LPC1766_0.02
© NXP B.V. 2008. All rights reserved.
Objective data sheet
Rev. 00.02 — 12 August 2008
31 of 70
NXP Semiconductors
LPC1766
Fast communication chip
7.29.2 Main PLL (PLL0)
The PLL0 accepts an input clock frequency in the range of 32 kHz to 24 MHz. The input
frequency is multiplied up to a high frequency, then divided down to provide the actual
clock used by the CPU and/or the USB block.
The PLL0 input, in the range of 32 kHz to 24 MHz, may initially be divided down by a
value ‘N’, which may be in the range of 1 to 256. This input division provides a wide range
of output frequencies from the same input frequency.
Following the PLL0 input divider is the PLL0 multiplier. This can multiply the input divider
output through the use of a Current Controlled Oscillator (CCO) by a value ‘M’, in the
range of 1 through 32768. The resulting frequency must be in the range of 275 MHz to
550 MHz. The multiplier works by dividing the CCO output by the value of M, then using a
phase-frequency detector to compare the divided CCO output to the multiplier input. The
error value is used to adjust the CCO frequency.
The PLL0 is turned off and bypassed following a chip Reset and by entering Power-down
mode. PLL0 is enabled by software only. The program must configure and activate the
PLL0, wait for the PL0L to lock, and then connect to the PLL0 as a clock source.
7.29.3 USB PLL (PLL1)
The LPC1766 contains a second, dedicated USB PLL1 to provide clocking for the USB
interface.
The PLL1 receives its clock input from the main oscillator only and provides a fixed
48 MHz clock to the USB block only. The PLL1 is disabled and powered off on reset. If the
PLL1 is left disabled, the USB clock will be supplied by the 48 MHz clock from the main
PLL0.
The PLL1 accepts an input clock frequency in the range of 10 MHz to 24 MHz only. The
input frequency is multiplied up the range of 48 MHz for the USB clock using a Current
Controlled Oscillators (CCO). It is insured that the PLL1 output has a 50% duty cycle.
7.29.4 RTC clock output
The LPC1766 features a clock output function intended mainly for use during system
development to allow checking the internal clocks CCLK, IRC clock, main crystal, RTC
clock, and USB clock in the outside world. The RTC clock output allows tuning the RTC
frequency without probing the pin, which would distort the results.
7.29.5 Wake-up timer
The LPC1766 begins operation at power-up and when awakened from Power-down mode
by using the 4 MHz IRC oscillator as the clock source. This allows chip operation to
resume quickly. If the main oscillator or the PLL is needed by the application, software will
need to enable these features and wait for them to stabilize before they are used as a
clock source.
When the main oscillator is initially activated, the wake-up timer allows software to ensure
that the main oscillator is fully functional before the processor uses it as a clock source
and starts to execute instructions. This is important at power on, all types of Reset, and
whenever any of the aforementioned functions are turned off for any reason. Since the
oscillator and other functions are turned off during Power-down mode, any wake-up of the
processor from Power-down mode makes use of the wake-up Timer.
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