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MPC82x54A Data Sheet

SFR: P3M1 (P3 Configuration 1)

Bit-7

Bit-6

Bit-5

Bit-4

Bit-3

Bit-2

Bit-1

Bit-0

P3M17

P3M16

P3M15

P3M14

P3M13

P3M12

P3M11

P3M10

Configuration of I/O port

PxM0n

PxM1n

Port Mode

Quasi-bidirectional (default)

Push-Pull output

Input Only (High-impedance)

Open-Drain Output

(x = 0, 1, 2 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 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.

MPC82E54AS Datasheet(PDF) 21 Page - Megawin Technology Co., Ltd

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