lpc2468fet208 NXP Semiconductors, lpc2468fet208 Datasheet - Page 41

• Manufacturer Part Number: lpc2468fet208
• Description: Lpc2468 Single-chip 16-bit/32-bit Micro; 512 Kb Flash, Ethernet, Can, Isp/iap, Usb 2.0 Device/host/otg, External Memory Interface
• Manufacturer: NXP Semiconductors

NXP Semiconductors
LPC2468_2
Preliminary data sheet
7.25.1.3 RTC oscillator
7.25.2 PLL
7.25.3 Wake-up timer
7.25.4 Power control
The RTC oscillator can be used as the clock source for the RTC and/or the WDT. Also, the
RTC oscillator can be used to drive the PLL and the CPU.
The PLL accepts an input clock frequency in the range of 32 kHz to 50 MHz. The input
frequency is multiplied up to a high frequency, then divided down to provide the actual
clock used by the CPU and the USB block.
The PLL input, in the range of 32 kHZ to 50 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 PLL input divider is the PLL 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 PLL is turned off and bypassed following a chip Reset and by entering Power-down
mode. PLL is enabled by software only. The program must configure and activate the PLL,
wait for the PLL to lock, then connect to the PLL as a clock source.
The LPC2468 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.
The Wake-up Timer monitors the crystal oscillator to check whether it is safe to begin
code execution. When power is applied to the chip, or when some event caused the chip
to exit Power-down mode, some time is required for the oscillator to produce a signal of
sufficient amplitude to drive the clock logic. The amount of time depends on many factors,
including the rate of V
electrical characteristics (if a quartz crystal is used), as well as any other external circuitry
(e.g., capacitors), and the characteristics of the oscillator itself under the existing ambient
conditions.
The LPC2468 supports a variety of power control features. There are three special modes
of processor power reduction: Idle mode, Sleep mode, and Power-down mode. The CPU
clock rate may also be controlled as needed by changing clock sources, reconfiguring PLL
DD(3V3)
Rev. 02 — 16 October 2007
ramp (in the case of power on), the type of crystal and its
Fast communication chip
LPC2468
© NXP B.V. 2007. All rights reserved.
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