Cooper LP800 Guia do Utilizador Página 445

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Propa
g
ation of Timin
g
Ambi
g
uity 16
-5
Propa
g
ation of Timin
g
Ambi
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uit
y
As signals propagate through the circuit, ambiguity is
contributed by each primitive having a nonzero MIN/MAX
delay spread. Consider the following example that uses the
delay values of the previous BUF model.
Fi
g
ure 16-2
Timing Ambiguity Example 2
This accumulation of ambiguity may have adverse effects on
proper circuit operation. In the following example, consider
ambiguity on the data input to a flip-flop.
Fi
g
ure 16-3
Timing Ambiguity Example 3
The simulator must predict an X output, because it is not known
with any certainty when the data input actually made the 0-1
transition. If the cumulative ambiguity present in the data signal
had been less, the 1 state would be latched-up correctly.
Figure 16-4 illustrates the case of unambiguous data change
(settled before the clock could transition) being latched-up by a
clock signal with some ambiguity. Note that the Q output will
change, but the time of its transition is a function of both the
clock’s ambiguity and that contributed by the flip-flop MIN/
MAX delays.
Fi
g
ure 16-4
Timing Ambiguity Example 4
5
20 45
35 85
QD
C
QD
C
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