AT87C5103-ICSIL datasheet(12/64 Pages) ATMEL

12 / 64 page

AT8xC5103

4134C–8051–09/04

Ports

The AT8xC5103 has 3 I/O ports, port 1, port 3 and port 4.

Except RST, and port 4, all port 1 and port 3 I/O port pins on the AT8xC5103 may be

software configured to one of four types on a bit-by-bit basis, as shown in Table 2 These

are: quasi-bi-directional (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.

Port Types

Quasi-Bi-directional Output

Configuration

The default port output configuration for standard AT8xC5103 I/O ports is the quasi-bi-

directional 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-bi-directional 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 cur-

rent for a quasi-bi-directional 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-bi-directional 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-bi-directional port configuration is shown in Figure 3.

PxM1.y BIt

PxM2.y Bit

Port Output Mode

Quasi bi-directional

Push-pull

Input Only (High Impedance)

Open Drain

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