© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 03 — 7 June 2005
21 of 139
Philips Semiconductors
UM10119
P89LPC938 User manual
2.6 External clock input option
In this configuration, the processor clock is derived from an external source driving the
XTAL1 / P3.1 pin. The rate may be from 0 Hz up to 18 MHz. The XTAL2 / P3.0 pin may be
used as a standard port pin or a clock output.
When using an oscillator frequency
above 12 MHz, the reset input function of P1.5 must be enabled. An external circuit
is required to hold the device in reset at power-up until V
DD
has reached its
specified level. When system power is removed V
DD
will fall below the minimum
specified operating voltage. When using an oscillator frequency above 12 MHz, in
some applications, an external brownout detect circuit may be required to hold the
device in reset when V
DD
falls below the minimum specified operating voltage.
Note: The oscillator must be configured in one of the following modes: Low frequency crystal,
medium frequency crystal, or high frequency crystal.
(1) A series resistor may be required to limit crystal drive levels. This is especially important for low
frequency crystals (see text).
Fig 6.
Using the crystal oscillator.
Fig 7.
Block diagram of oscillator control.
002aab008
P89LPC932A1
XTAL1
XTAL2
quartz crystal or
ceramic resonator
(1)
÷
2
002aaa891
RTC
CPU
WDT
DIVM
CCLK
UART
OSCCLK
I
2
C-BUS
PCLK
TIMER 0 AND
TIMER 1
HIGH FREQUENCY
MEDIUM FREQUENCY
LOW FREQUENCY
XTAL1
XTAL2
RC
OSCILLATOR
WATCHDOG
OSCILLATOR
(7.3728 MHz
±
1 %)
PCLK
RCCLK
SPI
CCU
(P89LPC932A1)
32
×
PLL
+
20%
−
30 %
(400 kHz )