Three-state gate

A three-state gate is modeled by assigning the value z to a variable under the control of a condition. Here is an example. 

The variable MainBus is three-state ed as long as ReadState is true. If ReadState is false, the value of CpuBus is assigned to MainBus. 

Here is a model that detects three l s in a data stream appearing on input Data. The input is checked on every falling edge of clock. If three consecutive 1 s are found on the input, the output is set to true, else it is set to false. 

Synthesis infers four flip-flops for this model, three for variable Previous and one for SeqFound. However, optimization reveals that one of the flip-flops for Previous is not necessary and hence it is removed. In this model, the output is latched since it is assigned a value under the control of a clock edge. If a latched output is not desired, then the assignment to SeqFound must be done outside the always statement. Such a module is shown next. 

In this module, the output SeqFound is not latched. Synthesis infers three flip-flops for variable Previous note that in this case, all the bits of Previous have to be latched. 

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The value z is used to generate a three-state gate. The value z can be assigned to a variable in an assignment statement however for synthesis, such an assignment must occur under the control of a condition, either in an if statement, or in a case statement. Here is an example. 

Figure 2-62 A conditional z value assignment produces a three-state gate.

A three-state gate can also be obtained by assigning the value z in a conditional expression, as shown in the next example. 

Figure 2-63 Three-state gate using a conditional expression.

Whenever a signal is assigned die value Z a three-state gate will be syn-diesized, as shown in the example below. 

Like other voltage-gated cation channels, Ca2+ channels exist in at least three states A resting state stabilized at negative potentials (such as the resting potentials of most electrically excitable cells) that is a closed state from which the channel can open. The open state is induced by depolarization. Channels do not stay open indefinitely because they are turned off during prolonged depolarization by transition into an inactivated state. Inactivation is driven both by depolarization... 

Three NOT gates (model 4), and one AND gate (model 3). Inputs concentrations of T and D. Output steady-state concentration of D3. 

Figure 2 A simplified three-state model of sodium channel gating.

Fig. 5. Density of states of the Au-C60-Au junction versus energy, for electrode separation of 13.7 A, V(, = 0. The curves are for three different gate voltages. Other system parameters are the same as in Fig. 4. Positions of the molecular levels are depicted over the peaks of IK )S( the triangles, circles, and squares correspond to Vg = 2.7 V, Vg = 0 V, and Vg = —2.7 V, respectively. The degeneracy of the original LUMO level is removed due to the presence of Au leads. Inset excess charge SQ (in units of e) in the C60 versus gate voltage. Due to the wider electrode separation, the equilibrium excess charge is smaller, 0.5e, and the DOS features are sharper due to a weaker coupling of the C60 to the leads.

Fig. 1. An equivalent circuit of three transfer gates of a-Si H charge-coupled devices. A central transfer electrode represented by a capacitor is assumed to be in a high state and to store signal electrons. Transfer electrodes of both sides are assumed to be in a low state.

Figure 5.3. The three states of a voltage-gated Na channel, and their inhibition by ligands binding to the open and inactive states, respectively.

The thyristor is a semiconductor device made of germanium or silicon wafers and comprises three or more Junctions, which can be switched from the OFF state to the ON state or vice versa. Basically it is a ptipn junction, as shown in Figure 6.20(a) and can be considered as composed of two transistors with npn and pnpjunctions, as illustrated in Figure 6.20(b). It does not turn ON when it is forward biased, unlike a diode, unless there is a gate firing pulse. Thyristors are forced commutated (a technique... 

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