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CHAPTER 13
INPUT/OUTPUT PORTS
Many applications do not require full use of all the on-chip peripheral functions. For example, the
Chip-Select Unit provides a total of ten chip-select lines; only a large design would require all
ten. For smaller designs that require fewer than ten chip-selects, these pins would be wasted.
The input/output ports give system designers the flexibility to replace the functions of unused pe-
ripheral pins with general-purpose I/O ports. Many of the on-chip peripheral pin functions are
multiplexed with an I/O port. If a particular peripheral pin function is unnecessary in an applica-
tion, that pin can be used for I/O. The 80C186EC/80C188EC has three types of ports: bidirection-
al, output-only and open-drain bidirectional.
13.1 FUNCTIONAL OVERVIEW
All port pin types are derived from a common bidirectional port logic module. Unidirectional and
open-drain ports are a subset of the bidirectional module. The following sections describe each
port type. The bidirectional port is described in detail, as it is the basis for all of the other port
types. The descriptions for the unidirectional and open-drain ports only highlight their specific
differences from the common bidirectional module.
13.1.1 Bidirectional Port
Figure 13-1 shows a simplified schematic of a bidirectional port pin. The overall function of a
bidirectional port pin is controlled by the state of the Port Control Latch. The output of the Port
Control Latch selects the source of output data and the source of the control signal for the three-
state output driver. When the port is programmed to act as a peripheral pin, both the data for the
pin and the directional control signal for the pin come from the associated integrated peripheral.
When a bidirectional port pin is programmed as an I/O port, all port parameters are under soft-
ware control.
The output of the Port Direction latch enables (or disables) the three-state output driver when the
pin is programmed as an I/O port. The three-state output driver is enabled by clearing the Port
Direction latch. The data driven on an output port pin is held in the Port Data latch. Setting the
Port Direction latch disables the three-state output driver, making the pin an input.
The signal present on the device pin is routed through a synchronizer to a three-state latch that
connects to the internal data bus. The state of the pin can be read at any time, regardless of wheth-
er the pin is used as an I/O port or for a peripheral function.
13-1
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