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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
203 of 1441
NXP Semiconductors
UM10503
Chapter 13: LPC43xx/LPC43Sxx Clock Generation Unit (CGU)
the PLL is not in lock. When the Power-down mode is terminated, the PLL will resume its
normal operation and will make the lock signal high once it has regained lock on the input
clock.
13.7.6.5 Selectable feedback divider clock
To allow a trade-off to be made between functionality and power consumption, the
feedback divider can be connected to either the CCO clock by setting FBSEL to 0 or to the
output clock by setting FBSEL to 1. If the post-divider is used to divide down the CCO
clock the current consumption of the feedback divider can be reduced by making it run on
the lower output clock instead of the CCO clock, but doing so will limit the relation
between output and phase detector clock frequencies to integer values.
13.7.6.6 Direct output mode
In normal operating mode (with DIRECT set to 0) the CCO clock is divided by 2, 4, 8 or 16
depending on the value of PSEL[1:0], automatically giving an output clock with a 50%
duty cycle. If a higher output frequency is needed, the CCO clock can be sent directly to
the output by setting DIRECT to 1. Since the CCO was designed to directly generate a
clock with a 50% duty cycle, the output clock duty cycle will also be 50% in direct mode.
13.7.6.7 Divider ratio programming
Pre-divider
The pre-divider’s division ratio is controlled by the NSEL[1:0] input. The division ratio
between PLL’s input clock and the phase detector clock is the decimal value on NSEL[1:0]
plus one.
Post-divider
The division ratio of the post divider is controlled by the PSEL bits. The division ratio is two
times the value of P selected by PSEL bits. This guarantees an output clock with a 50%
duty cycle.
Feedback divider
The feedback divider’s division ratio is controlled by the MSEL bits. The division ratio
between the PLL’s output clock and the input clock is the decimal value on MSEL bits plus
one.
13.7.6.8 Frequency selection
The PLL frequency equations use the following parameters (also see
Integer mode
In this mode the post divider is enabled and the feedback divider is set to run on the PLL
output clock, giving the following frequency relations:
(1)
FCLKOUT
M
FCLKIN
N
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