UM11029
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© NXP Semiconductors N.V. 2017. All rights reserved.
User manual
Rev. 1.0 — 16 June 2017
97 of 515
NXP Semiconductors
UM11029
Chapter 8: LPC84x System configuration (SYSCON)
8.6.20 External clock source select register
The EXTCLKSEL register selects the external clock, which can be the system oscillator or
clk_in (direct from external IO).
8.6.21 System clock control 0 register
The SYSAHBCLKCTRL0 register enables the clocks to individual system and peripheral
blocks. The system clock (bit 0) provides the clock for the AHB, the APB bridge, the ARM
Cortex-M0+, the SYSCON block, and the PMU. This clock cannot be disabled.
Table 144. SCT clock divider register (SCTCLKDIV, address 0x4004 8070) bit description
Bit
Symbol
Value
Description
Reset value
7:0
DIV
SCT clock divider values.
0: SCT clock disabled.
1: Divide by 1.
…
255: Divide by 255.
0x0
31:8
-
Reserved
-
Table 145. External clock source select register (EXTCLKSEL, address 0x4004 8074) bit
description
Bit
Symbol
Value
Description
Reset value
0
SEL
Clock source for external clock
0x0
0x0
System oscillator
0x1
CLK_IN
31:1
-
Reserved
-
Table 146. System clock control 0 register (SYSAHBCLKCTRL0, address 0x4004 8080) bit
description
Bit
Symbol
Value
Description
Reset
value
0
SYS
Enables the clock for the AHB, the APB bridge, the
Cortex-M0+ core clocks, SYSCON, and the PMU.
This bit is read only and always reads as 1.
1
1
ROM
Enables clock for ROM.
1
0
Disable
1
Enable
2
RAM0_1
Enables clock for SRAM0 and SRAM1.
1
0
Disable
1
Enable
3
Reserved
0
4
FLASH
Enables clock for flash.
1
0
Disable
1
Enable
5
I2C0
Enables clock for I2C0.
0
0
Disable
1
Enable