Philips 8-bit microcontroller with two-clock 80C51 core UM10109 User Manual

 

© Koninklijke Philips Electronics N.V. 2005. All rights reserved.

All but three I/O port pins on the P89LPC932A1 may be configured by software to one of 
four types on a pin-by-pin basis, as shown in 

. These are: quasi-bidirectional 

(standard 80C51 port outputs), push-pull, open drain, and input-only. Two configuration 
registers for each port select the output type for each port pin. 

P1.5 (RST) can only be an input and cannot be configured.

P1.2 (SCL/T0) and P1.3 (SDA/INT0) may only be configured to be either input-only or 
open drain.

Quasi-bidirectional outputs can be used both 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 is driven 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 latch 
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 latch 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 an external device, the 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 overpower 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 latch 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.

The quasi-bidirectional port configuration is shown in 

.

Although the P89LPC932A1 is a 3 V device most of the pins are 5 V-tolerant. If 5 V is 
applied to a pin configured in quasi-bidirectional mode, there will be a current flowing from 
the pin to V

DD

 causing extra power consumption. Therefore, applying 5 V to pins 

configured in quasi-bidirectional mode is discouraged.

A quasi-bidirectional port pin has a Schmitt-triggered input that also has a glitch 
suppression circuit

0

0

Quasi-bidirectional

0

1

Push-pull

1

0

Input only (high-impedance)

1

1

Open drain