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UM10601
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© NXP B.V. 2012. All rights reserved.
Preliminary user manual
Rev. 1.0 — 7 November 2012
24 of 313
NXP Semiconductors
UM10601
Chapter 4: LPC800 System configuration (SYSCON)
4.6.9 System PLL clock source update 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.
4.6.10 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
) must be toggled from 0 to 1 for the
update to take effect.
4.6.11 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.
Table 14.
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
0
0
No change
1
Update clock source
31:1
-
-
Reserved
-
Table 15.
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 16.
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
0
0
No change
1
Update clock source
31:1
-
-
Reserved
-