Basics of Analog Simulation
1. LOG and AnaLOG are simulating your circuit constantly, even
as you edit. But unlike the digital simulator, AnaLOG requires
your circuit to be "complete" before it can simulate. That is,
every pin must be connected to another pin, not left dangling.
If the circuit is incomplete, i.e., there are still unconnected pins
on some of the gates, AnaLOG will display a message to that effect
on its scoreboard. As soon as the circuit is complete, the
scoreboard will switch to "Simulation in Progress".
2. The gate that looks like a box with an arrow on it is the METER.
To monitor your simulation, sprinkle meters liberally around on
all of the interesting voltages. There is no limit to the
number of meters you can have. Remember that the meter's red
dot is at the tip of the arrow, so the tip must be exactly
touching the wire or pin of interest. If the meter is
unconnected or unhappy, it will display a "~" sign.
3. The "clock icon" is the round thing in the upper-right corner
that looks vaguely like a clock. Its importance is in the two
numbers directly below it. These are the current time and
current timestep, respectively. At the beginning, these are
both zero. As soon as the simulation begins, the current
time will start advancing. How fast it is advancing is shown
as the current time-step, and may vary depending on how fast
things are happening in the circuit.
4. A common error is to watch, say, a capacitor charging, then
go in and double the capacitance, and complain that it still
seems to be charging "at the same rate." If you look at the
clock icon, though, you will find that the timestep is twice
what it used to be: AnaLOG can simulate the ramp function
just as quickly in "real time," but the circuit is (correctly)
half as fast in "simulated time." Make a habit of paying
attention to the clock so that your observations have the
correct frame of reference in time.
5. Although simulation happens continuously, it is sometimes
necessary to "reset" the circuit back to its starting conditions.
For example, suppose you remove a wire from a running circuit and
the voltages that were in the circuit at the time are "impossible"
for the new circuit. The simulator will work as hard as it can
to figure out what to do, but may eventually give up and print a
message. Another example is that you may want to reset various
nodes to the initial voltages that you have programmed for them,
as described in the next paragraph. In any case, all you have to
do is select RESET in the Misc menu, or press the "R" key. This
sets the simulation clock back to 0 seconds, and returns the
voltages to their initial values.
6. If you open any AnaLOG gate for configuration, one thing you will
find is a set of three attributes for every pin on the gate.
These attributes are present voltage, reset voltage, and parasitic
capacitance, respectively. Every pin has a slight parasitic
capacitance to ground, initially 10fF. This capacitance must
exist for the simulation to run, but you can set it lower if
you find that it's in your way. The "present voltage" is an
actual indicator of the voltage currently on that pin, and it
changes while you watch during simulation. You can enter new
values here if you wish, but the simulator may not like you if you
go around making instantaneous changes in the circuit's voltages.
The "reset voltage" is used by the RESET command. If you leave
it blank, AnaLOG will solve for an appropriate initial voltage
at RESET time. Notice that Present and Reset voltages are for
nodes, not gates: if you change a pin's reset voltage, then
open up another gate connected to that same node, the reset
voltage shows up there, too.
7. When you reset, you will notice that the timestep jumps down,
usually to about 1E-18 seconds, then starts increasing. This
is partly because AnaLOG is cautiously trying to discover how
fast it can safely proceed in the simulation, and partly
because it has to solve for unspecified initial voltages by
simulating the brief charge-up that would occur when you
powered up a real circuit from scratch. If you push RESET
and the voltages seem to be frozen, don't depair -- wait for
the timestep to get out of the femtosecond range!
8. The AnaLOG scoreboard has some words on its left and right
sides. On the left is Memory, with indicators Set and Erase.
If you tap the word "Erase," all of your "reset voltages" will
be erased. You might do this if editing has caused your reset
voltages to be no longer appropriate, and you want AnaLOG to
solve the circuit from scratch. If you tap the word "Set,"
all of the present voltages in the circuit are copied into the
reset voltages. For example, you might turn your input
waveforms off, let the circuit settle to a stable state, then
push Set so that next time you RESET, the circuit will start
out at that stable state and you won't have to wait again.
9. AnaLOG is normally very cautious about voltage changes. If it
is going along at a particular timestep and some node suddenly
starts changing more than about .1V per timestep, AnaLOG will
reduce the timestep and simulate in smaller chunks. This is
because trying to simulate fast voltage changes all at once
results in ugly and inaccurate waveforms.
10. Sometimes all you are looking for is a DC point anyway, and
you don't care how ugly your transient waveforms are. In this
case, you can touch the word "Relaxed" on the scoreboard. This
tells AnaLOG to take timesteps as large as it can regardless of
how fast the voltages are changing. Touch "Exact" to return to
an accurate simulation. Often what people do is touch Relaxed
right after a RESET so that the initial charge-up proceeds
quickly, then touch Exact when the timestep gets up to the
- (510) 643 4005
- UC Berkeley / CS Division / 387 Soda Hall / Berkeley CA 94720