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UM10375
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 3 — 14 June 2011
49 of 368
NXP Semiconductors
UM10375
Chapter 3: LPC13xx System configuration
3.11.2 Power-down control
To reduce the power consumption when the PLL clock is not needed, a Power-down
mode has been incorporated. This mode is enabled by setting the SYSPLL_PD (or
USBPLL_PD) bits to one in the Power-down configuration register (
). 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 the PLL is not in lock. When
the Power-down mode is terminated by setting the SYSPLL_PD (or USBPLL_PD) bits 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.
3.11.3 Divider ratio programming
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 as shown in
and
. 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, as specified in
Changing the divider values
Changing the divider ratio while the PLL is running is not recommended. As there is no
way to synchronize the change of the MSEL and PSEL values with the dividers, the risk
exists that the counter will read in an undefined value, which could lead to unwanted
spikes or drops in the frequency of the output clock. The recommended way of changing
between divider settings is to power down the PLL, adjust the divider settings and then let
the PLL start up again.
3.11.4 Frequency selection
The PLL frequency equations use the following parameters (also see
):
Table 57.
PLL frequency parameters
Parameter
System PLL
USB PLL
FCLKIN
Frequency of sys_pllclkin (input clock
to the system PLL) from the
SYSPLLCLKSEL multiplexer (see
).
Frequency of usb_pllclkin (input clock
to the USB PLL) from the
USBPLLCLKSEL multiplexer (see
).
FCCO
Frequency of the Current Controlled
Oscillator (CCO); 156 to 320 MHz.
Frequency of the Current Controlled
Oscillator (CCO); 156 to 320 MHz.