C8051F320 Silicon Laboratories Inc, C8051F320 Datasheet - Page 124

• Manufacturer Part Number: C8051F320
• Description: IC 8051 MCU 16K FLASH 32LQFP
• Manufacturer: Silicon Laboratories Inc
• Series: C8051F32xr

Specifications of C8051F320

• Core Processor: 8051
• Core Size: 8-Bit
• Speed: 25MHz
• Connectivity: SMBus (2-Wire/I²C), SPI, UART/USART, USB
• Peripherals: Brown-out Detect/Reset, POR, PWM, Temp Sensor, WDT
• Number Of I /o: 25
• Program Memory Size: 16KB (16K x 8)
• Program Memory Type: FLASH
• Ram Size: 2.25K x 8
• Voltage - Supply (vcc/vdd): 2.7 V ~ 3.6 V
• Data Converters: A/D 17x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-LQFP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Eeprom Size: -

C8051F320/1
13.4. System and USB Clock Selection
The internal oscillator requires little start-up time and may be selected as the system or USB clock immediately fol-
lowing the OSCICN write that enables the internal oscillator. External crystals and ceramic resonators typically
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.
13.4.1. System Clock Selection
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 external oscillator may still
clock certain peripherals (timers, PCA, 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 oscillator, and 4x Clock Multiplier so long
as the selected oscillator is enabled and has settled.
13.4.2. 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 4x Clock Multiplier output, a divided version of the internal 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 Figure 13.6 for USB clock selec-
tion options.
Some example USB clock configurations for Full and Low Speed mode are given below:
124
Clock Signal
USB Clock
Clock Multiplier Input
Internal Oscillator
Clock Signal
USB Clock
Clock Multiplier Input
External Oscillator
†
Clock Signal
USB Clock
Internal Oscillator
Clock Signal
USB Clock
External Oscillator
Clock Recovery must be enabled for this configuration.
Table 13.2. Typical USB Low Speed Clock Settings
Table 13.1. Typical USB Full Speed Clock Settings
Input Source Selection
Clock Multiplier
Internal Oscillator
Divide by 1
Input Source Selection
Clock Multiplier
External Oscillator
Crystal Oscillator Mode
12 MHz Crystal
Input Source Selection
Internal Oscillator / 2
Divide by 1
Input Source Selection
External Oscillator / 4
Crystal Oscillator Mode
24 MHz Crystal
External Oscillator
External Oscillator
Internal Oscillator
Internal Oscillator
Rev. 1.1
†
Register Bit Settings
USBCLK = 000b
MULSEL = 00b
IFCN = 11b
Register Bit Settings
USBCLK = 000b
MULSEL = 01b
XOSCMD = 110b
XFCN = 111b
Register Bit Settings
USBCLK = 001b
IFCN = 11b
Register Bit Settings
USBCLK = 101b
XOSCMD = 110b
XFCN = 111b