Silicon Laboratories C8051F342, C8051F347 System and USB Clock Selection, System Clock Selection

C8051F340/1/2/3/4/5/6/7

14.5. System and USB Clock Selection

The internal oscillator requires little start-up time and may be selected as the system or USB clock immedi- ately following the OSCICN write that enables the internal oscillator. External crystals and ceramic resona- tors typically require a start-up time before they are settled and ready for use. The Crystal Valid Flag (XTLVLD in register OSCXCN) is set to ‘1’ by hardware when the external oscillator is settled. To avoid reading a false XTLVLD, in crystal mode software should delay at least 1 ms between enabling the external oscillator and checking XTLVLD. RC and C modes typically require no startup time.

14.5.1. System Clock Selection

The CLKSL[1:0] bits in register CLKSEL select which oscillator source is used as the system clock. CLKSL[1:0] must be set to 01b for the system clock to run from the external oscillator; however the exter- nal oscillator may still clock certain peripherals (timers, PCA, USB) when the internal oscillator is selected as the system clock. The system clock may be switched on-the-fly between the internal oscillator, external oscillator, and 4x Clock Multiplier so long as the selected oscillator is enabled and has settled.

14.5.2. USB Clock Selection

The USBCLK[2:0] bits in register CLKSEL select which oscillator source is used as the USB clock. The USB clock may be derived from the 4x Clock Multiplier output, a divided version of the internal oscillator, or a divided version of the external oscillator. Note that the USB clock must be 48 MHz when operating USB0 as a Full Speed Function; the USB clock must be 6 MHz when operating USB0 as a Low Speed Function. See SFR Definition 14.6 for USB clock selection options.

Some example USB clock configurations for Full and Low Speed mode are given below:

Internal Oscillator

Clock Signal	Input Source Selection	Register Bit Settings
USB Clock	Clock Multiplier	USBCLK = 000b
Clock Multiplier Input	Internal Oscillator*	MULSEL = 00b
Internal Oscillator	Divide by 1	IFCN = 11b
	External Oscillator
Clock Signal	Input Source Selection	Register Bit Settings
USB Clock	Clock Multiplier	USBCLK = 000b
Clock Multiplier Input	External Oscillator	MULSEL = 01b
External Oscillator	Crystal Oscillator Mode	XOSCMD = 110b
	12 MHz Crystal	XFCN = 111b
*Note: Clock Recovery must be enabled for this configuration.
	Internal Oscillator
Clock Signal	Input Source Selection	Register Bit Settings
USB Clock	Internal Oscillator / 2	USBCLK = 001b
Internal Oscillator	Divide by 1	IFCN = 11b
	External Oscillator
Clock Signal	Input Source Selection	Register Bit Settings
USB Clock	External Oscillator / 4	USBCLK = 101b
External Oscillator	Crystal Oscillator Mode	XOSCMD = 110b
	24 MHz Crystal	XFCN = 111b

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