CAE/CAD Tools
Marin Hristov
Advanced level study programme in
Electronics Design and Integration Technologies
28213-IC-1-2005-1-BE-ERASMUS-PROGUC-3 2006-2322 / 001-001 SO2
Technical University of Sofia
Faculty of Electronics
ECAD Laboratory
2008
CADENCE Design Framework
CAE/CAD Tools
Cadence Design Framework II is a common interface to the
complete range of Cadence IC design tools. Using a common
database and user interface, the framework allows easy cross
checking between the various stages of the design flow, e.g.
comparing the layout and the schematic.
The following describes the components of the Cadence Design
Framework II which are used for digital design.
CAE/CAD Tools
Design
Specifications
Cadence FLOW
Schematic capture
Simulation
Layout and DRC
Extraction and LVS
Post-layout Simulation
CAE/CAD Tools
Design Specifications
Design Specification typically describe the expected functionality
(Boolean operations) of the designed block, as well as the maximum
allowable delay times, the silicon area and other properties such as
power dissipation. Usually, the design specifications allow
considerable freedom to the circuit designer on issues concerning
the choice of a specific circuit topology, individual placement of the
devices, the locations of input and output pins, and the overall
aspect ratio (width-to-height ratio) of the final design.
In a large-scale design, the initial design specifications may also
evolve during the design process to accomodate other specs or
limitations.
CAE/CAD Tools
Design Specifications
As an example, the initial design specs of a one-bit binary full adder
circuit are listed below:





Technology: 0.8 um twin-well CMOS
Propagation delay of "sum" and "carry_out" signals < 1.2 ns
(worst case)
Transition times of "sum" and "carry_out" signals <1.2 ns (worst
case)
Circuit area < 1500 um^2
Dynamic power dissipation (at VDD=5 V and fmax=20 MHz) < 1
mW
CAE/CAD Tools
Schematic Capture
The traditional method for capturing (i.e. describing) your transistorlevel or gate-level design is via the schematic editor.
Schematic editors provide simple, intuitive means to draw, to place
and to connect individual components that make up your design.
The resulting schematic drawing must accurately describe the main
electrical properties of all components and their interconnections.
Also included in the schematic are the power supply and ground
connections, as well as all "pins" for the input and output signals of
your circuit. This information is crucial for generating the
corresponding netlist, which is used in later stages of the design.
The generation of a complete circuit schematic is therefore the first
important step of the design flow.
CAE/CAD Tools
Schematic Capture
In order to setup your environment to run Cadence applications you
need to open an xterm window and type
. cdscdk
this script modifies your environment (sets PATH and exports
variables). To see your current environment type the following at the
prompt:
set
Running the Cadence tools
Now you should be able to run the Cadence tools. Never run
Cadence from your root directory, it creates many extra files that will
clutter your root. Instead please create a directory (e.g. cadence)
and start Cadence there by typing:
mkdir cadence
cd cadence
icfb &
CAE/CAD Tools
Schematic Capture
The command icfb & starts Cadence in the background and you
should get a window with the icfb Command Interpreter Window
(CIW) as below:
With the CIW you can launch other applications and you can also
manage your files and libraries. For more information read here…
CAE/CAD Tools
Schematic Capture
In order to create new libraries go to File -> New -> Library from the
File menu of the Library Manager. Then fill in the name of the new
library (e.g. Tutorial) in the dialog window, and leave the Path empty.
Click on Attach to existing tech library and choose AMI 060u C5N
(#M, 2P, high-res) from all the options. Leave I/O Pad Type as
Perimeter then click OK.
CAE/CAD Tools
Schematic Capture
Start by clicking on the Tutorial library in the Library Manager
window once, then go to File -> New -> Cell View and fill in with
IVcurves as the cell name, schematic as the view name, and
Composer - Schematic as the tool, then press OK.
