UM10429
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User manual
Rev. 1 — 20 October 2010
17 of 258
NXP Semiconductors
UM10429
Chapter 3: LPC1102 System configuration
3.5.10 System PLL clock source update enable register
This register updates the clock source of the system PLL with the new input clock after the
SYSPLLCLKSEL register has been written to. In order for the update to take effect, first
write a zero to the SYSPLLUEN register and then write a one to SYSPLLUEN.
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated.
3.5.11 Main clock source select register
This register selects the main system clock which can be either any input to the system
PLL, the output from the system PLL (sys_pllclkout), or the watchdog or IRC oscillators
directly. The main system clock clocks the core, the peripherals, and the memories.
The MAINCLKUEN register (see
) must be toggled from LOW to HIGH for
the update to take effect.
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated.
Remark:
When using the C_CAN controller with baudrates above 100 kbit/s, the system
oscillator must be selected.
3.5.12 Main clock source update enable register
This register updates the clock source of the main clock with the new input clock after the
MAINCLKSEL register has been written to. In order for the update to take effect, first write
a zero to the MAINCLKUEN register and then write a one to MAINCLKUEN.
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated.
Table 15.
System PLL clock source update enable register (SYSPLLUEN, address 0x4004
8044) bit description
Bit
Symbol
Value
Description
Reset value
0
ENA
Enable system PLL clock source update
0x0
0
No change
1
Update clock source
31:1
-
-
Reserved
0x00
Table 16.
Main clock source select register (MAINCLKSEL, address 0x4004 8070) bit
description
Bit
Symbol
Value
Description
Reset value
1:0
SEL
Clock source for main clock
0x00
0x0
IRC oscillator
0x1
Input clock to system PLL
0x2
WDT oscillator
0x3
System PLL clock out
31:2
-
-
Reserved
0x00