Switch Level Modeling

The switch level modeling is used to model digital circuits at the MOS level transistor. In this era, digital circuits have become more complex and involve millions of transistors, so modeling at the transistor level is rarely used by the designer. Hence, mostly higher abstraction levels are used for design description.

NMOS and PMOS switches

The keywords ‘nmos’ and ‘pmos’ are used to model NMOS and PMOS respectively.

Example:

module switch_modeling (
  input d_in, ctrl,
  output p_out, n_out);
  
  pmos p1(p_out, d_in, ctrl);
  nmos n1(n_out, d_in, ctrl);
  
endmodule

Output:

At time = 0: ctrl = 0, d_in = 0, p_out = 0, n_out = z
At time = 5: ctrl = 1, d_in = 0, p_out = z, n_out = 0
At time = 10: ctrl = 1, d_in = 1, p_out = z, n_out = 1
At time = 15: ctrl = 0, d_in = 1, p_out = 1, n_out = z
At time = 20: ctrl = 0, d_in = 0, p_out = 0, n_out = z

CMOS switch

A CMOS is modeled with a combination of NMOS and PMOS devices.

cmos

module cmos_modeling (
  input d_in, p_ctrl, n_ctrl,
  output out);
  
  cmos p1(out, d_in, p_ctrl, n_ctrl);
  
endmodule

output:

At time = 0: d_in = 0, p_ctrl = 0, n_ctrl = 0, out = 0
At time = 5: d_in = 0, p_ctrl = 1, n_ctrl = 0, out = 0
At time = 10: d_in = 1, p_ctrl = 1, n_ctrl = 0, out = 1
At time = 15: d_in = 0, p_ctrl = 1, n_ctrl = 0, out = 0
At time = 20: d_in = 0, p_ctrl = 1, n_ctrl = 1, out = 0
At time = 25: d_in = 1, p_ctrl = 1, n_ctrl = 1, out = 1
At time = 30: d_in = 0, p_ctrl = 1, n_ctrl = 1, out = 0

Power and ground

The power (Vcc) and ground (GND) sources need to be defined in transistor level modeling to provide the supply to the signals.

Keywords used: supply1(Vcc) and supply0 (GND)

module power_modeling (
  output vcc, gnd
);
  
  supply1 high;
  supply0 low;
  
  assign vcc = high;
  assign gnd = low;
endmodule

Output:

At time = 0: vcc = 1, gnd = 0

Bidirectional switches

As of now, we have discussed unidirectional switches like PMOS, NMOS, and CMOS that conduct from drain to source. Sometimes in the design, there is a need to have a bi-directional switch that can be driven from any of the device sides.

Keywords used: tran, tranif0, and tranif1

All tran, tranif0, and tranif1 behaves as a buffer from in either of the device side,

Switch types	Description
tran	Connects in_out1 and in_out2
tranif0	Connects in_out1 and in_out2 if control signal is 0. If the control signal is 1, then If in_out1 is a driver signal, in_out2 = z (high impedence) If in_out2 is a driver signal, in_out1 = z (high impedence)
tranif1	Connects in_out1 and in_out2 if control signal is 1. If the control signal is 0, then If in_out1 is a driver signal, in_out2 = z (high impedence) If in_out2 is a driver signal, in_out1 = z (high impedence)

module bidirectional_modeling (
  input in_out1, ctrl,
  output in_out2, in_out2_if0, in_out2_if1);
  
  tran t1(in_out1, in_out2);
  tranif0 t2(in_out1, in_out2_if0, ctrl);
  tranif1 t3(in_out1, in_out2_if1, ctrl);
endmodule

Output:

At time = 0: ctrl = 0, in_out1 = 0, in_out2 = 0, in_out2_if0 = 0, in_out2_if1 = z
At time = 5: ctrl = 0, in_out1 = 1, in_out2 = 1, in_out2_if0 = 1, in_out2_if1 = z
At time = 10: ctrl = 1, in_out1 = 1, in_out2 = 1, in_out2_if0 = z, in_out2_if1 = 1
At time = 15: ctrl = 1, in_out1 = 0, in_out2 = 0, in_out2_if0 = z, in_out2_if1 = 0

Resistive switches

The resistive switches provide a high impedance from source to drain with a reduction in signal strength as compared to regular switches. All PMOS, NMOS, CMOS, and bidirectional switches can be modeled as resistive devices.

Keyword used: ‘r’ as a prefix to the regular switches.

Resistive switches	Resistive keywords
Resistive pmos	rpmos
Resistive nmos	rnmos
Resistive cmos	rcmos
Resistive tran	rtran
Resistive tranif0	rtranif0
Resistive tranif1	rtranif1

module resistive_modeling (
  input in_out1, ctrl,
  output in_out2, in_out2_if0, in_out2_if1);
  
  rtran t1(in_out1, in_out2);
  rtranif0 t2(in_out1, in_out2_if0, ctrl);
  rtranif1 t3(in_out1, in_out2_if1, ctrl);
endmodule

Output:

At time = 0: ctrl = 0, in_out1 = 0, in_out2 = 0, in_out2_if0 = 0, in_out2_if1 = z
At time = 5: ctrl = 0, in_out1 = 1, in_out2 = 1, in_out2_if0 = 1, in_out2_if1 = z
At time = 10: ctrl = 1, in_out1 = 1, in_out2 = 1, in_out2_if0 = z, in_out2_if1 = 1
At time = 15: ctrl = 1, in_out1 = 0, in_out2 = 0, in_out2_if0 = z, in_out2_if1 = 0