock domains can be a challenging task.
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It is common to employ several clock signals in a digital system.