Philips LPC900 User Guide
Introduction
AN10220
ABSTRACT
which allows only 3V operation. This application note describes how
2003 May 21
Philips
Semiconductors
INTEGRATED CIRCUITS
To be able to offer the LPC900 family of microcontrollers with so
many on-chip features at low prices, they are made in a process,
these microcontrollers can be adapted to 5V-systems.
Philips LPC900 family in 5 V environments
Wolfgang Schwartz
Application note
Philips Semiconductors
Philips LPC900 family in 5 V environments
INTRODUCTION
Philips LPC900 family of microcontrollers1 is made in
an advanced CMOS process, which allows the costeffective implementation of a broad variety of on-chip
features. Like in many other highly integrated ICs, the
small structures of such technologies limit the maximum supply voltages to values below 5V. The
LPC900 family is specified to work from 2.4V to 3.6V.
Although the supply voltage is lower, the I/O-pins are
5V-tolerant. I.e., the I/O stages cannot actively drive
the outputs higher than the supply voltage, but they
may be pulled to 5V externally.
Several aspects have to be taken into account, when
an LPC900 is to be used in a 5V-system:
• 3.3V power supply from a 5V source
• Interfacing inputs and outputs to 5V logic
levels.
• Driving external loads requiring higher cur-
rents and voltages above Vdd.
POWER SUPPLY CONSIDERATIONS
The LPC900 can be supplied with voltages between
2.4V and 3.6V. If the internal brownout detection circuit is used to detect power-failures, its trip-voltage
lies between 2.4V and 2.7V. In this case the nominal
supply voltage should be above 2.7V. This allows the
use in 3.0V- and 3.3V-systems with appropriate tolerances.
AN10220
In 5V-systems power for the 3V-microcontroller can
either be derived directly from the unregulated supply
or from the 5V supply.
The LPC932 e.g. consumes 25 mA maximum from
the 3V supply. The I/Os each can source or sink currents up to -3.2mA or +20mA respectively. The power
supply needs to be able to additionally handle the
total of these currents, particularly when this is drawn
from the 3V supply as well.
Smaller systems can use an LED with a typical voltage drop of 1.8V to generate the 3V from a 5V supply
(see Figure 1), while bigger systems will require an
LDO (Low Drop-Out regulator) like the Philips
TDA3663. The LED solution has the advantage of
not drawing any additional current, which is particularly important, when the microcontroller is in powerdown mode. To achieve the same with a voltage
regulator, a type with an enable input (e.g. TDA3673)
has to be chosen2.
Because the outputs of the LPC900 are 5V-tolerant,
driving loads from the 5V-supply directly can reduce
the current drawn from the 3V-supply (see below).
I/O PORT CHARACTERISTICS
An outstanding feature of the LPC900 family is that it
can be run without external components and all but
the power supply pins can be used as I/O. I.e., the
28-pin LPC932 has up to 26 I/Os. The I/O-pins are
(individually) configurable via SFR and can be
grouped as follows:
Figure 1
1
Please check Philips Semiconductors’ microcon-
trollers web site for product data [1].
2003-May-21 2 of 6
• 23 general-purpose I/O ports. These are bi-
directional I/Os. They are 5V tolerant with the
exception of two pins, which alternatively can
be used to build a crystal oscillator
• 1 Reset input, which can also be used as a
general-purpose input, when the internal reset function is used. It is not 5V tolerant.
• 2 I²C serial clock and data I/O. They are 5V
tolerant and can alternatively be used as input or open-drain output.
An I/O configured as bi-directional performs like a
standard 80C51-I/O: Writing a “0” to the port switches
on a very strong pull-down transistor, which is capa-
2
Please check Philips Semiconductors’ standard
analog web site for LDO product data [2].
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