You should get the Virtuoso Schematic Editing window. You also have
access to these commands (and others) from the menu. For more
information read here…
CAE/CAD Tools
Schematic Capture
Our first schematic will be used
to plot IV curves. Click on the
Instance button (which looks
somewhat like an IC, or go to
Add -> Instance), this will pop-up
two small windows, one being a
Component Browser window. In
this
window
choose
NCSU_Analog_Parts as the
library, click on N_Transistors,
then on nmos4 (an NMOS
transistor with all 4 terminals, G,
S, D, B):
CAE/CAD Tools
Schematic Capture
In Add Instance window you can
place the 5 transistors by clicking
on the left mouse button for the
first transistor and then moving
the mouse down and clicking
again. Do that right now. If you
make mistakes you can always
go to Edit -> Undo and try again.
You can press the ESC key on
the keyboard to get out of the
place instance mode or you can
keep placing other parts.
CAE/CAD Tools
Schematic Capture
You can also move, delete parts,
please explore the different
editing functions, you will only
learn by making mistakes and
then correcting them. Now we
also need to add ports, wires and
power supply. First let's add
ground by clicking on Instance
again and then choosing SupplyNets and then gnd in the
component browser window,
then place one gnd below the 5
transistors.
For more How to Edit information
read here…
CAE/CAD Tools
Schematic Capture
Then add wires (narrow) to
connect all transistor sources
and bodies to the ground.
Now add 6 DC voltage sources,
one for VDS and one for each
VGS and then connect them with
wires
to
the
transistors.
Unfortunately we also need to
add 5 more "dummy" voltage
sources (with a value of 0 V) so
that we can plot the currents in
the transistors. The DC voltage
sources that we are going to use
are in the Voltage_Sources
directory with the name vdc.
CAE/CAD Tools
Schematic Capture
Finally add the 5 dummy 0 V
sources in series with the drains,
and a voltage source vdc of 5 V
for VDS. Press ESC to get out of
the add instance mode. In case
you made a mistake you can
always go to Edit -> Undo, or
you can correct your mistake by
some form of edit.
For more information about Edit
Properties, read here…
CAE/CAD Tools
Schematic Capture
For example, if you filled in the wrong value for the DC voltage for
vdc you can always change that later by first selecting the instance
(click on it in the schematic) and then go to Edit -> Properties ->
Objects, then a pop-up window will appear where you can change
what you want.
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CAE/CAD Tools
SIMULATION
After the description of a circuit is completed using the Schematic
Editor, the electrical performance and the functionality of the circuit
must be verified using a Simulation tool. The detailed transistor-level
simulation of your design will be the first in-depth validation of its
operation. Based on simulation results, the designer usually
modifies some of the device properties in order to optimize the
performance.
The initial simulation phase also serves to detect some of the design
errors that may have been created during the schematic entry step.
It is quite common to discover errors such as a missing connection
or an unintended crossing of two signals in the schematic.
CAE/CAD Tools
Simulation
For our first schema with 5 transistors in the Virtuoso Schematic
window go to Tools -> Analog Environment. The design should be
set to the right Library, Cell and View. First we need to choose the
simulator , we will choose Spectre.
Go to Setup ->
Simulator/Directo
ry/Host,
and
choose Spectre
in the pop-up
window,
then
click OK.
For
more
information
about Spectre,
read here…
CAE/CAD Tools
Simulation
Now you need to choose the
type of simulation, go to
Analyses -> Choose... In this
case we will choose a dcsweep so click on the dc radio
button, then on Component
Parameter, then on Select
Component (all of these in the
Choosing Analyses dialog
window) then on the VDS
component in the schematic
window and choose dc in the
Select Component Parameter
pop-up window and click OK.
CAE/CAD Tools
Simulation
After saving the file we can
finally simulate! Click on the
Netlist (more read here…) and
Run button on the right or go
to Simulation -> Netlist and
Run. Click OK on the
Welcome to Spectre window
which
should
start
the
simulation:
For more detailed information,
read here…
CAE/CAD Tools
Simulation
Next step is to go to Results > Direct Plot -> DC which will
pop-up
your
schematic
window. Now you have to click
on the signals you want to
see. Since this is a dc-sweep
we want to see the drain
currents into the 5 transistors.
In order to do this you have to
click on the small red square
at + terminal of each of the
dummy power supplies in
series with each drain.
