UM10346_1
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 01 — 2 November 2009
24 of 132
NXP Semiconductors
UM10346
LPC980/982 User manual
2.8 Clock source switching on the fly
P89LPC980/982 can implement clock switching on any sources of watchdog oscillator,
7/14MHz IRC oscillator, crystal oscillator and external clock input during code is running.
CLKOK bit in register CLKCON is read only and used to indicate the clock switch status.
When CLKOK is ‘0’, clock switch is processing, not completed. When CLKOK is ‘1’, clock
switch is completed. When start new clock source switch, CLKOK is cleared
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).
(2) C1 and C2 are suggested to be equal in middle speed oscillator or high speed oscillator
mode. Higher capacitance is required on C1 in low speed oscillator mode.
For example, when crystal frequency is 32 Khz, the typical values of C1 and C2 are 68 pF and
22 pF.
Considering the diversity of crystals and resonators and differences in PCB layout, the
capacitances of C1 and C2 may need optimization to guarantee the oscillator system performance.
Note: The oscillator must be configured in one of the following modes: Low frequency crystal,
medium frequency crystal, or high frequency crystal.
Fig 6.
Using the crystal oscillator
002aad364
XTAL1
C1
C2
XTAL2
quartz crystal or
ceramic resonator
(1)
(1)
(1) ± 10 % at 400 kHz.
Fig 7.
Block diagram of oscillator control
÷
2
002aae978
RTC
CPU
WDT
DIVM
CCLK
UART
OSCCLK
TIMER 2/
TIMER 3/
TIMER 4
PCLK
TIMER 0 AND
TIMER 1
HIGH FREQUENCY
MEDIUM FREQUENCY
LOW FREQUENCY
XTAL1
XTAL2
RC OSCILLATOR
WITH CLOCK DOUBLER
WATCHDOG
OSCILLATOR
(7.3728 MHz/14.7456 MHz
±
1 %)
PCLK
RCCLK
SPI
I
2
C-BUS
(400 kHz/25 kHz)
(1)