C8051T322-GQR Silicon Labs, C8051T322-GQR Datasheet - Page 123

• Manufacturer Part Number: C8051T322-GQR
• Description: 8-bit Microcontrollers - MCU USB-OTP-16K-LQFP32
• Manufacturer: Silicon Labs

Specifications of C8051T322-GQR

• Rohs: yes
• Core: 8051
• Processor Series: C8051
• Data Bus Width: 8 bit

21.1. 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.
21.2. 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 21.1 for USB clock selection options.
Some example USB clock configurations for Full and Low Speed mode are given below:
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
External Oscillator
Note: Clock Recovery must be enabled for this configuration.
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
External Oscillator
USB Full Speed (48 MHz)
Input Source Selection
Internal Oscillator*
Divide by 1
Input Source Selection
External Oscillator
CMOS Oscillator Mode
48 MHz Oscillator
Input Source Selection
Internal Oscillator / 8
Divide by 1
Input Source Selection
External Oscillator / 4
CMOS Oscillator Mode
24 MHz Oscillator
Crystal Oscillator Mode
24 MHz Oscillator
USB Low Speed (6 MHz)
External Oscillator
External Oscillator
Internal Oscillator
Internal Oscillator
C8051T620/621/320/321/322/323
Rev. 1.1
Register Bit Settings
USBCLK = 000b
IFCN = 11b
Register Bit Settings
USBCLK = 010b
XOSCMD = 010b
Register Bit Settings
USBCLK = 001b
IFCN = 11b
Register Bit Settings
USBCLK = 101b
XOSCMD = 010b
XOSCMD = 110b
XFCN = 111b
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