AT89LP213 datasheet(24/98 Pages) ATMEL

24 / 98 page

3538D–MICRO–10/09

AT89LP213/214

13.1

Port Configuration

All port pins on the AT89LP213/214 may be configured to one of four modes: quasi-bidirectional

(standard 8051 port outputs), push-pull output, open-drain output, or input-only. Port modes may

be assigned in software on a pin-by-pin basis as shown in Table 13-2. The Tristate-Port User

Fuse determines the default state of the port pins. When the fuse is enabled, all port pins default

to input-only mode after reset, with the exception of P1.4 which starts in quasi-bidirectional

mode. When the fuse is disabled, all port pins, with the exception of P1.0 and P1.1, default to

quasi-bidirectional mode after reset and are weakly pulled high. Each port pin also has a

Schmitt-triggered input for improved input noise rejection. During Power-down all the Schmitt-

triggered inputs are disabled with the exception of P1.3, P3.2 and P3.3, which may be used to

wake up the device. Therefore P1.3, P3.2 and P3.3 should not be left floating during Power-

down. It is recommended that P3.1–0 on AT89LP213 and P3.4–5 on AT89LP214 be configured

for either quasi-bidirectional or push-pull output mode.

13.1.1

Quasi-bidirectional Output

Port pins in quasi-bidirectional output mode function similar to standard 8051 port pins. A Quasi-

bidirectional port can be used both as 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 driven low, it is driven strongly and able to

sink a large current. 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”. This 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 this pin is pulled low by an external

device, this 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 pull the port pin below its input threshold voltage.

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 two CPU clocks quickly pulling the

port pin high. The quasi-bidirectional port configuration is shown in Figure 13-1. The input cir-

cuitry of P1.3, P3.2 and P3.3 is not disabled during Power-down (see Figure 13-3).

Table 13-2.

Configuration Modes for Port x, Bit y

PxM0.y

PxM1.y

Port Mode

Quasi-bidirectional

Push-pull Output

Input Only (High Impedance)

Open-drain Output

AT89LP213 Datasheet(PDF) 24 Page - ATMEL Corporation

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