1、英文资料及中文翻译 FLIP-FLOPS 1 Intorduce In this passage, we show how to design flip-flops, which operate as one-bit memory cells. Flip-flops are also called latches. Logic circuits constructed using flip-flops can have the present output be a function of both the past and present inputs. Such circuits are
2、called senfiential logic circuits. All flip-flops are based on the same principle: Positive feedback is used to produce a circuit that is bistable . A bistable circuit is one that has two stable operating points. Which operating point the circuit is in is called the state of the circuit. If the stat
3、e can be sensed and changed, then the circuit can function as a one-bit memory element. The simplest bistable circuit is constructed using two inverters in a loop as shown in Figure 1 1.This circuit only has two nodes, A and B. Because of the inverters, if A is high, B must be low and vice versa； he
4、nce, the circuit has two stable states. The operation of the bistable circuit can also be viewed using a plot of the transfer characteristic of the two inverters in series, as shown in Figure 1 2. Part (a) of the figure shows the static transfer characteristic of one of the inverters. When the input
5、 voltage is below the threshold (a logical ZERO), the output voltage is high (a logical ONE). When the input voltage is greater than the threshold, the output voltage is low. In part (b) of the figure, we show the transfer characteristic that results from putting both inverters in series. Any soluti
6、on of the equations for this circuit must also lie on this characteristic. Because of the external connection, the input and output voltages of the series connection of the two inverters must be the same. Therefore, we draw a line with a slope of unity on the plot as well. This line is called the lo
7、ad line, because it represents the external load connection for the two inverters in series. Any solution of the equations for this circuit must also lie on the load line. Therefore, when the equations are simultaneously solved, the only possible operating points are found where the straight line in
8、tersects the transfer characteristic. There are three intersections on the plot, but only two of them are stable, as we will now demonstrate. The point where the load line intersects the middle of the transfer characteristic is not stable. To see that this statement is true, suppose for the moment t
9、hat the circuit is at this point. If the input voltage increases at all (due to noise or some change in the circuit), the output voltage of the inverters must also increase. But the output is input, so as it increases, it causes further increases in the output, and the original change is magnified.
10、This positive feedback will quickly drive the circuit to the top operating point shown. At that point, the input and output of the two-inverter chain are high and the midpoint (B in Figure 1 1) is low, so the circuit is stable and can remain in this state forever. If we started at the midpoint and l
11、et the input voltage decrease a bit, we would end up at the lower operating point, which is again stable. In the sections that follow, we show how we can move this bistable circuit from one operating point to the other. The internal positive feedback will then hold the circuit at that state until we
12、 deliberately change it； hence, the circuit has memory. Figure 1 1A bistable circuit (a) (b) Figure 1 2 (a) One inverter and its transfer characteristic (b) The transfer characteristic for two inverters in series and the load line for the circuit 2 The Set-Reset Flip-Flop A set-reset (SR) flip-flop
13、is shown in Figure 2 1(a). A table describing the function of the circuit is shown in part (b) of the figure, and the schematic symbol is A BVi VoViVoVoViVi Vo=shown in part (c). This function table is similar to a truth table, but it describes a dynamic situation, not a static one. The output is th
14、e output at some discrete time, denoted by Qn, and the table includes an entry for the previous state of the flip-flop (Qn-1). Although the circuit is drawn differently, the two NOR gates are in series, just like the inverters in Figure 1 2(b). The configuration shown here is usually described as cr
15、oss coupled. The flip-flop has two outputs that are complements of each other. We usually consider the Q output to be the state of the flip-flop. (a) S R Qn 0 0 Qn-1 0 1 0 1 0 1 1 1 不允许的 （ b） (c) Figure 2 1 (a) An SR flip-flop, (b) a table describing the circuits function (c) the schematic symbol. T
16、he circuit operates in the following way: If both inputs (S and R) are zero, the previous state is retained. Suppose, for example, that Qn-1 is high (i.e., ONE). Then the output of the bottom NOR, which is Q n-1 , will be low (i.e., ZERO), independently of what S is. In this case, both inputs to the top NOR are low, so its output is high, as originally assumed. Now suppose that Qn-1 is low. In this case, both inputs to the bottom NOR are low, so Q n-1 is high. Therefore, the output of the top NOR, Qn-1, will be low, as assumed. RSQQQQSR
