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UM10375
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 3 — 14 June 2011
25 of 368
NXP Semiconductors
UM10375
Chapter 3: LPC13xx System configuration
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated.
3.5.17 System AHB clock divider register
This register divides the main clock to provide the system clock to the core, memories,
and the peripherals. The system clock can be shut down completely by setting the DIV
bits to 0x0.
3.5.18 System AHB clock control register
The SYSAHBCLKCTRL register enables the clocks to individual system and peripheral
blocks. The system clock (sys_ahb_clk[0], bit 0 in the SYSAHBCLKCTRL register)
provides the clock for the AHB to APB bridge, the AHB matrix, the ARM Cortex-M3, the
Syscon block, and the PMU. This clock cannot be disabled.
Table 23.
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
0x0
0
No change
1
Update clock source
31:1
-
-
Reserved
0x00
Table 24.
System AHB clock divider register (SYSAHBCLKDIV, address 0x4004 8078) bit
description
Bit
Symbol
Description
Reset value
7:0
DIV
System AHB clock divider values
0: System clock disabled.
1: Divide by 1.
to
255: Divide by 255.
0x01
31:8
-
Reserved
0x00
Table 25.
System AHB clock control register (SYSAHBCLKCTRL, address 0x4004 8080) bit
description
Bit
Symbol
Value
Description
Reset
value
0
SYS
Enables clock for AHB to APB bridge, to the AHB
matrix, to the Cortex-M3 FCLK and HCLK, to the
SysCon, and to the PMU. This bit is read only.
1
0
Reserved
1
Enabled
1
ROM
Enables clock for ROM.
1
0
Disabled
1
Enabled
2
RAM
Enables clock for RAM.
1
0
Disabled
1
Enabled