Transcript PPT - School of Engineering and Applied Science

The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
ECE122 – 30
Lab 5: Layout of a Buffer
Ritu Bajpai
October 1, 2008
Objective
• Our objective today is to learn to get the
layout for our module, in L- Edit.
• For this we first design a buffer in S-Edit.
• Obtain it’s layout in L-Edit.
• Simulate the extracted view in T-Spice.
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Buffer
• What is a buffer?
• A buffer is a circuit which does not affect
the voltage of a circuit (voltage buffer) or
current of a circuit (current buffer) yet
alters the desired parameters to improve
the performance of the circuit.
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Why a buffer?
• For example a buffer can be used as an
impedance transforming state at the
output. It can be used to reduce the output
impedance of a circuit while keeping the
voltage response unchanged.
• Or a buffer can also be used as a wave
shaping stage for digital circuits.
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Layout Basics1
• Layout of the chip defines the various
layers associated with the masks used in
fabrication.
• The goal of layout process is to implement
the design in a compact area while
satisfying the design rules set by the
foundry.
1. Analysis and design of digital integrated circuits by Hodges Jackson and Saleh.
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Schematic Using S-Edit
• A buffer module is created using 2 SCMOS
inverters, placed in series and terminated on 2
sides by input and output ports as shown below.
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Creating a symbol
• Once you have your schematic ready, go
to the symbol mode for your module in SEdit and create a symbol for the buffer.
• Next you can set up a test bench to
simulate your buffer and get the transient
simulation results.
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Test bench for the buffer
Adding input output pads to the
buffer
• Instance your buffer into a new module
and add pads to the inputs and outputs.
• I saved this module as Buffer_wPads
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Exporting the design
• Export your design as a tpr file.
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Exporting the design
Open L-Edit
• Open L-Edit
• Replace Setup: C:\My Documents\Tanner EDA\Tanner
tools v13.0\L-Edit and LVS\SPR\Lights\Layout\lightslb
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Replace set up
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SPR Setup in L-Edit
• Go to SPR->Setup
• Fill in the paths for the tech library and your netlist
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SPR Place and Route in L-Edit
• Go to SPR->Place and Route, Hit Run
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Layout Using L-Edit
• If it completes correctly you will see this:
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Top view of layout with pad frame
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Note the pads on the pad frame
• We see that there are total 4 pads on the
pad frame.
• Two of them correspond to the pads that
we attached at the input and the output of
the buffer.
• Other two correspond to Vdd and Gnd.
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Obtaining cross-sectional view of the layout
• Cross sections
• Process Definition File is C:\My Documents\Tanner EDA\Tanner
Tools v13.0\L-Edit and LVS\SPR\Lights\Layout\lights.xst
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Cross sectional view of the layout
•You will not see the exactly same cross-sectional view as shown above
because it depends on the co-ordinate you choose.
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Cross-sectional view
• You can choose pick option from the
cross-section window instead of defining
the co-ordinate and use the cursor to
choose the co-ordinate on top view where
you want to see the cross-sectional view.
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Homework
• Implement the following functions as a single
module using only NAND gates from the
SCMOS library.
• Test and layout.
• Extra points for optimal designs
X = ((!A)B + A(!B)C)
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