CAE/CAD Tools
Simulation
Make sure you click on the
red square (the pin) which
means current, versus any
other part which means net, or
voltage. Click on all 5 power
supplies. If you are pressing
right on the pins a circle
should appear around each
chosen pin.
CAE/CAD Tools
Simulation
Now press on the ESC key (to
finish choosing the signals)
and you should get the
desired simulation results, 5
IV curves as in the textbook.
For detailed information about
Spectre, read here…
CAE/CAD Tools
Hierarchical Schematic Capture
If a certain circuit design consists of smaller hierarchical
components (or modules), it is usually very beneficial to identify
such modules early in the design process and to assign each such
module a corresponding symbol (or icon) to represent that circuit
module. This step largely simplifies the schematic representation of
the overall system.
A symbol view of the circuit is also required for some of the
subsequent simulation steps, thus, the schematic capture of the
circuit topology is usually followed by the creation of a symbol to
represent the entire circuit. The shape of the icon to be used for the
symbol may suggest the function of the module (e.g. logic gates AND, OR, NAND, NOR), but the default symbol icon is a simple
rectangular box with input and output pins. Note that this icon can
now be used as the building block of another module, and so on,
allowing the circuit designer to create a system-level design
consisting of multiple hierarchy levels.
CAE/CAD Tools
Hierarchical Schematic Capture
In case you are going to use
input,
output
and
bidirectional ports (CMOS
inverter), you can place
these either by pressing the
PIN button on the left or by
going to Add -> Pin... First
add one nmos4 transistor
and one pmos4 transistor, a
gnd and a vdd symbol (from
the Supply_Nets directory in
the
NCSU_Analog_Parts
library) and then an input
port IN and and output port
OUT.
For more information, read
here…
CAE/CAD Tools
Hierarchical Schematic Capture
Now connect everything with
wires as an inverter and
change
the
transistor
properties such that the ratio
of pmos to nmos is the "rule
of thumb = 2", i.e. make the
pmos 3um in width. Don't
forget
to
connect
the
transistor bodies to the
proper voltages (gnd for
nmos and vdd for pmos).
Your final schematic should
look like this:
CAE/CAD Tools
Hierarchical Schematic Capture
Now we can create a
hierarchical schematic that
uses the symbols that we
just created. Go to the
Library Manager and with
the
Tutorial
library
highlighted do File -> New ->
Cell View and create a
schematic view for a cell
named
InvRing.
The
Composer schematic editor
window should open up.
Let's create a schematic of a
ring
oscilator
with
11
inverters. Click on instance
and choose the symbol view
of the cell inverter in the
Tutorial library.
We are going to place 6 inverters first and we can do that with just
one command by filling the Array inputs, let's say Rows 1 and
Columns 6 (all 6 inverters will be arranged horizontally).
CAE/CAD Tools
Hierarchical Schematic Capture
Now click once to place the left most inverter, then move the mouse
to the right and click again to place the remaining ones.
CAE/CAD Tools
Hierarchical Schematic Capture
Now we only need to wire the 11 inverters into a ring and add a vdd
and a gnd symbol from the NCSU_Analog_Parts -> Supply Nets
library (leave them unconnected on the top).
CAE/CAD Tools
Hierarchical Schematic Capture
Many times when you have a complex hierarchical schematic you
may want to make modifications on different cells without having to
close and open different windows. You can do that by traversing
the hierarchy. You can traverse both up and down, for example
you can go down from the InvRing schematic and make
modifications in the inverter schematic, then go back up the
hierarchy. To do that you have to click on the symbol that you want
to descend to (in the Virtuoso Schematic window), then go to
Design -> Hierarchy -> Descend Edit, then click OK on the pop-up
dialog box.
CAE/CAD Tools
Hierarchical Schematic Capture
Now we are going to create a
hierarchical schematic at the
logic (gate) level by using
symbols for lower level
schematics. When creating
such a schematic it is
sometimes necessary to use
different gates with the same
logic but with different
transistor sizes (e.g. a "weak"
inverter and a "strong"
inverter). For these purposes
it is good if the sizes of the
transistors
are
parameterized. We will use
the schematic and symbol for
the inverter that we created
earlier.
