Datasheet

UM10398 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2014. All rights reserved.
User manual Rev. 12.3 — 10 June 2014 31 of 547
NXP Semiconductors
UM10398
Chapter 3: LPC111x/LPC11Cxx System configuration (SYSCON)
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 Section 3.5.12
) 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 17. System PLL clock source update enable register (SYSPLLCLKUEN, 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 18. 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