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C8051F380/1/2/3/4/5/6/7/C
19.1. System Clock Selection
The CLKSL[2:0] bits in register CLKSEL select which oscillator source is used as the system clock.
CLKSL[2:0] must be set to 001b for the system clock to run from the external oscillator; however the exter-
nal oscillator may still clock certain peripherals (timers, PCA) when the internal oscillator is selected as the
system clock. The system clock may be switched on-the-fly between the internal oscillators and external
oscillator so long as the selected clock source is enabled and running.
The internal high-frequency and low-frequency oscillators require little start-up time and may be selected
as the system clock immediately following the register write which enables the oscillator. The external RC
and C modes also typically require no startup time.
19.2. USB Clock Selection
The USBCLK[2:0] bits in register CLKSEL select which oscillator source is used as the USB clock. The
USB clock may be derived from the internal oscillators, a divided version of the internal High-Frequency
oscillator, or a divided version of the external oscillator. Note that the USB clock must be 48 MHz when
operating USB0 as a Full Speed Function; the USB clock must be 6 MHz when operating USB0 as a Low
Speed Function. See SFR Definition 19.1 for USB clock selection options.
Some example USB clock configurations for Full and Low Speed mode are given below:
USB Full Speed (48 MHz)
Internal Oscillator
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
External Oscillator
Input Source Selection
Internal Oscillator*
Divide by 1
External Oscillator
Input Source Selection
External Oscillator
CMOS Oscillator Mode
Register Bit Settings
USBCLK = 000b
IFCN = 11b
Register Bit Settings
USBCLK = 010b
XOSCMD = 010b
48 MHz Oscillator
Note: Clock Recovery must be enabled for this configuration.
USB Low Speed (6 MHz)
Internal Oscillator
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
External Oscillator
Input Source Selection
Internal Oscillator / 8
Divide by 1
External Oscillator
Input Source Selection
External Oscillator / 4
CMOS Oscillator Mode
24 MHz Oscillator
Crystal Oscillator Mode
24 MHz Oscillator
Rev. 1.4
Register Bit Settings
USBCLK = 001b
IFCN = 11b
Register Bit Settings
USBCLK = 101b
XOSCMD = 010b
XOSCMD = 110b
XFCN = 111b
143
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