Silicon Laboratories C8051F341, C8051F347, C8051F346 USB Clock Configuration, CRE Crssen Crlow

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

16.4. USB Clock Configuration

USB0 is capable of communication as a Full or Low Speed USB function. Communication speed is selected via the SPEED bit in SFR USB0XCN. When operating as a Low Speed function, the USB0 clock must be 6 MHz. When operating as a Full Speed function, the USB0 clock must be 48 MHz. Clock options are described in Section “14. Oscillators” on page 135. The USB0 clock is selected via SFR CLKSEL (see SFR Definition 14.6).

Clock Recovery circuitry uses the incoming USB data stream to adjust the internal oscillator; this allows the internal oscillator (and 4x Clock Multiplier) to meet the requirements for USB clock tolerance. Clock Recovery should be used in the following configurations:

Communication Speed	USB Clock	4x Clock Multiplier Input
Full Speed	4x Clock Multiplier	Internal Oscillator
Low Speed	Internal Oscillator / 2	N/A

When operating USB0 as a Low Speed function with Clock Recovery, software must write ‘1’ to the CRLOW bit to enable Low Speed Clock Recovery. Clock Recovery is typically not necessary in Low Speed mode.

Single Step Mode can be used to help the Clock Recovery circuitry to lock when high noise levels are present on the USB network. This mode is not required (or recommended) in typical USB environments.

USB Register Definition 16.5. CLKREC: Clock Recovery Control

R/W		R/W	R/W	R/W	R/W	R/W	R/W	R/W	Reset Value
CRE		CRSSEN	CRLOW			Reserved			00001001
Bit7		Bit6	Bit5	Bit4	Bit3	Bit2	Bit1	Bit0	USB Address:
									0x0F
Bit7:	CRE: Clock Recovery Enable.
	This bit enables/disables the USB clock recovery feature.
	0: Clock recovery disabled.
	1: Clock recovery enabled.
Bit6:	CRSSEN: Clock Recovery Single Step.
	This bit forces the oscillator calibration into ‘single-step’ mode during clock recovery.
	0: Normal calibration mode.
	1: Single step mode.
Bit5:	CRLOW: Low Speed Clock Recovery Mode.
	This bit must be set to ‘1’ if clock recovery is used when operating as a Low Speed USB
	device.
	0: Full Speed Mode.
	1: Low Speed Mode.
Bits4–0:	Reserved. Read = Variable. Must Write = 01001b.

Note: The USB transceiver must be enabled before enabling Clock Recovery.

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