c8051f410 Silicon Laboratories, c8051f410 Datasheet - Page 174

• Manufacturer Part Number: c8051f410
• Description: 2.0 V, 32/16 Kb Flash, Smartclock, 12-bit Adc
• Manufacturer: Silicon Laboratories

C8051F410/1/2/3
19.4. System Clock Selection
The internal oscillator requires little start-up time and may be selected as the system clock immediately fol-
lowing the OSCICN write that enables the internal oscillator. External crystals and ceramic resonators typ-
ically require a start-up time before they are settled and ready for use. The Crystal Valid Flag (XTLVLD in
register OSCXCN) is set to ‘1’ by hardware when the external oscillator is settled. To avoid reading a
false XTLVLD, in crystal mode software should delay at least 1 ms between enabling the external
oscillator and checking XTLVLD. RC and C modes typically require no startup time.
The CLKSL[1:0] bits in register CLKSEL select which oscillator source is used as the system clock.
CLKSL[1:0] must be set to 01b for the system clock to run from the external oscillator; however the exter-
nal oscillator may still clock certain peripherals (timers, PCA) when another oscillator is selected as the
system clock. The system clock may be switched on-the-fly between the internal oscillator, external oscilla-
tor, smaRTClock oscillator, and Clock Multiplier, as long as the selected clock source is enabled and has
settled.
174
Bits7–6: Unused. Read = 00b; Write = don’t care.
Bits5–4: CLKDIV1–0: Output /SYSCLK Divide Value
Bit3:
Bit2:
Bits1–0: CLKSL1–0: System Clock Select
Bit7
R
-
These bits can be used to pre-divide the /SYSCLK output before it is sent to a port pin
through the Crossbar.
00: Output will be SYSCLK.
01: Output will be SYSCLK/2.
10: Output will be SYSCLK/4.
11: Output will be SYSCLK/8.
Unused. Read = 0b; Write = don’t care.
Reserved. Read = 0b; Must write 0b.
These bits select the system clock source.
Bit6
R
-
CLKSL
00
01
10
11
SFR Definition 19.5. CLKSEL: Clock Select
R/W
Bit5
CLKDIV
R/W
Bit4
Internal Oscillator (as determined by the
IFCN bits in register OSCICN)
Rev. 1.0
smaRTClock Oscillator
Bit3
External Oscillator
R
-
Selected Clock
Clock Multiplier
Reserved
R/W
Bit2
R/W
Bit1
CLKSL
SFR Address:
R/W
Bit0
0xA9
00000000
Reset Value