Switch level modeling

Switch level modeling - chapter 10 –
padmanabhan book
P Devi Pradeep
 Designers familiar with logic gates and their configurations at the circuit
level may choose to do their designs using MOS transistors.
 Verilog has the provision to do the design description at the switch level
using such MOS transistors, which is the theme of the present chapter.
 Switch level modeling forms the basic level of modeling digital circuits
 The switches are available as an primitives in Verilog; they are central to
design description at this level.
 Basic gates can be defined in terms of such switches.
By repeated and successive instantiation of such switches, more involved
circuits can be modeled – on the same lines as was done with the gate level
primitives in Chapters 4 and 5.

Basic Switch Primitives:
 Different switch primitives are available in Verilog. Consider an nmos
switch. A typical instantiation has the form
nmos (out, in, control);
pmos (out, in, control);

Observations:
 When in the on state, the switch makes its output available at the same
strength as the input.
 There is only one exception to it: When the input is of strength supply,
the output is of strength strong. It is true of supply1 as well as supply0.
 When instantiating an nmos or a pmos switch, a name can be assigned
to the switch. But the name is not essential. (The same is true of the other
primitives discussed in the following sections as well.)
 The nmos and pmos switches function as unidirectional switches.

Resistive Switches:
 nmos and pmos represent switches of low impedance in the on-state. rnmos
and rpmos represent the resistive counterparts of these respectively.
 Typical instantiations have the form
rnmos (output1, input1, control1);
rpmos (output2, input2, control2);
 With rnmos if the control1 input is at 1 (high) state, the switch is ON and
functions as a definite resistance. It connects input1 to output1 through a
resistance. When control1 is at the 0 (low) state, the switch is OFF and
leaves output1 floating. The set of output values for all combinations of
input1 andcontrol1 values remain identical to those of the nmos switch
given in Table .
 The rpmos switch is ON when control2 is at 0 (low) state. It inserts a
definite resistance between the input and the output signals but retains the
signal value. The output values for different input values remain identical to
those in Table for the pmos switch.

Example 10.1 home work
module swt_aa (o1,o2,a1,a2,b1,b2,c1,c2);
output o1,o2;
input a1,a2,b1,b2,c1,c2;
wire o2;
tri1 o1;
bufif1 ctt1(o1,a1,c1), ctt2(o1,a2,c2);
bufif1 (weak1, weak0) ctp1(o2,b1,c1),ctp2(o2,b2,c2);
pullup pp(o2);
endmodule

CMOS inverter:
module inv (in, out );
output out;
input in;
supply0 a;
supply1 b;
nmos (out, a, in );
pmos (out, b, in);
endmodule

module npnor_2(out, in1, in2 );
output out;
input in1, in2;
supply1 a;
supply0 c;
wire b;
pmos(b, a, in2), (out, b, in1);
nmos (out, c, in1), (out, c, in2) ;
endmodule
* Here do some another examples like orgate nandgate and andgates….

NMOS Inverter with Pull up Load:
module NMOSinv(out,in);
output out;
Input in;
supply0 a;
pullup (out);
nmos(out,a,in);
endmodule
module tst_nm_in();
reg in;wire out;
NMOSinv nmv(out,in);
initial
in =1'b1;
always
#3 in =~in;
initial $monitor($time , " in = %b, output
= %b ",in,out);
initial #30 $stop;
endmodule

An NMOS Three Input NOR Gate:
module nor3NMOS(in1,in2,in3,b);
output b;
input in1,in2,in3;
supply0 a; wire b;
nmos(b,a,in1),(b,a,in2),(b,a,in3);
pullup(b);
endmodule

CMOS SWITCH:
x N_control turns ON the NMOS transistor and keeps it ON when it is in the 1
state.
x P_control turns ON the PMOS transistor and keeps it ON when it is in the 0
state.
The CMOS switch is instantiated as shown below.
cmos csw (out, in, N_control, P_control );
Significance of the different terms is as follows:
x cmos:The keyword for the switch instantiation
x csw: Name assigned to the switch in the
instantiation
x out: Name assigned to the output variable in the
instantiation
x in: Name assigned to the input variable in the
instantiation
x N_control: Name assigned to the control variable of
the NMOS transistor in the instantiation
x P_control: Name assigned to the control variable of
the PMOS transistor in the instantiation

Example 10.6 CMOS Switch – 1
module CMOSsw(out,in,n_ctr,p_ctr);
output out;
input in,n_ctr,p_ctr;
nmos gn(out,in,n_ctr);
pmos gp(out,in,p_ctr);
endmodule
module tst_CMOSsw();
reg in,n_ctr,p_ctr; wire out;
CMOSsw cmsw(out,in,n_ctr,p_ctr);
initial begin
in=1'b0;n_ctr=1'b1;p_ctr=~n_ctr; end
always #5 in =~in;
always begin #3 n_ctr=~n_ctr; #0p_ctr
=~n_ctr; end
initial $monitor($time , "in = %b , n_ctr =
%b , p_ctr = %b , output = %b
",in,n_ctr,p_ctr,out);
initial #39 $
# 0in = 0 , n_ctr = 1 , p_ctr = 0 , output = 0
# 3in = 0 , n_ctr = 0 , p_ctr = 1 , output = z
# 6in = 1 , n_ctr = 1 , p_ctr = 0 , output = 1
# 12in = 0 , n_ctr = 1 , p_ctr = 0 ,output = 0

