Motorola MC68VZ328 - Manual (Page 171)

I/O Ports 10-1
Chapter 10
I/O Ports
This chapter describes the 10 multipurpose ports of the MC68VZ328. It also describes how to use the ports
for external I/O control and to determine the status of the external signals. All 10 ports (A–G, J, K, and M)
are programmable I/O ports with pull-up and pull-down capability. Each port can be used as a
general-purpose I/O (GPIO) port, or it can be connected to its dedicated I/O function. Every signal line
connects to an external pin. Although each port consists of a group of five to eight signal lines, all
commands and actions occur at the pin level because each pin of a port is individually configured. The pin
name reflects the functions assigned to the pin. For example, the name PB/CSB1/SDWE indicates that the
pin is used for any of three separate signals: Port B data, Chip-Select B 1, and SDRAM Write-Enable. This
chapter describes pin assignments either programmed as GPIO or programmed to dedicated I/O functions.
When pins are programmed as GPIO, the direction of individual pins (input or output) can be configured,
and pull-up resistors (or pull-down resistors in some ports) can be enabled or disabled. When pins are
programmed as dedicated I/O, a pin’s direction cannot be controlled. A few exceptions to this rule are
noted in the programming information about the specific ports.
10.1 Port Configuration
With the exception of Port A, every port is multiplexed with at least one other dedicated I/O function.
Several ports have pins that can be configured for one of several dedicated I/O functions. Table 10-1 on
page 10-2 shows the I/O functions available for each port.
Ports are programmed by four dedicated 8-bit registers: direction, data, pull-up enable, and select. The
exceptions are Port A and Port D. Port A does not have a select register since it can only be used as a
GPIO. The remaining registers have select registers controlling whether the pin is assigned as a GPIO or a
dedicated I/O function. Some pins have multiple dedicated functions assigned to them. Selection of these
functions is controlled by other registers in the MC68VZ328. Port D is unique in that it is used for handling
external interrupts. It has four dedicated interrupt control registers in addition to the previously referenced
four registers.
The I/O drive control register (IOCR) in system control controls the drive strength (in mA) of all I/O
signals, including all of the ports. By default, all I/O pins on the MC68VZ328 default to a 4 mA driving
current. After reset, it is recommended the user select 2 mA drive strength for those signals not requiring
high current to ensure maximum power savings.