In the USB specification, the transmission rate of low-speed, full-speed and high-speed devices are 1.5 MB, 12 MB and 480 MB respectively. For the NuMicroTM series, some chips support full-speed device, full-speed host and high-speed device. In accordance with the signal sampling method, USB clock source must be set to a multiple of transmission rates. The full-speed device must be set to 4x frequency. Accordingly, the clock source of full-speed device and full-speed host device must be set to 48 MHz. For high-speed devices, the requirement is the frequency needs to be the same. Therefore, the clock source of the high-speed device must be set to 480 MHz.

In the NUC123 series, for example, the maximum CPU frequency it can run is 72 MHz. In USB applications, the PLL frequency can be set to 144 MHz so that system can run the highest frequency. When PLL is divided by 2 through CPU and PLL is divided by 3 through USB, the highest system frequency can be achieved.

Products:	Microcontrollers ,Arm Cortex-M0 MCUs ,Nano120 USB Series ,NUC120/122/123/220 USB Series ,NUC140/240 Connectivity Series ,Arm Cortex-M4 MCUs ,M452 USB Series ,M453 CAN Series ,NUC442/472 Series ,NUC505 Series
Applications:
Function:	Peripherals,Connectivity,USB

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How to configure the USB clock source? Issue Date：2015-02-13

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