The LPC2300 and LPC2400 series have several clocking options including the main oscillator, IRC, RTC, and WDT. The main oscillator allows the controller to be driven by 1 MHz to 25 MHz external crystals. The CPU can be driven by the external crystal directly or via the PLL. The MCU can also be clocked from the IRC, either directly or via the PLL. The IRC is a 4 MHz oscillator with a tolerance of 1% over the entire voltage and temperature range. This clock option is used as the CPU clock during power up to ensure that the part resumes operation as quickly as possible from power down states.