UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
131 of 1269
NXP Semiconductors
UM10503
Chapter 11: LPC43xx Clock Generation Unit (CGU)
11.7.6.2 PLL1 description
The block diagram of this PLL is shown in
. The input frequency range is 10 MHz
to 25 MHz. The input clock is fed directly to the Phase-Frequency Detector (PFD). This
block compares the phase and frequency of its inputs, and generates a control signal
when phase and/ or frequency do not match. The loop filter filters these control signals
and drives the current controlled oscillator (CCO), which generates the main clock. The
CCO frequency range is 156 MHz to 320 MHz.These clocks are either divided by 2xP by
the programmable post divider to create the output clocks, or are sent directly to the
outputs. The main output clock is then divided by M by the programmable feedback
divider to generate the feedback clock. The output signal of the phase-frequency detector
is also monitored by the lock detector, to signal when the PLL has locked on to the input
clock.
11.7.6.3 Lock detector
The lock detector measures the phase difference between the rising edges of the input
and feedback clocks. Only when this difference is smaller than the so called “lock
criterion” for more than eight consecutive input clock periods, the lock output switches
from low to high. A single too large phase difference immediately resets the counter and
causes the lock signal to drop (if it was high). Requiring eight phase measurements in a
row to be below a certain figure ensures that the lock detector will not indicate lock until
both the phase and frequency of the input and feedback clocks are very well aligned. This
effectively prevents false lock indications, and thus ensures a glitch free lock signal.
11.7.6.4 Power-down control
To reduce the power consumption when the PLL clock is not needed, a Power-down
mode has been incorporated. In this mode, the internal current reference will be turned
off, the oscillator and the phase-frequency detector will be stopped and the dividers will
enter a reset state. While in Power-down mode, the lock output will be low to indicate that
Fig 29. PLL1 block diagram
LOCK
DETECT
PFD
FCLKOUT
pd
analog section
pd
cd
/M
/2P
/N
cd
PSEL<1:0>
pd
2
MSEL<7:0>
8
NSEL<1:0>
2
FCLKIN
FCLKIN
FCCO
LOCK
1
0
1
0
FBSEL
CCO
DIRECT
1
0
BYPASS