LPC1766 datasheet(32/66 Pages) NXP

32 / 66 page

LPC1769_68_67_66_65_64_4

Product data sheet

Rev. 04 — 1 February 2010

32 of 66

NXP Semiconductors

LPC1769/68/67/66/65/64

32-bit ARM Cortex-M3 microcontroller

7.29.2 Main PLL (PLL0)

The PLL0 accepts an input clock frequency in the range of 32 kHz to 25 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 25 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 PLL0 to lock, and then connect to the PLL0 as a clock source.

7.29.3 USB PLL (PLL1)

The LPC17xx contain 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 25 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 LPC17xx feature a clock output function intended for synchronizing with external

devices and 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 LPC17xx begin 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

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