Example 10.7 CMOS Switch – 2: module
CMOSsw1(out,in,con);
output out;
input in,con;
wire p_ctr;
not gn(p_ctr,con);
cmos gc(out,in,con,p-ctr);
endmodule
module tst_CMOSsw1();
reg in,con; wire out;
CMOSsw1 cmsw(out,in,con);
initial begin in=1'b0;con=1'b1; end
always #5 in =~in;
always #3 con=~con;
initial $monitor($time , "in = %b , con =
%b , output = %b " ,in,con,out);
initial #40 $stop;
endmodule

Example 10.8: A RAM Cell
 Figure shows a basic ram cell with facilities for writing data, storing
data,
and reading data.
 When switch sw2 is on, qb – the output of inverter g1 – forms the input
to the inverter g2 and vice versa. The g1-g2 combination functions as a
latch and freezes the last state entry before sw2 turns on.
 When wsb (write/store) is high, switch sw1 is ON, and switch sw2 OFF.
With sw1 on, input Din is connected to the input of gate g1 and remains so
connected.
When wsb goes low, din is isolated, since sw1 is OFF. But sw2 is ON and
the data remains latched in the latch formed by g1-g2. In other words the
data Din is stored in the RAM cell formed by g1-g2.
 When RD (Read) goes active (=1), the latched state is available as output
Do. Reading is normally done when the latch is in the stored state.

module csw(out,in,n_ctr);
output out; input in,n_ctr; wire p_ctr;
assign p_ctr =~n_ctr;
cmos csw(out,in,n_ctr,p_ctr);
endmodule
module ram_cell(do,din,wsb,rd);
output do;
input din,wsb,rd;
wire sb;
wire q,qq;
tri do;
csw sw1(q,din,wsb),sw2(q,qq,sb),sw3(do,q,rd);
not n1(sb,wsb),n2(qb,q),n3(qq,qb);
endmodule

An Alternate RAM Cell Realization
module ram1(do,din,wr,rd);
output do;
input din,wr,rd;
wire qb,q; tri do;
scw
sww(q,din,wr),swr(do,q,rd);
not(pull1,pull0)n1(qb,q),n2(q,q);
endmodule
module scw(out,in,n_ctr);
output out;
input in,n_ctr;
wire p_ctr;
assign p_ctr =~n_ctr;
cmos sw(out,in,n_ctr,p_ctr);
endmodule

A Dynamic Shift Register:
The shift register can be modified to suit a variety of needs:
---------Dynamic logic incorporating NAND / NOR gates.
---- Dynamic RAM with row and column select lines and refresh functions.
----- A shift register to function as a right- or a left-shift-type shift register; a
direction select bit can be used to alter the shift direction.

module shreg1(dout,din,phi1);
output dout;
input din,phi1;
wire phi2;
trireg[3:0] x,y;
trireg dout;
assign phi2=~phi1;
cmos switch0(x[0],din,phi1,phi2),
switch1(x[1],y[0],phi2,phi1),
switch4(dout,y[3],phi1,phi2);
cell cc0(y[0],x[0]), cc1(y[1],x[1]), cc2(y[2],x[2]), cc3(y[3],x[3]);
endmodule
module cell(op,ip);
output op;
input ip;
supply1 pwr;
supply0 gnd;
nmos(op,gnd,ip);
pmos(op,pwr,ip);
endmodule

BI-DIRECTIONAL GATES:
 The gates discussed so far (nmos, pmos, rnmos, rpmos, rcmos) are all
unidirectional gates.
 When turned ON, the gate establishes a connection and makes the signal
at the input side available at the output side.
 Verilog has a set of primitives for bi-directional switches as well. They
connect the nets on either side when ON and isolate them when OFF.
 The signal flow can be in either direction.
 None of the continuous-type assignments at higher levels dealt with so far
has a functionality equivalent to the bi-directional gates.
There are six types of bidirectional gates.
tran , rtran , tranif1, and rtranif1 tranif0 and
rtranif0

tran and rtran:
 The tran gate is a bi-directional gate of two ports. When instantiated, it
connects the two ports directly.
 Thus the instantiation tran (s1, s2);
 connects the signal lines s1 and s2. Either line can be input, inout or
output.
rtran is the resistive counterpart of tran.
tranif1 and rtranif1:
 tranif1 is a bi-directional switch turned ON/OFF through a control line. It is in
the ON-state when the control signal is at 1 (high) state. When the control line is
at state 0 (low), the switch is in the OFF state.
 A typical instantiation has the form tranif1 (s1, s2, c );
 Here c is the control line. If c=1, s1 and s2 are connected and signal
transmission can be in either direction
 rtranif1 is the resistive counterpart of tranif1.

tranif0 and rtranif0:
 tranif0 and rtranif0 are again bi-directional switches.
 The switch is OFF if the control line is in the 1 (high) state, and it is ON
when the control line is in the 0 (low) state.
 A typical instantiation has the form tranif0 (s1, s2, c);
With the above instantiation, if c = 0, s1 and s2 are connected and signal
transmission can be in either direction.
 If c = 1, the switch is OFF and s1 and s2 are isolated from each other.
rtranif0 is the resistive counterpart of tranif0.

