Each oscillator, except the low-frequency oscillator, can be used for more than one
purpose as required in a particular application.
Following reset, the LPC81xM will operate from the IRC until switched by software. This
allows systems to operate without any external crystal and the bootloader code to operate
at a known frequency.
See Figure 9 for an overview of the LPC81xM clock generation.
Internal RC Oscillator (IRC)
The IRC may be used as the clock source for the WWDT, and/or as the clock that drives
the PLL and subsequently the CPU. The nominal IRC frequency is 12 MHz. The IRC is
trimmed taccuracy over the entire voltage and temperature range.
The IRC can be used as a clock source for the CPU with or without using the PLL. The
IRC frequency can be boosted to a higher frequency, up to the maximum CPU operating
frequency, by the system PLL.
Upon power-up or any chip reset, the LPC81xM use the IRC as the clock source.
Software may later switch to one of the other available clock sources.
Crystal Oscillator (SysOsc)
The crystal oscillator can be used as the clock source for the CPU, with or without using
the PLL.
The SysOsc operates at frequencies of 1 MHz to 25 MHz. This frequency can be boosted
to a higher frequency, up to the maximum CPU operating frequency, by the system PLL.
Internal Low-power Oscillator and Watchdog Oscillator (WDOsc)
The nominal frequency of the WDOsc is programmable between 9.4 kHz and 2.3 MHz.
The frequency spread over silicon process variations is
The WDOsc is a dedicated oscillator for the windowed WWDT.
The internal low-power 10 kHz (
accuracy) oscillator serves a the clock input to the
WKT. This oscillator can be configured to run in all low power modes.
8.21.2 Clock input
A 3.3 V external clock source (25 MHz typical) can be supplied on the selected CLKIN pin
or a 1.8 V external clock source can be supplied on the XTALIN pin (see Section 14.1).
8.21.3 System PLL
The PLL accepts an input clock frequency in the range of 10 MHz to 25 MHz. The input
frequency is multiplied up to a high frequency with a Current Controlled Oscillator (CCO).
The multiplier can be an integer value from 1 to 32. The CCO operates in the range of
156 MHz to 320 MHz, so there is an additional divider in the loop to keep the CCO within
its frequency range while the PLL is providing the desired output frequency. The output
divider may be set to divide byr 16 to produce the output clock. Since the
minimum output divider value is 2, it is insured that the PLL output has a 50 % duty cycle.
The PLL is turned off and bypassed following a chip reset and may be enabled by
software. The program must configure and activate the PLL, wait for the PLL to lock, and
then connect to the PLL as a clock source. The PLL settling time is nominally 100