CAE/CAD Tools
Hierarchical Schematic Capture
Let's copy the existing inverter cell to 4 other cells, let's call them:
Invx1, Invx4, Invx16 and Loadx64, respectively. These will be a
parameterized inverter with minimum size (x1), 4 times the
minimum (x4), 16 times the minimum (x16) a fix load 64 times the
minimum. In order to copy the cells first click on the library, then on
the inverter cell, then go to Edit -> Copy, and fill the new name of
the cell in the To unfilled part, and make sure that Copy All Views is
checked. In this way you should get 4 more cells in the Tutorial
library.
CAE/CAD Tools
Hierarchical Schematic Capture
Open the schematic view by
double clicking on the
schematic view in the Library
Manager window. We now
need
to
change
the
properties Width and Length
for the transistors such that
they are parameterized. Edit
properties for the nmos and
change Length to: Len and
Width to: Wid, then pmos and
change Length to: Len and
Width to: a*Wid.
CAE/CAD Tools
Hierarchical Schematic Capture
When we use this inverter in a hierarchical schematic we can now
keep the default values or change the default values to go to a
different technology (e.g. change Len to 0.25u for a 0.25u CMOS
technology) or have different transistor strengths for different
inverters.
CAE/CAD Tools
Hierarchical Schematic Capture
Now change the sizes for Invx4, Invx16 and Loadx64. For all of
them make the length Len, both for PMOS and NMOS. This is the
standard practice for digital design where transistors are minimum
length for high performance. Then make the widths:



a*b*Wid for PMOS, b*Wid for NMOS for Invx4
a*b*b*Wid for PMOS, b*b*Wid for NMOS for Invx16
a*c*Wid for PMOS, c*Wid for NMOS for Loadx64
CAE/CAD Tools
Hierarchical Schematic Capture
We should also edit the
symbols of the 4 new cells to
reflect
their
different
characteristics. Let's do it
though, go to the Library
Manager and double click on
the symbol view of the Invx1
cell. Go to Edit -> Properties > Objects... and then clik on
the text inv on the symbol
itself. In the pop-up window
fill Invx1 as the Label, then
clik OK.
CAE/CAD Tools
Hierarchical Schematic Capture
Now let's assign sizes to the
4 inverters. We are interested
in the inverter delay for a
fanout of 4 and assume that
an inverter 4 times larger
than the base size is
equivalent
to
4
base
inverters. This means that we
can keep the first inverter at
the base size, make the
second one 4 times larger,
the 3rd one 16 times larger
and the 4th one 32 times
larger, simply by making the
parameter b = 4.
CAE/CAD Tools
Hierarchical Schematic Capture
The last skill in the schematic entry will be to traverse hierarchy. For
this go to Design -> Hierarchy -> Descend Edit and then click on the
first inverter and then clik OK. You should get to the inverter
schematic where you can verify that the sizes are actually as you
would expect.
CAE/CAD Tools
Simulation Hierarchical Schema
For Simulation we use two
inverters for delay such that
we can take both tpLH and
tpHL into account without
having to do manual addition.
In the Composer window go
to
Tools
->
Analog
Environment. The design
should be set to the right
Library, Cell and View.
We also need to set up inputs
and power supply since we
don't have explicit voltage
sources. Go to Setup ->
Stimuli.
CAE/CAD Tools
Simulation Hierarchical Schema
We need to setup both inputs (IN1) and the global sources (power
supply). For IN1 use a pulse with amplitude 5 (Voltage 1=0, Voltage
2=5) with 0.4n rise and fall times and 1.6n pulse width and 4n
period, make sure you enable it. Click on the Global Sources, you
should have only one (vdd!). Click on Enabled, Function dc, Type
Voltage, DC voltage 5, Source type dc, and click on Apply. Now you
need to choose the type of simulation, go to Analyses -> Choose...
In this case we will choose tran which is the default, 8n as the Stop
time and moderate as the accuracy default. Save All.
The only other settings that we need are the variables a, b, c, Len
and Wid, for now let's set a=2, b=4, c=64, Len=0.6u, Wid=1.5u.
