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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
403 of 1441
NXP Semiconductors
UM10503
Chapter 17: LPC43xx/LPC43Sxx System Control Unit (SCU)/ IO
17.3.2 Digital pin mode
) in the SFS registers allow the selection of weak
on-chip pull-up or pull-down resistors with a typical value of 50 k
for each pin or the
selection of the repeater mode.
The possible on-chip resistor configurations are pull-up enabled, pull-down enabled, or no
pull-up/pull-down. The default value is pull-up enabled.
The repeater mode enables the pull-up resistor if the pin is at a logic HIGH and enables
the pull-down resistor if the pin is at a logic LOW. This causes the pin to retain its last
known state if it is configured as an input and is not driven externally. Repeater mode may
typically be used to prevent a pin from floating (and potentially using significant power if it
floats to an indeterminate state) if it is temporarily not driven.
To select the repeater mode, enable both the pull-up and the pull-down resistor in the SFS
registers.
17.3.3 Input buffer
To be able to receive a digital signal, the input buffer must be enabled through bit EZI in
the pin configuration registers (see
). By default, the input buffer is disabled.
For pads that support both a digital and an analog function, the input buffer must be
disabled before enabling the analog function (see
17.3.4 Programmable glitch filter
All digital pins support a programmable glitch filter (bit ZIF), which can be switched on or
off (see
). By default, the glitch filter is on. The glitch filter should be disabled for
clocking signals with frequencies higher than 30 MHz.
17.3.5 Programmable slew rate
Normal-drive and high-speed pins support a programmable slew rate (bit EHS) to select
between lower noise and speed or higher noise and speed (see
). The typical
frequencies supported are 50 MHz/80 MHz for normal-drive pins and 75 MHz/204 MHz for
high-speed pins.
17.3.6 High-speed pins
The clock pins CLK0 to CLK3 and P3_3 support a programmable high-speed output with
typical frequencies of 75 MHz or 204 MHz depending on the slew rate setting (see
17.3.7 High-drive pins
) support a high-drive output with four programmable
levels.
High-drive pins support the programmable glitch filter but not the programmable slew rate.