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
0
0
1
0
1
Quasi-bidirectional (default)
Push-Pull output
0
1
Input Only (High-impedance)
Open-Drain Output
1
(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.
MEGAWIN
MPC82x54A Data Sheet
21
MEGAWIN [ MEGAWIN TECHNOLOGY CO., LTD ]
