Intel 8253 - Open Drain Output; Push-Pull Output

4/17/98 7-19 External Bus
Open Drain Output
Another port output configuration provided by the standard XA I/O ports is open drain. This
configuration turns off all pullups and only drives the pulldown transistor of the port driver when
the port latch contains a logic 0. To be used as a logic output, a port configured in this manner
must have an external pullup, typically a resistor tied to Vdd. The pulldown for this mode is the
same as for the quasi-bidirectional mode.
An advantage of the open drain output is that is may be used to create wired AND logic. Several
open drain outputs of various devices can be tied together, and any one of them can drive the
wire low, creating a logical AND function without using a logic gate. The figure below show the
structure of the open drain output.
Figure 7.16 Structure of the Open Drain Output Configuration
Push-Pull Output
The push-pull output mode has the same pulldown structure as both the open drain and the quasi-
bidirectional output modes, but provides a continuous strong pullup when the port latch contains
a logic 1. This mode uses the same pullup as the strong pullup for the quasi-bidirectional mode.
The push-pull mode may be used when more source current is needed from a port output. The
output structure for this mode is shown below.
Figure 7.17 Structure of the Push-Pull Output Configuration
port
pin
input
data
port latch
data
N
port
pin
Vdd
input
data
port latch
data
N
P