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[ Team LiB ] 11.1 Switch-Modeling Elements Verilog provides various constructs to model switch-level circuits. Digital circuits at MOS-transistor level are described using these elements.[1] Array of instances can be defined for switches. | Team LiB 11.1 Switch-Modeling Elements Verilog provides various constructs to model switch-level circuits. Digital circuits at MOS-transistor level are described using these elements.111 1 Array of instances can be defined for switches. Array of instances is described in Section 5.1.3 Array of Instances. 11.1.1 MOS Switches Two types of MOS switches can be defined with the keywords nmos and pmos. MOS switch keywords nmos pmos Keyword nmos is used to model NMOS transistors keyword pmos is used to model PMOS transistors. The symbols for nmos and pmos switches are shown in Figure 11-1. Figure 11-1. NMOS and PMOS Switches In Verilog nmos and pmos switches are instantiated as shown in Example 11-1. Example 11-1 Instantiation of NMOS and PMOS Switches nmos n1 out data control instantiate a nmos switch pmos p1 out data control instantiate a pmos switch Since switches are Verilog primitives like logic gates the name of the instance is optional. Therefore it is acceptable to instantiate a switch without assigning an instance name. nmos out data control instantiate an nmos switch no instance name pmos out data control instantiate a pmos switch no instance name The value of the out signal is determined from the values of data and control signals. Logic tables for out are shown in Table 11-1. Some combinations of data and control signals cause the gates to output to either a 1 or 0 or to an z value without a preference for either value. The symbol L stands for 0 or z H stands for 1 or z. Table 11-1. Logic Tables for NMOS and PMOS Thus the nmos switch conducts when its control signal is 1. If the control signal is 0 the output assumes a high impedance value. Similarly a pmos switch conducts if the control signal is 0. 11.1.2 CMOS Switches CMOS switches are declared with the keyword cmos. A cmos device can be modeled with a nmos and a pmos device. The symbol for a cmos switch is shown in Figure 11-2. Figure 11-2. CMOS Switch A cmos switch is instantiated as shown in Example .
