Parameterized Cells (P-Cells) with Xschem

Introduction

Xschem allows the use of P-Cells by means of executing a script which is specified as a property of a component in a schematic. Inputs for the script are also declared as properties. To design these scripts, we first build a set of schematic and symbol templates representing each of the building blocks. Then, we use these templates to create specific scripts which output the blocks’ components as a set of text strings analogous to the process of reading the schematic view of a component. Thus, Xschem produces a netlist which includes the parametrized cells, ready for simulation with Ngspice.

Requirements

• WSL-2
• Docker Desktop
• UNIC-CASS or IIC-JKU Open-Source IC Tools docker container.
• Python
• VS Code

Example: P-Cell of Inverter

Schematic & Symbol Templates

1. Create a new schematic inv_gen.sch in your design folder. The schematic should look like the figure below.

2. Create a new symbol inv_gen.sym in your design folder. The symbol should look like the figure below.

Note

It is important that both the sch and sym files have the same name.

3. Now, let's take a look at both files in a text editor. We can see that both the schematic and symbol views are text files which can be generated by any script.

inv_gen.schinv_gen.sym

v {xschem version=3.4.6 file_version=1.2}
G {}
K {}
V {}
S {}
E {}
N -40 0 -40 50 {lab=A}
N -60 0 -40 0 {lab=A}
N -40 -50 -40 0 {lab=A}
N 0 0 0 20 {lab=xxx}
N 0 0 70 0 {lab=xxx}
N 0 -20 0 0 {lab=xxx}
N 0 -100 0 -50 {lab=VDD}
N -0 50 -0 100 {lab=VSS}
C {sg13g2_pr/sg13_lv_nmos.sym} -20 50 0 0 {name=M1
l=0.13u
w=1u
ng=1
m=1
model=sg13_lv_nmos
spiceprefix=X
}
C {sg13g2_pr/sg13_lv_pmos.sym} -20 -50 0 0 {name=M2
l=0.13u
w=2u
ng=1
m=1
model=sg13_lv_pmos
spiceprefix=X
}
C {iopin.sym} 0 -100 0 0 {name=p1 lab=VDD}
C {iopin.sym} 0 100 0 0 {name=p2 lab=VSS}
C {iopin.sym} -60 0 0 1 {name=p3 lab=A}
C {iopin.sym} 70 0 0 0 {name=p4 lab=Z}

v {xschem version=3.4.6 file_version=1.2}
G {}
K {type=subcircuit
format="@name @pinlist @symname"
template="name=x1"
}
V {}
S {}
E {}
L 4 -20 -20 -20 20 {}
L 4 -20 -20 20 0 {}
L 4 -20 20 20 0 {}
L 4 30 0 50 0 {}
L 4 -40 0 -20 0 {}
L 7 0 -20 0 -10 {}
L 7 0 10 0 20 {}
B 5 -2.5 -22.5 2.5 -17.5 {name=VDD dir=inout}
B 5 -42.5 -2.5 -37.5 2.5 {name=A dir=inout}
B 5 47.5 -2.5 52.5 2.5 {name=Z dir=inout}
B 5 -2.5 17.5 2.5 22.5 {name=VSS dir=inout}
A 4 25 0 5 180 360 {}
T {@symname} 35 4 0 0 0.3 0.3 {}
T {@name} 35 -32 0 0 0.2 0.2 {}
T {VDD} 5 -6 2 1 0.2 0.2 {}
T {A} -35 -14 0 0 0.2 0.2 {}
T {Z} 45 -14 0 1 0.2 0.2 {}
T {VSS} 25 6 0 1 0.2 0.2 {}

Python Scripts Outputing Schematic Instances

4. The goal is to create a Python script which output the inverter instances as a set of text strings. With this in mind, we create the file inv_gen.py with the following content:

#!/usr/bin/python3

import sys

# ****************************************************************
# Functions
def nmos_sg13_lv_nmos(name,w_um,l_um,ng,m,position):
    libraryname = 'sg13g2_pr'
    modelname = 'sg13_lv_nmos'
    print('C {' + libraryname + '/' + modelname + '.sym} ' + str(position[0]) + ' ' + str(position[1]) + ' ' + str(position[2]) + ' ' + str(position[3]) + ' {name=' + name)
    print('l=' + str(l_um) + 'u')
    print('w=' + str(w_um) + 'u')
    print('ng=' + str(ng))
    print('m=' + str(m))
    print('model=' + modelname)
    print('spiceprefix=X')
    print('}')


def pmos_sg13_lv_pmos(name,w_um,l_um,ng,m,position):
    libraryname = 'sg13g2_pr'
    modelname = 'sg13_lv_pmos'
    print('C {' + libraryname + '/' + modelname + '.sym} ' + str(position[0]) + ' ' + str(position[1]) + ' ' + str(position[2]) + ' ' + str(position[3]) + ' {name=' + name)
    print('l=' + str(l_um) + 'u')
    print('w=' + str(w_um) + 'u')
    print('ng=' + str(ng))
    print('m=' + str(m))
    print('model=' + modelname)
    print('spiceprefix=X')
    print('}')

def iopin(name,label,position):
    pintype = 'iopin'
    print('C {' + pintype + '.sym} ' + str(position[0]) + ' ' + str(position[1]) + ' ' + str(position[2]) + ' ' + str(position[3]) + ' {name=' + name + ' lab=' + label +'}')


# ****************************************************************
# parsing arguments. These are strings.
wn_um = float(sys.argv[1])
wp_um = float(sys.argv[2])

# print schematic
print('v {xschem version=3.4.6 file_version=1.2}')
print('G {}')
print('K {}')
print('V {}')
print('S {}')
print('E {}')
print('N -40 0 -40 50 {lab=A}')
print('N -60 0 -40 0 {lab=A}')
print('N -40 -50 -40 0 {lab=A}')
print('N 0 0 0 20 {lab=xxx}')
print('N 0 0 70 0 {lab=xxx}')
print('N 0 -20 0 0 {lab=xxx}')
print('N 0 -100 0 -50 {lab=VDD}')
print('N -0 50 -0 100 {lab=VSS}')
nmos_sg13_lv_nmos('M1',wn_um,0.13,1,1,[-20,50,0,0])
pmos_sg13_lv_pmos('M2',wp_um,0.13,1,1,[-20,-50,0,0])
iopin('p1','VDD',[0,-100,0,0])
iopin('p2','VSS',[0,100,0,0])
iopin('p3','A',[-60,0,0,1])
iopin('p4','Z',[70,0,0,0])

End of Line characters in Linux and Windows

Make sure you create the Python file from your Linux terminal to make sure of the end-of-line character which could affect the execution of the script.

Setting Up the P-Cell in Another Schematic

5. Let's create a testbench to test our P-Cell inv_gen. Create a new schematic tb_inv_ge.sch to perform a transient simulation. It should look like the figure below.

6. Select one of the inv_gen instances and open its properties by pressing q. Edit the properties according to the figure below.

Instance Properties

name=x1
wn_um=1
wp_um=2
schematic="inv_gen.py(@wn_um\,@wp_um\)"
tclcommand="edit_file [abs_sym_path inv_gen.py]"

7. The table below shows each of the properties. Notice how we define the width of each transistor as inputs of the Python script, and pass those variables to the script using @[variable]\.

Property	Description
wn_um	Width of nmos in um.
wp_um	Width of pmos in um.
schematic	File name of the Python script with parsed variables.
tclcommand	TCL command that executes the script.

When is the Script Executed?

8. The Python scripts is executes during: * Netlist process. * Descending into the schematic.

Simulation with Two Inverter Sizes