Silicon Laboratories C8051F345, C8051F347, C8051F346 Clock Multiplier, Mulen Mulinit Mulrdy Mulsel

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

14.4. 4x Clock Multiplier

The 4x Clock Multiplier allows a 12 MHz oscillator to generate the 48 MHz clock required for Full Speed USB communication (see Section “16.4. USB Clock Configuration” on page 170). A divided version of the Multiplier output can also be used as the system clock. See Section 14.5 for details on system clock and USB clock source selection.

The 4x Clock Multiplier is configured via the CLKMUL register. The procedure for configuring and enabling the 4x Clock Multiplier is as follows:

1.Reset the Multiplier by writing 0x00 to register CLKMUL.

2.Select the Multiplier input source via the MULSEL bits.

3.Enable the Multiplier with the MULEN bit (CLKMUL = 0x80).

4.Delay for >5 µs.

5.Initialize the Multiplier with the MULINIT bit (CLKMUL = 0xC0).

6.Poll for MULRDY => ‘1’.

Important Note: When using an external oscillator as the input to the 4x Clock Multiplier, the exter- nal source must be enabled and stable before the Multiplier is initialized. See Section 14.5 for details on selecting an external oscillator source.

SFR Definition 14.5. CLKMUL: Clock Multiplier Control

R/W		R/W		R	R/W		R/W	R/W	R/W	R/W	Reset Value
MULEN		MULINIT		MULRDY	-		-	-	MULSEL		00000000
Bit7		Bit6		Bit5	Bit4		Bit3	Bit2	Bit1	Bit0	SFR Address
											0xB9
Bit7:	MULEN: Clock Multiplier Enable
	0: Clock Multiplier disabled.
	1: Clock Multiplier enabled.
Bit6:	MULINIT: Clock Multiplier Initialize
	This bit should be a ‘0’ when the Clock Multiplier is enabled. Once enabled, writing a ‘1’ to
	this bit will initialize the Clock Multiplier. The MULRDY bit reads ‘1’ when the Clock Multiplier
	is stabilized.
Bit5:	MULRDY: Clock Multiplier Ready
	This read-only bit indicates the status of the Clock Multiplier.
	0: Clock Multiplier not ready.
	1: Clock Multiplier ready (locked).
Bits4–2:	Unused. Read = 000b; Write = don’t care.
Bits1–0:	MULSEL: Clock Multiplier Input Select
	These bits select the clock supplied to the Clock Multiplier.
		MULSEL				Selected Clock
			00			Internal Oscillator
			01			External Oscillator
			10		External Oscillator / 2
			11				RESERVED

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