Philips LPC900 User Guide

INTEGRATED CIRCUITS

ABSTRACT

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, which allows only 3V operation. This application note describes how these microcontrollers can be adapted to 5V-systems.

AN10220

Philips LPC900 family in 5 V environments

Wolfgang Schwartz

2003 May 21

Philips

Semiconductors

Philips Semiconductors	Application note
Philips LPC900 family in 5 V environments	AN10220

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 currents 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 3.0Vand 3.3V-systems with appropriate tolerances.

Figure 1

1 Please check Philips Semiconductors’ microcontrollers web site for product data [1].

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:

∙23 general-purpose I/O ports. These are bidirectional 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].

2003-May-21

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