The LPC17xx includes three independent oscillators: the Main Oscillator, Internal RC Oscillator, and the RTC oscillator. Each oscillator can be used for more than one purpose as required in a particular application. Following Reset, the LPC17xx will operate from the Internal RC Oscillator until switched by software. This allows systems to operate without an external crystal, and allows the bootloader code to operate at a known frequency. The Internal RC Oscillator (IRC) may be used as the clock source for the watchdog timer, and/or as the clock that drives PLL0 and subsequently the CPU. The nominal IRC frequency is 4 MHz with an accuracy of ± 1%. The accuracy of the IRC is a KEY feature which may not be available with a lot of competitor devices. The main oscillator can also be used as the clock source for the CPU, with or without using the main PLL. The main oscillator 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 main PLL (PLL0). The main oscillator also provides the clock source for the dedicated USB PLL RTC oscillator. The RTC oscillator provides a 1 Hz clock to the RTC and a 32 kHz clock output that can be used as the clock source for PLL0, CPU, and/or the watchdog timer.