On the SAM3U, there are four different clock sources possible: slow clock, main clock, PLLA clock and UPLL clock. For the slow clock or SLCK, the slowest clock of the system, there are three different sources for this clock: the on-chip 32 KHz RC oscillator, an external 32 KHz crystal if better accuracy is required, or an external 32 K square wave signal if available on the board. Main clock, or MAINCK, is used as input of the PLL clock. There are two different sources possible: an embedded fast RC oscillator that can be selected as a 4, 8 or 12 MHz oscillator. The 8 and 12 MHz oscillators are calibrated in production, which means that their value is + or – 1% at 25 degrees, or + or – 5% in worst conditions. Instead of the on-chip fast oscillator, an external 3 to 20 MHz crystal can be used, or if this crystal is bypassed, an external clock can be input. PLLA clock, main clock is used as an input for this clock and the PLLA clock generates the highest frequency of the system. In the SAM3U, the RC filter for this clock and for the UPLL clock is integrated: this means that if the customer wants to use the 4 MHz on-chip RC oscillator on the SAM3U, no external components such as crystals or RC filters are necessary on the board. This really reduces BOM cost. Lastly, the UPLL Clock is used to generate the 480 MHz for the high-speed USB device.