EE141 Fall 2004 Discussion 1

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EE141 Fall 2004Discussion 1

• Tips for Using SPICE

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Discussion Operation

• Discussion talks about technical details that
  could help understanding the class material
• Tips in using the circuit design tools
• Advices on preparing for the projects and
  homeworks
• Discussing common mistakes / misunderstandings in
  the graded homeworks
• Review on some typical old exam problems
• Open to questions and discussions

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Get Your Self Ready
Computer Accounts for StudentsYou must have an
Instructional UNIX account, using your existing
account on the Instructinal systems, including
c199.eecs and quasar.eecs in a few weeks.For
more information, http//www-inst.eecs.berkeley.ed
u/
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SPICE

• The main computer-aided analysis program used in
  circuit design for the last 30 years

• Originated from the work of a number of talented
  grad. Students in EECS Dept. of UC, Berkeley,
  under the guidance of D. Pederson and R. Rohrer.

• The universal acceptance is due not only to its
  robustness and ease of use but also to its
  original free distribution by UCB.

• In nowadays research is still going on to make
  SPICE faster, more reliable and extending the
  area of its application.

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Where to Use

• HSPICE is available for instructional use on (in
  order of preference)
• c199.eecs.berkeley.edu
• quasar.eecs.berkeley.edu
• cory.eecs.berkeley.edu

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Getting Started

• Run the simulator on your input file
• hspice filename.sp gt! filename.lis
• Use the waveform viewer to see the output awaves

• Musts
• Input files must have the extension .sp for the
  waveform viewer to work.
• The input file must have .OPTION POST specified.

• Documental helps
• Resources section on EE141 website
• Do not print it out! (hspice manual is 10MB, over
  1900 pages)
• The SPICE Book by Andrei Vladimirescu

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Netlist Format

• Modeling of nonlinear elements
• Circuit netlist
• Control statements
• Analysis

• The input files are case insensitive.
• The first line is always a comment. Other lines
  are commented with a leading or
• All nonlinear devices must have a .MODEL
  statement.

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Names

• Can contain letters, numbers, and the characters
  ! - / lt gt _
• Can be 1024 characters long
• Node names can begin with letters, numbers, or
  the characters _ !
• Trailing alphabetic characters are ignored in net
  names. For example a node named 1A is considered
  to be equivalent to node 1
• Nodes named 0, GND, GND!, and GROUND all refer to
  the global ground node.

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Values

• Scientific notation e.g. 1.1e-17
• Use a suffix e.g. 2.3u (xmega, kkilo, mmili,
  umicro, nnano, ppico, ffemto)
• Use a parameter e.g. Kratio (parameters must be
  declared with a .PARAM statement)
• Evaluated expressions e.g. 500mKratio

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Control

• .INCLUDE
• includes a file, usually a model file, or a
  subcircuit file
• .OPTION
• sets simulation options, e.g .OPTION POST
• .ALTER
• but allows another simulation to be performed
  with the same setup except the changes that
  follow the .ALTER statement
• .END
• marks the end of an input file

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Analysis

• OP Operating point, DC circuit solution
• .DC Sweep of DC operating points (capacitances
  are ignored)
• .DC var startval stopval incr - performs a DC
  sweep on the independent source or parameter var,
  varying its value from startval to stopval using
  the increment incr.
• .TRAN Perform a transient analysis (differential
  equation solver)
• .TRAN tincr tstop finds the operating point
  (.OP) and then performs a transient analysis of
  duration tstop seconds with a maximum time step
  of tincr.
• .AC Frequency sweep used in analog design to
  find the frequency responds of a circuit

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Measurement

• .MEASURE TRAN t_delay TRIG v(in) VAL2.5 CROSS1
  TARG v(out) VAL2.5 CROSS1
• Measures the propagation delay between the nodes
  in and out, where the signals first cross 2.5
  volts.
• .MEAS TRAN t_rise TRIG v(out) VAL0.5 RISE1 TARG
  v(out) VAL4.5 RISE1
• Measures the first 10-90 rise time of a 5V
  signal
• .MEAS TRAN max_current MAX I(Vdd)
• Measures the maximum current through the
  independent source Vdd
• .MEAS peak_power PARAMmax_current5
• Calculates the peak power, assuming that
  max_current has been measured

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Trouble Shooting Failure to Converge

• Non-convergence happens in OP, DC and TRAN
  analysis when SPICE cant find solution to the
  circuit calculation with the input control and
  circuit setup

• Typical error messages
• ERROR NO CONVERGENCE IN DC ANALYSIS
• ERROR INTERNAL TIME STEP TOO SMALL IN
  TRANSIENT ANALYSIS

• To deal with it
• In digital circuit simulation, CHECK THE
  CIRCUIT CONNECTIONS.
• In analog circuit simulation, do hand analysis
  / change control options / simplify models

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Trouble Shooting No DC Path to Ground

• (OP and DC)
• This often happens with floating MOSFET gates.
  Just add a resistor between the node and ground,
  or use the .IC or .NODESET commands to create an
  initial condition. Be warned, however, that
  .NODESET and .IC can cause convergence problems.

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Trouble Shooting Stability Problems

• (TRAN)
• Sometimes a transient analysis shows a
  ringing or oscillation that shouldnt be there.
  This can often be solved by reducing the maximum
  time step or using slower rise and fall times for
  independent sources

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Next The Hand Analysis

• A good designer must be equipped with two weapons

• The ability to get the most help from tools
• The intuition into the design

• System definition / integration
• Architecture management
• Circuit optimization
• Device modeling
• Simplified models of device operation with only
  first-order effect suffices to help us develop
  intuitions into the circuit functions /
  performances design.