DS80C320-MCG Datasheet, PDF (117/175 Pages) Dallas Semiconductor – High-Speed Microcontroller User Guide

PORT 2 FUNCTIONAL CIRCUITRY Figure 10-2
ADDRESS
A8-A15
ADDRESS
CONTROL
INTERNAL
DATA BUS
DQ
Q
WRITE
ENABLE
DELAY
=2Tclk
READ
ENABLE
READ
LATCH/PIN
High-Speed Microcontroller User’s Guide
VCC
VCC
PORT
0.n
VCC
POWER
DOWN
PORTS 1 AND 3
Ports 1 and 3 are general purpose I/O ports with optional special functions associated with each pin.
Enabling the special function automatically converts the I/O pin to that function. To insure proper
operation, each alternate function pin should be programmed to a logic 1. For example, enabling the
UART converts P3.0 and P3.1 to the serial I/O functions.
The drive characteristics of these pins do not change when the pin is configured for general I/O or as the
special function associated with that pin. The exceptions are pins P3.6 and P3.7, which employ the
current-limited transition drivers described later when used as RD and WR signals. The drive
characteristics of Port 1 and Port 3 are the same as for Port 2 (non-bus mode). That is, the logic 0 is
created by a strong pull-down. The logic 1 is created by a strong transition pull-up that changes to a weak
pull-up.
Using one or more I/O pins of a port as special function pins will not effect the remaining port pins. An
extreme example is as follows. P3.6 has the alternate function of WR and P3.7 of RD. These strobes are
used for expanded data memory access. If a system used only the RD signal, then P3.6 would still be
available as an I/O port. This is not a practical suggestion, but it illustrates how the special functions are
independent.
A more practical application is the optional use of an interrupt. If INT0 (P3.2) is enabled, then an
externally imposed logic 0 will cause an interrupt. By then disabling the INT0, P3.2 can be used as a
general purpose I/O pin. This allows the INT0 to be used to “wake-up” the system, but does not
eliminate another use of the pin.
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