AT83C5111 datasheet(18/86 Pages) ATMEL

18 / 86 page

AT8xC5111

4190A–8051–11/02

Ports

The low pin count versions of the AT8xC5111 has 3 I/O ports, port 1, port 3, and port 4.

All port1, port3 and port4 I/O port pins on the AT8xC5111 may be software configured to

one of four types on a bit-by-bit basis, as shown in Table 13. These are: quasi bi-direc-

tional (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 1. Port Output Configuration Settings Using PxM1 and PxM2 Registers

Port Types

Quasi Bi-directional Output

Configuration

The default port output configuration for standard AT8xC5111 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-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 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 2.

PxM1.y Bit

PxM2.y Bit

Port Output Mode

Quasi bidirectional

Push-pull

Input Only (High Impedance)

Open Drain

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