AT83EC5123 ATMEL Corporation, AT83EC5123 Datasheet - Page 13

• Manufacturer Part Number: AT83EC5123
• Description: (AT8xx512x) Smart Card Reader ICs
• Manufacturer: ATMEL Corporation

Port Configuration
Standard I/O P0
Quasi-Bi-directional Output
Configuration
4202B–SCR–07/03
The P0 port is described in Figure 5.
Figure 5. Standard Input/Output P0
The default port output configuration for standard I/O ports is the quasi-bi-directional
output that is common on the 80C51 and most of its derivatives. The “Port51” output
type can be used as both 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 port outputs a logic low state, it is driven strongly and is able to sink a fairly
large current.
These features are somewhat similar to an open-drain output except that there are three
pull-up transistors in the quasi-bi-directional output that serve different purposes.
One of these pull-ups, called the weak pull-up, is turned on whenever the port latch for
the pin contains a logic 1. The weak pull-up sources a very small current that will pull the
pin high if it is left floating. The weak pull-up can be turned off by the DPU bit in AUXR
register.
A second pull-up, called the medium pull-up, is turned on when the port latch for the pin
contains a logic 1 and the pin itself is also at a logic 1 level. This pull-up provides the pri-
mary source current for a quasi-bi-directional pin that is outputting a 1. If a pin that has a
logic 1 on it is pulled low by an external device, the medium pull-up turns off, and only
the weak pull-up remains on. In order to pull the pin low under these conditions, the
external device has to sink enough current to overpower the medium pull-up and take
the voltage on the port pin below its input threshold.
The “Port51” is described in Figure 6.
Input
Data
Port latch
Data
ADDR/DATA
1
0
CONTROL
MUX
PMOS
NMOS
AT8xC5122/23
Vcc
Pin
13