C8051F327 View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

MFG CO.

C8051F327

Full Speed USB, 16 kB Flash MCU Family

Silicon Laboratories 

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C8051F326/7

10.4. 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 “12.4. USB Clock Configuration” on page 94). A divided version of the

Multiplier output can also be used as the system clock. See Section “10.5. System and USB Clock Selec-

tion” on page 76 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 clock as the input to the 4x Clock Multiplier, the external source

must be stable before the Multiplier is initialized. See Section “10.5. System and USB Clock Selection” on

page 76 for details on clock selection.

SFR Definition 10.4. 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:

Bit6:

Bit5:

Bits4–1:

Bit0:

MULEN: Clock Multiplier Enable

0: Clock Multiplier disabled.

1: Clock Multiplier enabled.

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.

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).

Unused. Read = 0000b. Write = don’t care.

MULSEL: Clock Multiplier Input Select

This bit selects the clock supplied to the Clock Multiplier.

MULSEL

0

1

Selected Clock

Internal Oscillator

External Clock

Rev. 1.1

75

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