C8051F327 View Datasheet(PDF) - Silicon Laboratories
Part Name
Description
MFG CO.
'C8051F327' PDF : 141 Pages View PDF
C8051F326/7
10.5. System and USB Clock Selection
The internal oscillator requires little start-up time and may be selected as the system or USB clock immedi-
ately following the OSCICN write that enables the internal oscillator. If the external clock is selected as the
system or USB clock, then startup times may vary based on the specifications of the external clock.
10.5.1. 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 clock; however the external
clock may still clock certain peripherals (timers, UART, USB) when the internal oscillator is selected as the
system clock. The system clock may be switched on-the-fly between the internal oscillator, external clock,
low frequency oscillator, and 4x Clock Multiplier so long as the selected oscillator is enabled and can pro-
vide a stable clock.
10.5.2. USB Clock Selection
The USBCLK[1:0] bits in register CLKSEL select which oscillator source is used as the USB clock. The
USB clock may be derived from the 4x Clock Multiplier output, internal oscillator divided by 2, or an exter-
nal clock. The USB clock source may also be turned off. 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 Figure 10.5 for USB clock selection options.
Some example USB clock configurations for Full and Low Speed mode are given below:
Table 10.1. Typical USB Full Speed Clock Settings
Clock Signal
USB Clock
Clock Multiplier Input
Internal Oscillator
Internal Oscillator
Input Source Selection
Clock Multiplier
Internal Oscillator*
Divide by 1
Register Bit Settings
USBCLK = 00b
MULSEL = 0b
IFCN = 11b
External Clock
Clock Signal
Input Source Selection Register Bit Settings
USB Clock
Clock Multiplier
USBCLK = 10b
Clock Multiplier Input
External Clock
MULSEL = 1b
Port I/O
12 MHz CMOS Clock
INPUTEN = 1b (GPI-
OCN.6)
*Note: Clock Recovery must be enabled for this configuration.
Table 10.2. Typical USB Low Speed Clock Settings
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
Port I/O
Internal Oscillator
Input Source Selection
Internal Oscillator / 2
Divide by 1
External Clock
Input Source Selection
External Clock
6 MHz CMOS Clock
Register Bit Settings
USBCLK = 01b
IFCN = 11b
Register Bit Settings
USBCLK = 10b
INPUTEN = 1b (GPI-
OCN.6)
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Rev. 1.1
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