AT89LP51 datasheet(41/113 Pages) ATMEL

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3709B–MICRO–12/10

AT89LP51/52 - Preliminary

10.1.1

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. When the pin is pulled low externally this pull-up will always source some

current.

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 one CPU clock, quickly pulling the port

pin high. The quasi-bidirectional port configuration is shown in Figure 10-1.

10.1.2

Input-only Mode

The input only port configuration is shown in Figure 10-2. The output drivers are tristated. The

input includes a Schmitt-triggered input for improved input noise rejection. The input circuitry of

P3.2, P3.3, P4.6 and P4.7 is not disabled during Power-down (see Figure 10-3) and therefore

these pins should not be left floating during Power-down when configured in this mode.

Input-only mode can reduce power consumption for low-level inputs over quasi-bidirectional

mode because the “very weak” pull-up is turned off and only very small leakage current in the

sub microamp range is present.

Table 10-3.

PMOD

– Port Mode Register

PMOD = C1H

Reset Value = 0000 0011B

Not Bit Addressable

P3M1

P3M0

P2M1

P2M0

P1M1

P1M0

P0M1

P0M0

Bit

543

Symbol

Function

P3M1-0

Port 3 Configuration Mode

P2M

1-0

Port 2 Configuration Mode

P1M1-0

Port 1 Configuration Mode

P0M1-0

Port 0 Configuration Mode

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