P4M0
(Address=B3H, Port 4 Mode Register 0)
7
6
5
4
3
2
1
0
P4M0.3
P4M1
(Address=B4H, Port 4 Mode Register 1)
7
6
5
4
3
P4M0.2
P4M0.1
P4M0.0
2
1
0
P4M1.3
P4M1.2
P4M1.1
P4M1.0
9.1.1. Quasi-bidirectional
Port pins in quasi-bidirectional mode are 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.
9.1.2. Open-Drain Output
The open-drain output configuration turns off all pull-ups and only drives the pull-down transistor of the port pin
when the port register contains a logic “0”. To use this configuration in application, a port pin must have an
external pull-up, typically a resistor tied to VDD. The pull-down for this mode is the same as for the quasi-
bidirectional mode. In addition, the input path of the port pin in this configuration is also the same as quasi-
bidirectional mode.
18
MG84FL54B Data Sheet
MEGAWIN
MEGAWIN [ MEGAWIN TECHNOLOGY CO., LTD ]