CAE/CAD Tools
Simulation Hierarchical Schema
The variable a should now appear in the Design Variables list. Do
the same for b, c, Wid, Len. All variables should now appear in the
Design Variables list.
CAE/CAD Tools
Simulation Hierarchical Schema
For the simulation click on
the Netlist and Run button
on the right Click OK on the
Welcome to Spectre window
which should start the
simulation. For simulation
results go to Results ->
Direct Plot -> Transient
Signal which will pop-up your
schematic window. Since
this is a transient analysis
we want to see a few
voltages. In order to do this
you have to click on the
desired nets, then the ESC
key. Click on OUT, IN1, IN2
and IN3.
CAE/CAD Tools
Simulation Hierarchical Schema
Now we can measure tpLH, tpHL for the second and third inverters
(signals IN2 and IN3). To do this accurately we are going to use the
waveform calculator. Go to Tools -> Calculator in the Analog
Environment window which should pop up the calculator.
CAE/CAD Tools
Simulation Hierarchical Schema
Now do Analysis -> Start in the Parametric Analysis window. Once
the simulations are over we can again plot the waveforms. Go to
Results -> Direct Plot -> Transient Signal and then choose IN2 and
OUT.
CAE/CAD Tools
Simulation Hierarchical Schema
The first observation that we
can make is that the delays
are quite close even if we
changed the pmos/nmos
ratio quite drastically (from 1
to 3), which should reinforce
the idea that complementary
static CMOS is non-ratioed.
If we zoom on the HL
transition you will discover
that the fastest solution in
this case is actually the a = 1
waveform where the pmos
and nmos are equal (412ps
vs. 468ps when a=2.8).
CAE/CAD Tools
Simulation Hierarchical Schema
On the other hand if you look
at the LH transition you can
see that the solution with a =
1 has a poor rise time while a
= 1.4 or a = 1.6 have much
better rise times and still have
small delays. From this
superficial analysis we can
conclude that indeed a value
around a = 1.5 (close to
sqrt(2-3) = 1.41-1.73 as
suggested in class) is close
to optimal for delay and
decent rise time.
CAE/CAD Tools
Layout and DRC (Design Rule Checking)
The creation of the mask layout is one of the most important steps in
the full-custom design flow, where the designer describes the
detailed geometries and the relative positioning of each mask layer
to be used in actual fabrication, using a Layout Editor.
The created mask layout must conform to a complex set of design
rules, in order to ensure a lower probability of fabrication defects. A
tool built into the Layout Editor, called Design Rule Checker, is used
to detect any design rule violations during and after the mask layout
design. The designer must perform DRC (in a large design, DRC is
usually performed frequently - before the entire design is
completed), and make sure that all layout errors are eventually
removed from the mask layout, before the final design is saved.
CAE/CAD Tools
Layout and DRC
Now we are going to create a
layout
for
our
inverter
schematic. We are going to
use
the
SCMOS_SUBM
scalable CMOS design rules
for submicron processes
available from MOSIS. The
AMI C5N process uses
LAMBDA = 0.3u which seems
to be in contradiction with the
claim that it is a 0.5u process.
LAMBDA = 0.3u is really
chosen for satisfying the
design rules for everything
but transistor length which
with extra step is done at
MOSIS to reduce it from the
drawn 0.6u to 0.5u. Change
the width of the nmos to 5
lambda and of the pmos to 10
lambda.
CAE/CAD Tools
Layout and DRC
Two windows should pop-up, the Virtuoso layout window screen and
the LSW which is used for choosing the layers to be used:
CAE/CAD Tools
Layout and DRC
Now let's draw the gate. We'll
draw another rectangle, 2
lambda wide, in the middle of
the active region so that it
overlaps the area by two
lambda on each side. Click
on poly in the LSW and then
start from the point X: 1.65, Y:
-3.90 to X: 2.25, Y: -1.20
CAE/CAD Tools
Layout and DRC
Now we need to add the two
contacts, both 2 lambda on
each side (0.60) and 2
lambda from poly and 1.5
lambda from the outside.
