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3.3. Using Expressions
Digital Operators
These are the operators ‘&’, ‘|’ and ‘~’. These were introduced in old SIMetrix version as a
simple means of implementing digital gates in the analog domain. Their function has largely been
superseded by gates in the event driven simulator but they are nevertheless still supported.
Although they are used in a digital manner the functions implemented by these operators are
continuous in nature. They are defined as follows:
Expression
Condition
Result
out = x & y
x<vtl OR y<vtl
out = vl
x>vth AND y>vth
out = vh
x>vth AND vth>y>vtl
out = (y-vtl)*(vh-vl)/(vth-vtl)+vl
vtl<x< vth AND y>vth
out = (x-vtl)*(vh-vl)/(vth-vtl)+vl
vtl<x< vth AND vth>y>vtl
out = (y-vtl)*(x-vtl)*(vh-vl)/(vth-vtl)2 +vl
x<vtl AND y<vtl
out = vl
x>vth OR y>vth
out = vh
x<vtl AND vth>y>vtl
out = vh-(vth-y)*(vh-vl)/(vth-vtl)
vtl<x< vth AND y<vtl
out = vh-(vth-x)*(vh-vl)/(vth-vtl)
vtl<x< vth AND vth>y>vtl
out = vh-(vth-y)*(vth-x)*(vh-vl)/(vth-vtl)2
x<vtl
out = vh
x>vth
out = vl
vtl<x< vth
out = (vth-x)/(vth-vtl)*(vh-vl)+vl
out = x|y
out = ~x
Where:
vth = upper input threshold
vtl = lower input threshold
vh = output high
vl = output low
These values default to 2.2, 2.1, 5 and 0 respectively. These values are typical for high speed CMOS
logic (74HC family). They can be changed with four simulator options set by the .OPTIONS
simulator statement. These are respectively:
LOGICTHRESHHIGH,
LOGICTHRESHLOW,
LOGICHIGH,
LOGICLW
To change the lower input threshold to 1.9, add the following line to the netlist:
.OPTIONS LOGICTHRESHLOW=1.9
To find out how to add additional lines to the netlist when using the schematic editor, refer to
“Adding Extra Netlist Lines” on page 2.
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SIMetrix Simulator Reference Manual