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].

Application note

Philips Semiconductors

Philips LPC900 family in 5 V environments

Table 1

Port 5V tolerant Input Quasi Bi-Directional Push-Pull Open-Drain

P0.0 - P0.7 X X X X X

P1.0, P1.1, P1.4, P1.6, P1.7 X X X X X

P2.0 - P2-7 X X X X X

P1.2, P1.3 X X - - X

P3.0 - P3.1 - X X X X

P1.5 - X - - -

ble of sinking up to 20mA in case of the LPC900­family.
Writing a “1” to the port switches the pull-down tran­sistor off and activates a strong pull-up transistor
momentarily. This rapidly charges external load ca­pacitances and allows a fast output transition. Two
weak pull-up devices sustain the high output level
capable of sourcing up to 20µA.

In this configuration the port also serves as an input.
Pulling down the input, the driving circuitry has to
sink the current supplied by the two pull-up devices.

high. Therefore no such devices3 are present in P1.2
and P1.3, which can be used as SCL and SDA in an
I²C bus system.

All ports can be configured as input by putting the
outputs into tri-state, no pull-up or pull-down are acti­vated. The high-impedance inputs (leakage currents
below 10µA) are equipped with Schmitt-Trigger cir­cuitry to suppress input noise. All but three inputs are
5V tolerant in this configuration (see Table 1).

INTERFACING INPUTS TO 5V LOGIC

AN10220

Once the input level is below about 1.5V, one of the
pull-up devices is switched off and only the current of
the remaining pull-up needs to be drawn. This static
current is lower than 50µA, while the transitional cur­rent at 1.5V may be as high as 250µA. By this means
a hysteresis is achieved, which is dependent on the
impedance of the driving circuitry.

In push-pull mode a “0” switches on the strong pull­down transistor like in bi-directional mode. For the
high-level output, however, the strong pull-up is
switched on as long as a “1” is output; the weak pull­up devices are not used. The output can source up to

3.2mA in this configuration.
In open-drain mode only the strong pull-down tran-

sistor is available, no pull-up devices are involved at
all.

The I²C-bus requires special open-drain drivers. An
un-powered microcontroller must not disturb the
communication of other devices on the I²C-bus. With
no power, Vdd would be 0V and any structure like a
pull-up or ESD protection device connected to V
would clamp the voltage level on the I²C-bus to about
1V and thus prevent the I²C-bus lines from going

dd

5V logic families have different trip points for their
logic levels. TTL-type levels, which were adopted by
some CMOS logic families, are rather asymmetrical,
while pure CMOS logic has symmetrical levels. Spe­cial care must be taken when driving CMOS devices
with TTL logic.

Figure 2

The inputs of the LPC900 can be driven directly from
outputs of 5V logic families, when they are configured

2003-May-21 3 of 6

3

Of course, a different kind of ESD protection is im-

plemented on these pins.

Application note

Philips Semiconductors

Philips LPC900 family in 5 V environments

as inputs4. The (worst-case) switching levels of the
Schmitt-Trigger inputs are specified as follows:

VIL: 0.528V @ Vdd=2.4V
VIH: 2.520V @ Vdd=3.6V

When driven from 5V CMOS logic families, the re­sulting noise immunity is worst at the low level, where
the CMOS output may be as high as 0.5V. This im­proves from 25mV at Vdd=2.4V to about 0.7V in typi­cal applications with supply voltages of 3.0V or 3.3V.
The output high level of the CMOS logic is close to
5V, thus not critical.

The low level output of 5V (and 3.3V) TTL-logic is
<0.4V, resulting in a worst case noise immunity of
125mV. More critical is its >2.4V output high level.
With the LPC900 it virtually gives no noise margin
(see Figure 2). Some measures have to be taken to
improve this.

AN10220

INTERFACING OUTPUTS TO 5V LOGIC

Driving a low-level the LPC900 can sink up to 20mA
at a voltage not higher than 0.3V. This gives an ex­cellent noise margin of > 500mV for TTL and >1.2V
for CMOS.

Because the LPC900 can drive a high-level close to
the supply voltage, an output can drive a TTL-input
(which are rather high impedance at high level) di­rectly; the noise margin still is >0.2V. The outputs
should be configured as bi-directional; the weak in­ternal pull-up devices pull the levels up to Vdd.

When no current is drawn from a TTL output, the
high level voltage is significantly higher than 2.4V
and should work well with the (high-impedance) input
of the LPC900. To be sure a pull-up resistor of some
kO (pull-up to either 5V or the microcontrollers Vdd)
should be added

5

Although the noise margin of TTL seems to be worse
than that of CMOS, it should be noted, that the output
impedance of TTL is much lower than that of CMOS.
This makes it less susceptible to capacitively coupled
noise.

