C8051F54X_14 Datasheet, PDF (135/275 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F54x
17. Oscillators and Clock Selection
C8051F54x devices include a programmable internal high-frequency oscillator, an external oscillator drive
circuit, and a clock multiplier. The internal oscillator can be enabled/disabled and calibrated using the
OSCICN, OSCICRS, and OSCIFIN registers, as shown in Figure 17.1. The system clock can be sourced
by the external oscillator circuit or the internal oscillator. The clock multiplier can produce three possible
base outputs which can be scaled by a programmable factor of 1, 2/3, 2/4 (or 1/2), 2/5, 2/6 (or 1/3), or 2/7:
Internal Oscillator x 2, External Oscillator x 2, or External Oscillator x 4.
OSCICRS OSCIFIN
OSCICN
CLKSEL
Option 3
XTAL2
Option 2
VDD
XTAL2
Option 4
XTAL2
Option 1
XTAL1
10M
XTAL2
CAL
EN
IOSC
Programmable Internal
n
Clock Generator
IOSC/2
EXOSC
EXOSC/2
IOSC
CLOCK MULTIPLIER
x4
n
Input
Circuit
OSC
EXOSC
SYSCLK
OSCXCN
CLKMUL
Figure 17.1. Oscillator Options
17.1. 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 has settled.
The internal oscillator requires little start-up time and may be selected as the system clock immediately fol-
lowing the register write which enables the oscillator. The external RC and C modes also typically require
no startup time.
External crystals and ceramic resonators however, 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 crystal or ceramic resonator is settled. In crystal mode, to avoid reading a false XTLVLD, soft-
ware should delay at least 1 ms between enabling the external oscillator and checking XTLVLD.
Rev. 1.1
135