MPC82E52AS datasheet(21/72 Pages) MEGAWIN

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MPC82x52A Data Sheet

Configuration of I/O port

PxM0n

PxM1n

Port Mode

Quasi-bidirectional(default)

Push-Pull output

Input Only (High-impedance)

Open-Drain Output

(x = 1 or 3

n = 7, 6, 5, 4, 3, 2, 1 or 0)

Quasi-bidirectional Mode

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 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.

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