Click on cc in the LSW and
then draw the first rectangle
from X: 0.45, Y: -2.85 to X:
1.05, Y: -2.25, then copy the
rectangle to the position of
the other contact by doing
Edit -> Copy.
CAE/CAD Tools
Layout and DRC
With this the active area for
the nmos is done but we still
need to put nselect around
the active. Before we do that
let's define the substrate
contact area. Let's draw a
pactive rectangle that is 5
lambda
on
each
side
adjacent
to
the
nmos
transistor, then copy a
contact into the middle of this
region.
CAE/CAD Tools
Layout and DRC
Now we need to surround the
active area with select
rectangles, nselect for the
transistor and pselect for the
substrate contact. These
areas need to be 2 lambda
larger than the active. Click
on nselect first and draw a
rectangle from X: 0.00 Y: 1.20 to X: 4.50 Y: -3.90
CAE/CAD Tools
Layout and DRC
Then
draw
a
pselect
rectangle
around
the
substrate contact.
CAE/CAD Tools
Layout and DRC
Next is the pmos. The pmos
is drawn in the same fashion
except that nactive becomes
pactive and viceversa and
nselect becomes pselect and
viceversa. Also the width is
twice larger and we need two
contacts on each side. Notice
that we have drawn the nmos
with the active area 6 lambda
below the Y: 0.00 axis, draw
the pmos 6 lambda above the
Y: 0.00 axis.
CAE/CAD Tools
Layout and DRC
There is just one more
element needed for the
transistors: the nwell for the
pmos. Draw a rectangle that
surrounds the pmos active
area by 6 lambda (1.8
microns), you should get:
CAE/CAD Tools
Layout and DRC
We also need to add metal 1
above the contacts that
needs to overlap the contacts
by 1 lambda.
CAE/CAD Tools
Layout and DRC
Now Run a preliminary DRC.
Go to Verify -> DRC... then
click on OK. Check your CIW
window, you should have no
errors, in case you have
errors you need to go back
and fix them.
CAE/CAD Tools
Layout and DRC
First let's route the output,
click on metal 1 and then
Create -> Path and draw a
path from the drain of the
pmos to the drain od the
nmos (right side). The path is
only 3 lambda wide so draw it
aligned to the right most side.
You have to double click to
end the path.
CAE/CAD Tools
Layout and DRC
Now draw another path
centered around the 0 axis
to the end of the nwell.
CAE/CAD Tools
Layout and DRC
Now let's connect the input.
Draw a poly path between
the two gates. Observe that
the default width of the poly
line is correct: 2 lambda.
CAE/CAD Tools
Layout and DRC
Now start another poly line
centered around the 0 axis
going to the left and after you
pass the 0,0 point and
change to metal 1 by using
the Change to Layer in the
Create Path window. This
should automatically insert a
contact between poly and
metal 1, click once to place
the
contact
structure
adjacent to the 0,0 point to
the right. Double click when
you reach the left-most edge
of the nwell to end the path.
In order to see all the layers
in the contact "pcell" type
Shift-F with the cursor in the
layout window.
CAE/CAD Tools
Layout and DRC
The only items left now are
the
vdd
and
gnd
connections, we are going to
use Create -> Polygon for
those (we could also use
rectangles or paths). Create
the polygon using the points:
X: -1.35 Y: 4.65
X: -1.35 Y: 5.55
X: -3.30 Y: 5.55
X: -3.30 Y: 7.35
X: 5.70 Y: 7.35
X: 5.70 Y: 5.55
X: 1.35 Y: 5.55
X: 1.35 Y: 4.65
CAE/CAD Tools
Layout and DRC
For the ground we will simply
copy this polygon, go to Edit
-> Copy and then click on
Upside Down in the Copy
window. Now place the
copied polygon at the bottom
making conatct with the
nmos source.
CAE/CAD Tools
Extraction and LVS (Layout versus
Schematic Check )
Circuit extraction is performed after the mask layout design is
completed, in order to create a detailed net-list (or circuit
description) for the simulation tool. The circuit extractor is capable of
identifying the individual transistors and their interconnections . The
extracted net-list file and parameters are subsequently used in
Layout-versus-Schematic comparison and in detailed transistor-level
simulations (post-layout simulation).
