T84C5112-RKFEL datasheet(23/97 Pages) ATMEL

23 / 97 page

Rev. B - November 10, 2000

Preliminary

T80C5112

9. Ports

The T80C5112 has 5 I/O ports, from port 0 to port 4.. The number of possible I/O on each package option is

shown on Table 9.

At least 39 pins of the T80C5112 may be used as I/Os when a two-pin external oscillator.

Table 9. Number of available I/O versus pin number

All port1, port3 and port4 I/O port pins on the T80C5112 may be software configured to one of four types on a

bit-by-bit basis, as shown in Table 10. These are: quasi-bidirectional (standard 80C51 port outputs), push-pull,

open drain, and input only. Two configuration registers for each port choose the output type for each port pin.

Table 10. Port Output Configuration settings using PxM1 and PxM2 registers

9.1. Ports types

9.1.1. Quasi-Bidirectional Output Configuration

The default port output configuration for standard T80C5112 I/O ports is the quasi-bidirectional output that is

common on the 80C51 and most of its derivatives. This output type can be used as both an input and output

without the need to reconfigure the port. This is possible because when the port outputs a logic high, it is weakly

driven, allowing an external device to pull the pin low. When the pin is pulled low, it is driven strongly and able

to sink a fairly large current. These features are somewhat similar to an open drain output except that there are

three pull-up transistors in the quasi-bidirectional output that serve different purposes. One of these pull-ups, called

the "very weak" pull-up, is turned on whenever the port latch for the pin contains a logic 1. The very weak pull-

up sources a very small current that will pull the pin high if it is left floating. A second pull-up, called the "weak"

pull-up, is turned on when the port latch for the pin contains a logic 1 and the pin itself is also at a logic 1 level.

This pull-up provides the primary source current for a quasi-bidirectional pin that is outputting a 1. If a pin that

has a logic 1 on it is pulled low by an external device, the weak pull-up turns off, and only the very weak pull-

up remains on. In order to pull the pin low under these conditions, the external device has to sink enough current

to overpower the weak pull-up and take the voltage on the port pin below its input threshold.

The third pull-up is referred to as the "strong" pull-up. This pull-up is used to speed up low-to-high transitions on

a quasi-bidirectional port pin when the port latch changes from a logic 0 to a logic 1. When this occurs, the strong

pull-up turns on for a brief time, two CPU clocks, in order to pull the port pin high quickly. Then it turns off again.

The quasi-bidirectional port configuration is shown in Figure 5.

Supply pins

I/O

Inputs

Maximun I/O count

PxM1.y bit

PxM2.y bit

Port Output Mode

Quasi bidirectional

Push-Pull

Input Only (High Impedance)

Open Drain

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