C8051F326 Datasheet, PDF (77/140 Pages) List of Unclassifed Manufacturers – Full Speed USB, 16 kB Flash MCU Family

C8051F326/7
10.3. 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 96). A divided version of
the Multiplier output can also be used as the system clock. See Section 10.4 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 10.4 for details on clock selec-
tion.
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
These bits select the clock supplied to the Clock Multiplier.
MULSEL
0
1
Selected Clock
Internal Oscillator
External Clock
Rev. 0.5
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