C8051F2XX Datasheet, PDF (97/146 Pages) Silicon Laboratories – Mixed Signal 8 kB ISP Flash MCU Family

C8051F2xx
13. Oscillator
The MCU includes an internal oscillator and an external oscillator drive circuit, either of which can generate
the system clock. The MCU boots from the internal oscillator after any reset. This internal oscillator can
be enabled/disabled and its frequency can be set using the Internal Oscillator Control Register (OSCICN)
as shown in SFR Definition 13.1. The internal oscillator's electrical specifications are given in Table 13.1.
Both oscillators are disabled when the RST pin is held low. The MCU can run from the internal oscillator
permanently, or it can switch to the external oscillator if desired using CLKSL bit in the OSCICN Register.
The external oscillator requires an external resonator, crystal, capacitor, or RC network connected to the
XTAL1/XTAL2 pins (see Figure 13.1). The oscillator circuit must be configured for one of these sources in
the OSCXCN register. An external CMOS clock can also provide the system clock by driving the XTAL1
pin. The XTAL1 and XTAL2 pins are NOT 5 V tolerant.
OSCICN
opt. 4
XTAL1
opt. 3
XTAL1
XTAL2
opt. 2
VDD
XTAL1
opt. 1
XTAL1
XTAL2
EN
Internal Clock
Generator
Input
Circuit
OSC
SYSCLK
OSCXCN
Figure 13.1. Oscillator Diagram
Rev. 1.6
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