NXP Semiconductors P89LPC9321 UM10310 - Manual (Page 29)

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User manual Rev. 2 — 1 November 2010 29 of 139
NXP Semiconductors
UM10310
P89LPC9321 User manual
4.1 Port configurations
All but three I/O port pins on the P89LPC9321 may be configured by software to one of
four types on a pin-by-pin basis, as shown in
Table 13 . 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.
4.2 Quasi-bidirectional output configuration
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 Figure 10 .
Although the P89LPC9321 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
Table 13. Port output configuration settings
PxM1.y PxM2.y Port output mode
0 0 Quasi-bidirectional
0 1 Push-pull
1 0 Input only (high-impedance)
1 1 Open drain