UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
38 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
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 P89LPC952/954 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
(Please refer to the
P89LPC952/954 data sheet, Dynamic characteristics
for glitch filter
specifications).
5.3 Open drain output configuration
The open drain output configuration turns off all pull-ups and only drives the pull-down
transistor of the port pin when the port latch contains a logic 0. To be used as a logic
output, a port configured in this manner must have an external pull-up, typically a resistor
tied to V
DD
. The pull-down for this mode is the same as for the quasi-bidirectional mode.
The open drain port configuration is shown in
.
An open drain port pin has a Schmitt-triggered input that also has a glitch suppression
circuit.
Please refer to the
P89LPC952/954 data sheet, Dynamic characteristics
for glitch filter
specifications.
Fig 10. Quasi-bidirectional output.
002aaa914
2 CPU
CLOCK DELAY
port latch
data
weak
strong
input
data
very
weak
P
P
P
V
DD
port
pin
glitch rejection