Also the use of the internal pull-ups of the LPC900
can be considered. To achieve this, the port has to
be configured as bi-directional I/O with a “1” written to
it. However, the following obstacles have to be taken
into account:

• the current flowing into the pull-up device
above Vdd is typically higher than the leakage
currents (and is not specified),

• this current adds to the power supply of the
microcontroller. Care must be taken, that the
sum of currents flowing into the pins does not
pull the supply voltage higher than specified
i.e., the power supply must be able to sink
current (particularly in power-down)!

4

This is the default mode after power-up.

5

For 3.3V TTL a pull-up resistor to 3.3V will also im-

prove the noise margin.

Figure 3

The input high level of 5V CMOS-logic cannot be
reached by any 3V part without external components.
Either a level-shifter6 or external pull-up resistors to
5V with the LPC900’s output in open-drain configura­tion can be used. The value of the pull-up resistor
depends on the load characteristics (capacitance,
current, speed) and the drive capability of the
LPC900; a resistor of some kO should do in most
cases.

DRIVING OTHER LOADS

The rather strong outputs of the LPC900 can sink
20mA or source 3.2mA. This combined with the op­tion to individually configure the outputs to bi­directional, push-pull or open drain gives a high de­gree of flexibility to drive any load.

The push-pull configuration cannot be used to drive
loads above Vdd; this would result in excessive cur­rents flowing into the port.

6

Please check Philips Semiconductors’ logic web

site for product selection [5].

2003-May-21 4 of 6

Application note

Philips Semiconductors

Philips LPC900 family in 5 V environments

The internal pull-up devices of the bi-directional
mode can be used to support an external pull-up re­sistor by actively pulling the output up to Vdd. How­ever, an unspecified current will flow into the output
above Vdd as described above.

The best solution to handle higher voltages than V
is the use of the open-drain configuration. An exter­nal resistor can pull the level up to 5.5V maximum
with only leakage currents flowing into the LPC900
output.

Like in most CMOS devices the LPC900 outputs’
drive capability is unsymmetrical; they can sink much
higher currents than they can source. Higher current
loads up to 20mA can preferably be switched to GND
or Vss. Tying the loads directly to the 5V supply also
reduces the load on the (regulated) 3V power supply.

LED e.g. should advantageously be connected to 5V
via a current limiting resistor. For higher currents than
20mA several outputs of one port can be used in
parallel; it only has to be made sure that all are
driven high or low at the same time and that the sum
of all currents does not exceed 80mA.

dd

AN10220

ANALOG INPUTS

The analog comparators of the LPC900 can be used
in different configurations. Their input voltage range
is specified from Vss to Vdd - 0.3V. To monitor volt­ages above this limit they need to be scaled down by
any kind of voltage divider. Because of the high im­pedance of the inputs - the maximum leakage current
is less than 10µA - a resistive divider can be applied
in many cases.

REFERENCES

1. Philips Semiconductors microcontrollers web
site: http://www.PhilipsMCU.com

2. Philips Semiconductors standard analog web
site:

http://www.semiconductors.philips.com/markets/mms/products/analog/index.html

3. Philips Semiconductors standard logic web site:

http://www.philipslogic.com/logic/

4. Ramin Kowssari, Mike Magdaluyo

Sorting through the low voltage logic maze

AN10156, Philips Semiconductors, 2002 June 06

http://www.philipslogic.com/support/appnotes/logic/pdf/an10156.pdf

If a higher-current load connected to ground needs to
be driven, some external components are required.

Figure 4

Either a p-channel logic-FET or a bipolar pnp­transistor will do the job (see Figure 4). R1 is
required to make sure T1 switches off when the
microcontroller’s (open-drain) output is also off. R2 is
needed to limit the current driving the transistor; its
value is determined by the transistor parameters and
the load-current. It is not necessary when a p­channel FET is used.

2003-May-21 5 of 6

Philips Semiconductors

Application note

Philips LPC900 family in 5 V environments AN10220

Revision history: 2003-05-21 Revision 01

Definitions

Short-form specification – The data in a short-form specification is extracted from a full data sheet with the same type number and title. For

detailed information, see the relevant datasheet or data handbook.
Limiting values definition – Limiting values given are in accordance with the Absolute Maximum Rating System (IEC134). Stress above one

or more of the limiting values may cause permanent damage to the device. These are stress ratings only and operation of the device at these
or at any other conditions above those given in the Characteristics sections of the specification is not implied. Exposure to limiting values for
extended periods may affect device reliability.

Application information – Applications that are described herein for any of these products are for illustrative purposes only. Philips Semicon­ductors make no representation or warranty that such applications will be suitable for the specified use without further testing or modification.

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Document order number: 9397 750 11538

 Koninklijke Philips Electronics N.V. 2003

All rights reserved. Printed in U.S.A

Date of release: 05-03