User`s guide

Input/Output Characteristics 7
-21
The digital primitives comprising the 74393 part, reference the
IO_STD I/O Model in the model libraries as shown:
.model IO_STD uio (
+ drvh=96.4 drvl=104
+ AtoD1="AtoD_STD" AtoD2="AtoD_STD_NX"
+ AtoD3="AtoD_STD" AtoD4="AtoD_STD_NX"
+ DtoA1="DtoA_STD" DtoA2="DtoA_STD"
+ DtoA3="DtoA_STD" DtoA4="DtoA_STD"
+ tswhl1=1.373ns tswlh1=3.382ns
+ tswhl2=1.346ns tswlh2=3.424ns
+ tswhl3=1.511ns tswlh3=3.517ns
+ tswhl4=1.487ns tswlh4=3.564ns
+)
Definin
g
Output Stren
g
ths
The goal of running simulations is to calculate values for each
node in the circuit. For analog nodes, the values are voltages.
For digital nodes, these values are states. The state of a digital
node is calculated from the output strengths of the devices
driving the node, and the logic level of the node.
The purpose of strengths is to allow the simulator to find the
value of a node when more than one output is driving it. A
common example is a bus line which is driven by more than one
tristate driver. Under normal circumstances, all drivers except
one are driving at the Z (high impedance) strength. Thus, the bus
line will take on the value of the one gate that is driving at a
higher strength (lower impedance).
Another example is a bus line connected to several open
collector output devices and a digital pullup resistor. The pullup
resistor outputs a 1 level at a weak (but non-Z) strength. If all of
the open-collector devices are outputting at Z strength, then the
node will have a 1 level because of the pullup resistor. If any of
the open collectors output a 0, at a higher strength than the
pullup resistor, then the 0 will overpower the weak 1 from the
pullup, and the node will be a 0 level.
Node stren
g
th calculations are
described in
Chapter 14,Digital
Simulation.