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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
38 of 571
NXP Semiconductors
UM10316
Chapter 3: LPC29xx Clock Generation Unit (CGU)
[1]
Do not change the BYPASS and ENABLE bits in one write-action: this will result in unstable device
operation!
[2]
For between 15 MHz to 20 MHz the state of the HF pin is don’t care, see also the crystal specification notes
in
. Section 11 (Oscillator).
5.5 PLL status register (CGU0 and CGU1)
The register PLL_STATUS reflects the status bits of the PLL.
5.6 PLL control register (CGU0 and CGU1)
The PLL_CONTROL register contains the control bits for the PLL. In the CGU0, only the
crystal oscillator is allowed as an input to the PLL. In the CGU1, both internal base clocks,
BASE_ICLK0_CLK and BASE_ICLK1_CLK, can be inputs to the PLL.
Post-divider ratio programming
The division ratio of the post-divider is controlled by PSEL[0:1] in the PLL_CONTROL
register. The division ratio is twice the value of P. This guarantees an output clock with a
50% duty cycle.
Table 18.
XTAL_OSC_CONTROL register bit description (XTAL_OSC_CONTROL, address
0xFFFF 8020)
* = reset value
Bit
Symbol
Access
Value
Description
31 to 3
reserved
R
-
Reserved
2
HF
R/W
Oscillator HF pin
1*
Oscillator high-frequency mode (crystal or
external clock source 15 to 25 MHz)
0
Oscillator low-frequency mode (crystal or
external clock source 1 to 20 MHz)
1
BYPASS
R/W
Configure crystal operation or external-clock
input pin XIN_OSC
0*
Operation with crystal connected
1
Bypass mode. Use this mode when an external
clock source is used instead of a crystal
0
ENABLE
R/W
Oscillator-pad enable
0
Power-down
1*
Enable
Table 19.
PLL_STATUS register bit description (PLL_STATUS, address 0xFFFF 8024
(CGU0) and 0xFFFF B024 (CGU1))
* = reset value
Bit
Symbol
Access
Value
Description
31 to 1
reserved
R
-
Reserved; do not modify. Read as logic 0, write
as logic 0
0
LOCK
R
Indicates if the PLL is in lock or not.
1
In lock
0*
Not in lock