After the mask layout design of the circuit is completed, the design
should be checked against the schematic circuit description created
earlier. The design called "Layout-versus-Schematic (LVS) Check"
will compare the original network with the one extracted from the
mask layout, and prove that the two networks are indeed equivalent.
A successful LVS will not guarantee that the extracted circuit will
actually satisfy the performance requirements. Any errors that may
show up during LVS should be corrected in the mask layout - before
proceeding to post-layout simulation.
CAE/CAD Tools
Extraction and LVS
First let's do some more
"cleanup" of our existing
layout. First observation is
that we can move the metal
1 to poly contact such that
we minimize the use of poly
for routing. Go to Edit ->
Stretch and move the
contact "flush" with the poly.
CAE/CAD Tools
Extraction and LVS
Go to Edit -> Merge and then
click on polygons and paths
that can be merged (e.g.
start with the output paths,
then you can merge the vdd
and gnd). Notice how they all
become as if we had drawn
polygons and there are no
"lines" artificially separating
the regions.
CAE/CAD Tools
Extraction and LVS
In order to prepare for
extraction and LVS we have
to define the input and power
supply pins in our layout as
in the schematic. Go to
Create -> Pin and enter IN
as Name, input as I/O type
and metal1 as Pin type.
CAE/CAD Tools
Extraction and LVS
Place the pin on the leftmost
side of the metal 1 shape
used as an input. Simlarly
place an output pin OUT on
the right most end of the
output metal 1 polygon and
two inputOutput pins called
vdd! and gnd! for power and
ground on the respective
metal shapes on the top and
bottom of the layout. Notice
the 4 squares that represent
the pins in the layout.
CAE/CAD Tools
Extraction and LVS
Go to Verify -> Extract... Click
on Set Switches in the popup window and then choose
Extract_parasitic_caps.
Notice that here you could
have generated pselect and
nselect layers automatically,
you may decide to do that in
the future to save effort. Now
click on OK
CAE/CAD Tools
Extraction and LVS
In the icfb window make sure
you have no errors. If there
are no errors you can now
open the newly created
extracted view from the
Library Manager by double
clicking. Press Shift-F to see
the symbols for the extracted
transistors. Notice that the
extracted view is a layout that
also has an underlying
schematic.
CAE/CAD Tools
Extraction and LVS
Now in the extracted window
go to Verify -> LVS...
Press on Run and wait until
the stop window pops up.
This window simply signifies
that LVS has terminated, not
that the comparison was
succesful. Click OK. In order
to see that LVS verified the
layout to correspond to the
schematic click on Info in the
LVS window.
CAE/CAD Tools
Extraction and LVS
The thing left is to simulate the extracted view of the inverter with the
associated parasitics. Since our cell layout is very small it is likely that
the parasitics are so small that no significant simulation differences
will be observed but in general the differences can be substantial for
large complicated layouts.
First you need to create a symbol for your cell, with Design -> Create
Cellview. Then create a new cell schematic that contains only your
cell (as a symbol) with input and output ports and supply symbols
(vdd and gnd). vdd and gnd need only be placed at the top level, no
need to connect them to anything else.
Now let's start Analog Artist, in the newly created cell window go to
Tools -> Analog Environment, then go to Setup -> Environment. This
entry is an ordered list of cell views that contain information that can
be simulated. The simulator will search until it finds one of these
cellviews. The default entry does not contain an extracted cellview.
We need to add an entry for extracted cellview in front of the
schematic cellview. As a result of this modification, the simulator will
use the extracted cell view of the cell, if one is available.
CAE/CAD Tools
Extraction and LVS
Now you can continue exactly
as in the simulation tutorial
and the only difference will be
that you are simulating the
extracted view (with the
parasitics) and not the
schematic view.
CAE/CAD Tools
Post-layout Simulation
The electrical performance of a full-custom design can be best
analyzed by performing a post-layout simulation on the extracted
circuit net-list. At this point, the designer should have a complete
mask layout of the intended circuit/system, and should have passed
the DRC and LVS steps with no violations.
