AT89LP2052 datasheet(21/94 Pages) ATMEL

21 / 94 page

3547J–MICRO–10/09

AT89LP2052/LP4052

A second pull-up, called the “weak” pull-up, is turned on when the port register for the pin con-

tains 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 over-

power 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 register changes from a logic “0”

to a logic “1”. When this occurs, the strong pull-up turns on for one CPU clock, quickly pulling the

port pin high.

When in quasi-bidirectional mode the port pin will always output a “0” when corresponding bit in

the port register is also “0”. When the port register is “1” the pin may be used either as an input

or an output of “1”. The quasi-bidirectional port configuration is shown in Figure 15-1. The input

circuitry of P3.2 and P3.3 is not disabled during Power-down (see Figure 15-3).

Figure 15-1. Quasi-bidirectional Output

15.2

Input-only Mode

The input port configuration is shown in Figure 15-2. It is a Schmitt-triggered input for improved

input noise rejection.

Figure 15-2. Input Only

Figure 15-3. Input Only for P3.2 and P3.3

1 Clock Delay

(D Flip-Flop)

Strong

Very

Weak

Port

VCC

From Port

Input

Data

PWD

Port

Input

Data

PWD

Port

Input

Data

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