NXP Semiconductors LPC11U1x, User Manual

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User manual Rev. 5.5 — 21 December 2016 30 of 523
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Chapter 3: LPC11U3x/2x/1x System control block
3.5.15 Main clock source select register
This register selects the main system clock, which can be the system PLL (sys_pllclkout),
or the watchdog oscillator, or the IRC oscillator. The main system clock clocks the core,
the peripherals, and the memories.
Bit 0 of the MAINCLKUEN register (see Section 3.5.16 ) must be toggled from 0 to 1 for
the update to take effect.
3.5.16 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 bit 0 of this register, then write a one.
3.5.17 System clock divider register
This register controls how the main clock is divided to provide the system clock to the
core, memories, and the peripherals. The system clock can be shut down completely by
setting the DIV field to zero.
Table 20. USB PLL clock source update enable register (USBPLLCLKUEN, address 0x4004
804C) bit description
Bit Symbol Value Description Reset value
0 ENA Enable USB PLL clock source update 0x0
0 No change
1 Update clock source
31:1 - - Reserved 0x00
Table 21. 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 0
0x0 IRC Oscillator
0x1 PLL input
0x2 Watchdog oscillator
0x3 PLL output
31:2 - - Reserved -
Table 22. Main clock source update enable register (MAINCLKUEN, address 0x4004 8074)
bit description
Bit Symbol Value Description Reset value
0 ENA Enable main clock source update 1
0 No change
1 Update clock source
31:1 - - Reserved -