C8051F352 Datasheet, PDF (135/234 Pages) Silicon Laboratories – 8 k ISP Flash MCU Family

C8051F350/1/2/3
17.3. Clock Multiplier
The Clock Multiplier generates an output clock which is 4 times the input clock frequency. The Clock Multi-
plier’s input can be selected from the external oscillator, or 1/2 the internal or external oscillators. This pro-
duces three possible outputs: Internal Oscillator x 2, External Oscillator x 2, or External Oscillator x 4. See
Section “17.4. System Clock Selection’ on page 136 for details on system clock selection.
The Clock Multiplier is configured via the CLKMUL register (SFR Definition 17.4). The procedure for con-
figuring and enabling the 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 Clock Multiplier, the external
source must be enabled and stable before the Multiplier is initialized. See Section “17.4. System
Clock Selection’ on page 136 for details on selecting an external oscillator source.
SFR Definition 17.4. 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: 0xBE
Bit7:
Bit6:
Bit5:
Bits4–2:
Bits1–0:
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 = 000b; Write = don’t care.
MULSEL: Clock Multiplier Input Select
These bits select the clock supplied to the Clock Multiplier.
MULSEL
00
01
10
11
Selected Input Clock
Internal Oscillator / 2
External Oscillator
External Oscillator / 2
RESERVED
Clock Multipler Output
Internal Oscillator x 2
External Oscillator x 4
External Oscillator x 2
RESERVED
Rev. 1.1
135