MOTOROLA
Chapter 13. External Signals
13-21
Part IV. Hardware Interface
13.2 Active Pull-Up Buffers
Active pull-up buffers are a special variety of bidirectional three-state buffer with the
following properties:
¥
When enabled as an output and driving low, they behave as a normal output driver
(that is, the pin is constantly driven low).
¥
When enabled as an output and driving high, drive high until an internal detection
circuit determines that the output has reached the logic high threshold and then stop
driving (that is, the pin switches to high-impedance).
¥
When disabled as an output or functioning as an input, it is not driven.
Due to the behavior of the buffer when being driven high, a pull-up resistor is required
externally to function as a Ôbus keepÕ for these shared signals in periods when no drivers
are active and to keep the buffer from oscillating when the buffer is driving high, because
if the voltage ever dips below the logic high threshold while the buffer is enabled as an
output, the buffer will reactivate. Further, external logic must not attempt to drive these
signals low while active pull-up buffers are enabled as outputs, because the buffers will
reactivate and drive high, resulting in a buffer Þght and possible damage to the MPC860, to
the system, or to both.
Figure 13-4 compares three-state buffers and active pull-up buffers graphically in general
terms. It makes no implication as to which edges trigger which events for any particular
signal.
Figure 13-4. Three-State Buffers and Active Pull-Up Buffers
Three-state
buffer
1
2
3
1 Drive high on one edge
2 Switch to Hi-Z on later edge
3 Pull-up resistor maintains
logic high state
1 2
3
1 Drive high on one edge
2 Switch to Hi-Z when
Active
pull-up
buffer
5
4
threshold voltage
(Voh+margin) is reached
3 Pull-up resistor maintains
logic high state
4 Disable buffer as output
5 Pull-up resistor maintains
logic high state; other
driver can drive signal
Note: Events 1 and 4 can be in quick succession.
Содержание MPC860 PowerQUICC
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