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4.2.1 Quasi-bidirectional I/O

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

port can be used 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

outputs 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 register 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 register 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 the 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 two CPU clocks, quickly pulling the port pin high.

Vcc

2 clock

delay

Vcc

PORT

Weak

Very weak

Strong

PORT

LATCH DATA

INPUT

DATA

Quasi-bidirectional output

4.2 I/O ports Modes

STC12LE5616AD Datasheet(PDF) 74 Page - STCmicro Technology Co,.Ltd