LPC1752 PHILIPS [NXP Semiconductors], LPC1752 Datasheet - Page 30

• Manufacturer Part Number: LPC1752
• Description: 32-bit ARM Cortex-M3 MCU up to 512 kB flash and 64 kB SRAM with Ethernet, USB 2.0 Host/Device/OTG, CAN
• Manufacturer: PHILIPS [NXP Semiconductors]

NXP Semiconductors
LPC1758_56_54_52_51_2
Objective data sheet
7.29.2 Main PLL (PLL0)
7.29.3 USB PLL (PLL1)
7.29.4 RTC clock output
7.29.5 Wake-up timer
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.
The LPC1758/56/54/52/51 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.
The LPC1758/56/54/52/51 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.
The LPC1758/56/54/52/51 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
Rev. 02 — 11 February 2009
LPC1758/56/54/52/51
32-bit ARM Cortex-M3 microcontroller
© NXP B.V. 2009. All rights reserved.
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