Observations:
With the bi-directional switches the signal on either side can be of input,
output, or inout type. They can be nets or appearing as ports in the module. But
the type declaration on the two sides has to be consistent.
 In the above instantiation s1 can be an input port in a module. In that case, s2
has to be a net forming an input to another instantiated module or circuit block.
s2 can be of output or inout type also. But it cannot be another input port.
ƒ s1 and s2 – both cannot be output ports.
ƒ s1 and s2 – both can be inout ports.
 With tran, tranif1, and tranif0 bi-directional switches if the input signal has
strength supply1 (supply0), the output side signal has strength
strong1 (strong0).
 For all other strength values of the input signal, the strength value of the
output side signal retains the strength of the input side signal.
With rtran, rtranif1 and rtranif0 switches the output side signal strength is
less than that of the input side signal.

Example 10.11 bus switching home work

Example 10.12 Another RAM Cell
module ram_cell1(do,di,wr,rd,a_d);
output do;
input di,wr,rd,a_d;
wire ddd,q,qb,wrb,rdb;
not(rdb,rd),(wrb,wr);
not(pull1,pull0)(q,qb),(qb,q);
tranif1 g3(ddd,q,a_d);
cmosg4(ddd,di,wr,wrb);
cmos g5(do,ddd,rd,rdb);
endmodule
 When wr = 1, cmos gate g4 turns ON; the data at the input port di (with
strength strong0 / strong1) are connected to q through ddd. It forces the
latch to its state – since q has strength pull0 / pull1 only – di prevails
here. This constitutes the write operation.
 When rd = 1, cmos gate g5 turns ON. The net ddd is connected to the
output net do. The data stored in the latch are made available at the output
port do. This constitutes the read operation.

TIME DELAYS WITH SWITCH PRIMITIVES
 The instantiation nmos g1 (out, in, ctrl );
Def: has no delay associated with it.
The instantiation nmos (delay1) g2 (out, in, ctrl );
Def : has delay1 as the delay for the output to rise, fall, and turn OFF.
 The instantiation nmos (delay_r, delay_f) g3 (out, in, ctrl );
Def : has delay_r as the rise-time for the output. delay_f is the fall-time for
the output. The turn-off time is zero.
 The instantiation nmos (delay_r, delay_f, delay_o) g4 (out, in, ctrl );
Def:has delay_r as the rise-time for the output. delay_f is the fall-time for the
output delay_o is the time to turn OFF
when the control signal ctrl goes from 0 to 1. Delays can be assigned to
the other uni-directional gates (rcmos, pmos, rpmos, cmos, and
rcmos) in a similar manner.
Bi-directional switches do not delay transmission – their rise- and
fall- times are zero. They can have only turn-on and turn-off delays
associated with them. tran has no delay associated with it.

Contd..
 The instantiation tranif1 (delay_r, delay_f) g5 (out, in, ctrl );
Def: represents an instantiation of the controlled bi-directional switch. When
control changes from 0 to 1, the switch turns on with a delay of delay_r. When
control changes from 1 to 0, the switch turns off with a delay of delay_f.
 The instantiation transif1 (delay0) g2 (out, in, ctrl );
Def: represents an instantiation with delay0 as the delay for the switch to
turn on when control changes from 0 to 1, with the same delay for it to turn off
when control changes from 1 to 0. When a delay value is not specified in an
instantiation, the turn-on and turn-off are considered to be ideal that is,
instantaneous. Delay values similar to the above illustrations can be associated
with rtranif1, tranif0, and rtranif0 as well.

INSTANTIATIONS WITH STRENGTHS AND DELAYS
 In the most general form of instantiation, strength values and delay values can
be combined.
 For example, the instantiation
nmos (strong1, strong0) (delay_r, delay_f, delay_o ) gg (s1, s2, ctrl) ;
 Means the following:
x It has strength strong0 when in the low state and strength strong1when
in the high state.
x When output changes state from low to high, it has a delay time of
delay_r.
x When the output changes state from high to low, it has a delay time of
delay_f.
x When output turns-off it has a turn-off delay time of delay _o.
rnmos, pmos, and rpmos switches too can be instantiated in the general
form in the same manner. The general instantiation for the bi-directional
gates too can be done similarly.
10.7 STRENGTH CONTENTION WITH TRIREG NETS- Home work

There are many ways to generate clock in Verilog; you could use one of
the following methods:

Switch level modeling

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Switch level modeling