UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
24 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
2.5 Watchdog oscillator option
The watchdog has a separate oscillator which has a frequency of 400 kHz. This oscillator
can be used to save power when a high clock frequency is not needed.
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 external clock input 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 external clock input 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. These requirements for clock
frequencies above 12 MHz do not apply when using the internal RC oscillator in
clock doubler mode.
Table 6.
On-chip RC oscillator trim register (TRIM - address 96h) bit description
Bit
Symbol
Description
0
TRIM.0
Trim value. Determines the frequency of the internal RC oscillator. During reset,
these bits are loaded with a stored factory calibration value. When writing to either
bit 6 or bit 7 of this register, care should be taken to preserve the current TRIM value
by reading this register, modifying bits 6 or 7 as required, and writing the result to
this register.
1
TRIM.1
2
TRIM.2
3
TRIM.3
4
TRIM.4
5
TRIM.5
6
ENCLK
when = 1,
CCLK
⁄
2
is output on the XTAL2 pin provided the crystal oscillator is not
being used.
7
RCCLK
when = 1, selects the RC Oscillator output as the CPU clock (CCLK). This allows for
fast switching between any clock source and the internal RC oscillator without
needing to go through a reset cycle.