mpc82x52a Megawin Technology, mpc82x52a Datasheet - Page 21

• Manufacturer Part Number: mpc82x52a
• Description: 8-bit Micro-controller
• Manufacturer: Megawin Technology

MEGAWIN
Configuration of I/O port
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.
(x = 1 or 3
PxM0n
0
0
1
1
n = 7, 6, 5, 4, 3, 2, 1 or 0)
PxM1n
0
1
0
1
Quasi-bidirectional(default)
Push-Pull output
Input Only (High-impedance)
Open-Drain Output
Port Mode
MPC82x52A Data Sheet
21