C8051T630 Datasheet, PDF (101/221 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051T630/1/2/3/4/5
19. Oscillators and Clock Selection
C8051T630/1/2/3/4/5 devices include a programmable internal high-frequency oscillator, a programmable
internal low-frequency oscillator, and an external oscillator drive circuit. The internal high-frequency oscilla-
tor can be enabled/disabled and calibrated using the OSCICN and OSCICL registers, as shown in
Figure 19.1. The internal low-frequency oscillator can be enabled/disabled and calibrated using the
OSCLCN register. The system clock can be sourced by the external oscillator circuit or either internal oscil-
lator. Both internal oscillators offer a selectable post-scaling feature.
RC Mode
VDD
EXTCLK
C Mode
EXTCLK
EXTCLK
CMOS Mode
EXTCLK
OSCICL
OSCICN
OSCLCN
EN
Programmable
Internal Clock
Generator
OSCLF
EN
Low Frequency
Oscillator
Input
Circuit
OSC
OSCLF OSCLD
n
n
OSCLD
SYSCLK
OSCXCN
CLKSEL
Figure 19.1. Oscillator Options
19.1. 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 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 oscillator, external oscilla-
tor, and Clock Multiplier 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.
Rev. 1.0
101