AT89C51RD2/ED2 I/O ports (P1, P2, P3, P4, P5) implement the quasi-bidirectional 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 "weak"

pull-up, is turned on whenever the port latch for the pin contains a logic 1. The 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 "medium" 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-bidi-

The DPU bit (bit 7 in AUXR register) allows to disable the permanent weak pull up of all ports

when latch data is logical 0.

The quasi-bidirectional port configuration is shown in Figure 5-1.