tspc860 ATMEL Corporation, tspc860 Datasheet - Page 25

• Manufacturer Part Number: tspc860
• Description: Integrated Communication Processor
• Manufacturer: ATMEL Corporation

Active Pull-up Buffers
Figure 6. Three-State Buffers and Active Pull-Up Buffers
Note:
2129B–HIREL–12/04
Events 1 and 4 can be in quick succession.
Three-state
pull-up
Active
buffer
buffer
Active pull-up buffers are a special variety of bidirectional three-state buffer with the fol-
lowing properties:
•
•
•
Due to the behavior of the buffer when being driven high, a pull-up resistor is required
externally to function as a ‘bus keep’ 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 fight and possible damage
to the TSPC860, to the system, or to both.
Figure 6 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.
1
1 2
When enabled as an output and driving low, they behave as normal output drivers
(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 should not be driven.
2
3
4
3
5
1 Drive high on one edge
2 Switch to Hi-Z on later edge
3 Pull-up resistor maintains logic high state
1 Drive high on one edge
2 Switch to Hi-Z when threshold voltage
3 Pull-up resistor maintains logic high state
4 Disable buffer as output
5 Pull-up resistor maintains logic high state;
(Voh + margin) is reached
other driver can drive signal
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