2-18
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Chapter 2
Digital I/O
When multiple lines are configured with the same filter settings they are considered a bus. Two
filtering modes for use with multiple lines:
•
Line filtering
—Each line transitions independently of the other lines in the bus and acts
like the behavior described above
•
Bus filtering
—When any one line in the bus has jitter, all lines in the bus will hold state
until the bus becomes stable. However, each individual line only waits one extra filter tick
before changing. This prevents a noisy line from holding a valid transition indefinitely. If
all the bus line transitions become stable in less than one filter clock period and the bus
period is more than two filter clock periods, then all the bus lines are guaranteed to be
correlated at the output of the filter, as shown in Figure 2-13.
The behavior for each transition can be thought of as a state machine. If a line transitions and
stays high for two consecutive filter clock edges, then one of two options occurs:
•
Case 1
—If no transitions have occurred on the other lines, the transition propagates on the
second filtered clock edge, as shown in Figure 2-14.
Figure 2-14.
Case 1
Digit
a
l Inp
u
t P0.A
Digit
a
l Inp
u
t P0.B
Filter Clock
Filtered Inp
u
t A
Filtered Inp
u
t B
S
t
ab
le
S
t
ab
le
S
t
ab
le