If the results of post-layout simulation are not satisfactory, the
designer should modify some of the transistor dimensions and/or the
circuit topology, in order to achieve the desired circuit performance
under "realistic" conditions.
CAE/CAD Tools
Post-layout Simulation
It is now time to run the
program called Encounter for
place and route.
The GUI will appear as in the
following picture. To zoom in,
draw a box with the right
mouse button. To go back to
full view, hit "f".
CAE/CAD Tools
Post-layout Simulation
Encounter uses configuration
files to neatly organize the
many pieces that make up a
design. Go to "Design” and
load in the file called
"encounter.conf". Next we
need to create a floorplan.
Do "Floorplan -> Specify
Floorplan". Set the Core
Utilization to 0.5 and the
space between the core and
the boundary to 30um on all
sides. A utilization of 50%
leaves enough room for
buffer
insertion
during
optimization. Inside the 30um
we will place supply rings.
CAE/CAD Tools
Post-layout Simulation
Hit OK and floor plan will look
as shown:
CAE/CAD Tools
Post-layout Simulation
Next we add supply rings. Do
"Floorplan -> Power Planning
-> Add Rings". Set the top
and bottom layer to Metal3
and set the ring width to.
Finally,
make
the
ring
centered in the channel. This
will create 2 rings, for VDD
and GND, around the core.
CAE/CAD Tools
Post-layout Simulation
Go to "Route -> Sroute" and
press OK. The default values
are okay. This will route all
power tracks in Metal1 and
will insert vias between the
stripes, rings and tracks. Your
layout will look as follows but
with no vertical red lines.
CAE/CAD Tools
Post-layout Simulation
To place the cells, go to
"Place -> Place".
CAE/CAD Tools
Post-layout Simulation
Clock Tree Insertion
The first step in timing
optimization is to insert a
clock tree. First we will have
Encounter
create
a
specification file. Do "Clock ->
Create Clock Tree Spec" and
put "buf" and "inv" as the
footprints for the buffer and
inverter. Encounter
uses
footprints to up- and downsize instances of the same
functionality.
CAE/CAD Tools
Post-layout Simulation
To create the clock tree do "Clock -> Synthesize Clock Tree" and
accept the default values. Next we can run timing optimization based
on the clock tree we inserted. Do "Timing -> Optimization" and check
"PostCTS". Hit OK and the result can be seen in the command
window.
CAE/CAD Tools
Post-layout Simulation
Routing
We use "Nanoroute" to
perform global and detail
routing. It is an extremely
powerful router and replaces
the older WRoute tool. To
route the design, to "Route ->
Nanoroute" and hit OK. To
get actual layout on the
screen in the gui window hit
the layout button in view
menu.
CAE/CAD Tools
Post-layout Simulation
Now we can run post-route optimization. This allows Encounter to
optimize the design based on actual wires. Any modified net will
automatically be re-routed by Nanoroute. To start, do "Timing ->
Optimization" and select "postRoute" mode. The result can be
seen in the command window.
CAE/CAD Tools
Post-layout Simulation
The final step is to add filler
cells. These are empty cells
that provide nwell continuity.
Do "Place -> Filler -> Add
Filler". Enter "FILL" as the cell
name (in the IIT cell library
this is the name of the filler
cell). The value "FILLER" will
be used as the prefix for the
instance name of each filler
cell added.
CAE/CAD Tools
Post-layout Simulation
After adding filler, hit "f" to
redraw the window. It can be
seen that the core area is
completely covered with cell
instances(your layout will be
slightly
different
than
following).
CAE/CAD Tools
Post-layout Simulation
The last step is to export the
design to GDS. This is the
format used by Layout Editors
(e.g. Cadence Virtuoso) and
IC foundries. It requires a map
file, to map layer names to
layer numbers. In this case we
use a map file that matches
Mosis layer numbers. Do
"Save -> GDS", choose any
name for the resulting file and
select the map file. Your GDS
Structure name should be
controller.
CAE/CAD Tools
Post-layout Simulation
Finally, go to "Design -> Save Design" to save the entire design
to disk. Then next time you run Encounter, you can do "Design ->
Restore Design".
CAE/CAD Tools