DR
AFT
DR
AFT
DRAFT
DR
D
RAFT
DRAFT
DRA
FT DRAF
D
RAFT DRAFT DRAFT DRAFT DRAFT D
DRAFT
D
RAFT DRA
FT DRAFT DRAFT DRAFT DRA
UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
28 of 571
NXP Semiconductors
UM10316
Chapter 3: LPC29xx Clock Generation Unit (CGU)
The PLL reference input clock is provided by the external oscillator (XO50M). The PLLs
accept an input clock frequency in the range of 10 MHz to 25 MHz only. The input
frequency can be directly routed to the post-divider using the BYPASS control. The
post-divider can be bypassed using the DIRECT control.
The post-divider is controlled by settings of the field PSEL in the output control register.
PSEL is a 2-bit value that selects a division between 1 and 8 in powers of 2.
The feedback divider is controlled by settings of the MSEL field in the output control
register. The MSEL is a 5-bit value corresponding to the feedback divider minus 1. Thus, if
MSEL is programmed to 0 the feedback divider is 1.
In normal mode the post-divider is enabled and the following relations are verified:
F
clkout
= MDIV
×
F
clkin
= F
cco
/ 2
×
PDIV
Values of the dividers are chosen with the following process:
1. Specify the input clock frequency F
clkin
2. Calculate M to obtain the desired output frequency F
clkout
with M = F
clkout
/ F
clkin
3. Find a value for P so that F
cco
= 2
×
P / F
clkout
4. Verify that all frequencies and divider values conform to the limits
In direct mode, the following relations are verified:
F
clkout
= M
×
F
clkin
= F
cco
Unless the PLL is configured in bypass mode it must be locked before using it as a clock
source. The PLL lock indication is read from the PLL status register.
Once the output clock is generated it is possible to use a three-phase output control which
generates three clock signals separated in phase by 120°. This setting is controlled by
field P23EN.
Settings to power down the PLL, controlled by field PD in the PLL control register, and
safe switch setting controlled by the AUTOBLOK field are not shown in the illustration.
Note that safe switching of the clock is not enabled at reset.
Fig 12. PLI60MPLL control mechanisms
P23
CCO
/ MDIV
clkout120 /
clkout240
/ 2PDIV
MSEL
PSEL
P23EN
Input clock
Bypass
Direct
clkout