Chapter 5: Switch-Level Descriptions

OVERVIEW

In This Chapter

Understand the concept of describing and simulating digital systems using transistors.
Identify the basic statements of switch-level description in Verilog, such as nmos, pmos, cmos, supply1, supply0, tranif0, tran, and tranif0.
Develop a counterpart VHDL switch-level package that matches the switch-level functions of the Verilog description.
Review and understand the fundamentals of transistors, and how they can be implemented as switches.
Review Boolean functions for combinational circuits.

Switch-level description implements switches (transistors) to describe relatively small-scale digital systems.
Switch-level description is usually implemented in Very Large-Scale Integrated (VLSI) circuit layouts.
Switch-level description is the lowest HDL logic level that can be used to simulate digital systems.
Only small-scale systems can be simulated using pure switch-level description. If the system is not small, a huge number of switches are needed that may render the simulation impractical.
Switch-level description is routinely used along with other types of modeling to describe digital systems.
The switches used in this chapter are assumed to be perfect; they are either open (high impedance) or closed (zero impedance).
In contrast to Verilog, basic VHDL does not have built-in statements such as nmos, pmos, and cmos. To use these statements in VHDL, user-built packages must be developed.

Valence electrons: Electrons in the outer shell of the atom that can interact with the valence electrons of another atom.
N-type semiconductor: The free carriers are negatively charged electrons.
P-type semiconductor:

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