UM10850
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 2.4 — 13 September 2016
71 of 464
NXP Semiconductors
UM10850
Chapter 4: LPC5410x System configuration (SYSCON)
The PLL lock indicator is not dependable when Fref is below 100 kHz or above 20 MHz.
In fractional mode and spread spectrum mode, the PLL will generally not lock, software
should use a time interval to insure the PLL will be stable. See
4.6.4.2.2
Power-down
To reduce the power consumption when the PLL clock is not needed, a PLL Power-down
mode has been incorporated. This mode is enabled by setting the SYSPLL_PD bit to one
in the power configuration register PDRUNCFG (
). 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 PLL Power-down
mode, the lock output will be low to indicate that the PLL is not in lock. When the PLL
Power-down mode is terminated by setting the SYSPLL_PD bit to zero, the PLL will
resume its normal operation and will make the lock signal high once it has regained lock
on the input clock.
4.6.4.3 Operating modes
The PLL includes several main operating modes, and a power-down mode. These are
summarized in
and detailed in the following sections.
[1]
Use 1 if the PLL output is used even though the PLL is not altering the frequency.
4.6.4.3.1
Normal modes
Typical operation of the PLL includes an optional pre-divide of the PLL input, followed by a
frequency multiplication, and finally an optional post-divide to produce the PLL output.
Notations used in the frequency equations:
•
Fin = the input to the PLL.
•
Fout = the output of the PLL.
•
Fref = the PLL reference frequency, the input to the phase frequency detector.
•
N = optional pre-divider value.
•
M = feedback divider value, which represents the multiplier for the PLL. Note that an
additional divide-by-2 may optionally be included in the divider path.
•
P = optional post-divider value. An additional divide-by-2 is included in the
post-divider path.
A block diagram of the PLL as used in normal modes is shown in
In all variations of normal mode, the following requirements must be met:
Table 101. PLL operating mode summary
Mode
PDEN_SYS_PLL
bit in PDRUNCFG
Bits in SYSPLLCTRL:
SEL_EXT bit in
SYSYPLLSSCTRL0
PD bit in
SYSYPLLSSCTRL1
BYPASS
UPLIMOFF BANDSEL
Normal
0
0
0
1
1
1
Fractional divider
0
0
1
0
0
0
Spread spectrum
0
0
1
0
0
0
Power-down
1
x
x
x
1