Boston University
SC571: VLSI Design Principles
Electrical and Computer Engineering
Lab #3: Buffer Chains
Due: March 19, 2002
GOAL: The goal of this lab is to achieve greater skill in layout using Cadence Virtuoso
and to study the performance characteristics of circuits using transient simulation in
Analog Environment. The student is recommended to use modularity concepts in this
design activity.
DESIGN: You are asked to build, simulate, and study each of the three circuits below.
Use the TSMC025 (min L = 0.3 um) device technology library for your layout and
simulations. Assume VDD = 2.5 volts.
1. A series of complementary CMOS buffers which are near optimal to drive a 15pF
load. Assume that Cg=14fF for a 2 x 1 input gate (W=2 um, L=1 um). Let the
pull-ups be such that rise and fall times are equal.
2. A 3-stage complementary CMOS buffer chain whose stage ratio is approximately
the cube root of the ratio of the load capacitance to the input gate capacitance.
3. A 3-stage pseudo-NMOS buffer chain to drive the same load capacitance. Instead
of designing this ratio circuit chain for equal rise and fall delays, design to
achieve a VOL of 0.3 volts or less.
Compare the layout area, delay, and power for all three buffer chains.
SIMULATION:
1. Drive the 15pF load for each circuit noting the rise and fall times and the delay
times for each internal node and the output for each buffer chain.
2. Use Extract in Cadence to build SPICE models based on your layouts and
perform the simulations in Analog Environment using these models. Compare
these results with simulations done from the schematics.
OUTPUT:
Demo your work to the GTF and make hard copies of circuit schematics, layouts, and
simulation results for your report.
WRITEUP: Make a file called lab3p*.txt (*=1,2,3) listing rise, fall, and delay times of
the signals at all nodes. Measure rise/fall times as 10-90% and delay times as 50% to
50% points on the circuit node waveforms. Explain the signals on the graphs, noting
risetimes, falltimes, and delay times. Also, compute the power for each circuit assuming
a square wave input waveform. List the total area of each of the three circuits.
NOTES:
1. Use risetimes and falltimes of ~ 0.1ns. When you line up the cursor on input
signals to make time measurements, line up on the midpoint values.
2. Use pulse values of 2.5V and 0V.
3. When laying out the inverter circuits, two options seem possible: 1. Brute force:
layout of everything separately, or 2. Make the large buffer stages with a number
of parallel transistor sections, with each section, in turn, containing a number of
CMOS device poly gates in parallel. Try to use copy and paste techniques for this
approach, as much as possible.
4. Use the handout (from class) on how to optimally choose sizing for the
complementary CMOS case as well as the pseudo-NMOS case.
5. You will use transistor sizes based on the minimum design dimension of 0.3 um.
Thus, the calculated optimal values may not be obtained precisely. You might
want to alternate going a little over and a little under on alternate stages.
R. W. Knepper 2/27/02