5.2.5
Clock Control
The system control module determines the control of clocks in this part.
5.2.5.1
Fundamental Clock Sources
There are multiple clock sources for use in the microcontroller. The
Run and Sleep Mode
Configuration Register (RSCLKCFG)
can be used to configure the required clock source for the
device after Power-On Reset, as well as the system clock divisor encodings. The available clock
sources are as follows:
■
Precision Internal Oscillator (PIOSC).
The precision internal oscillator is an on-chip clock
source that the microcontroller uses during and following POR. It is the clock source in effect at
the start of reset vector fetch and the start of code application execution. It does not require the
use of any external components and provides a clock that is 16 MHz ±F
PIOSC
across temperature
(see Table 27-19 on page 1837). The PIOSC allows for a reduced system cost in applications that
require an accurate enough clock source. If the main oscillator is required, software must enable
the main oscillator following reset and allow the main oscillator to stabilize before changing the
clock reference. If the Hibernation Module clock source is a 32.768-kHz oscillator, the precision
internal oscillator can be trimmed by software based on a reference clock for increased accuracy.
Regardless of whether or not the PIOSC is the source for the system clock, the PIOSC can be
configured to be an alternate clock source for some of the peripherals. See the section called
“Peripheral Clock Sources” on page 234 for more information on peripherals that can use the
PIOSC as an alternate clock.
■
Main Oscillator (MOSC).
The main oscillator provides a frequency-accurate clock source by
one of two means: an external single-ended clock source is connected to the
OSC0
input pin, or
an external crystal is connected across the
OSC0
input and
OSC1
output pins. If the PLL is being
used, the crystal value can be any frequency between 5 MHz to 25 MHz (inclusive). Refer to
Table 5-7 on page 238 for recommended crystal values and PLL register programming. If the PLL
is not being used, the crystal may be any one of the supported frequencies between 4 MHz to
25 MHz. The single-ended clock source range is from DC through the specified speed of the
microcontroller.
■
Low-Frequency Internal Oscillator (LFIOSC).
The Low-Frequency Internal Oscillator (LFIOSC)
provides a nominal frequency of 33 kHz with percentage variance specified in the Electrical
Characteristics section. It is intended for use during Deep-Sleep power-saving modes. This
power-savings mode provides reduced internal switching and the ability to power down the
MOSC and/or PIOSC while in Deep-Sleep mode through configuration of the
Deep Sleep Clock
Configuration Register (DSCLKCFG)
register.
■
Hibernation Module RTC Oscillator (RTCOSC) Clock Source.
The Hibernation Module provides
a muxed output of two clocks to the System Control Module, an external 32.768-kHz clock or a
low-frequency clock (HIB LFIOSC). The Hibernation module has the option of being clocked by
a 32.768-kHz oscillator connected to the
XOSC0
pin. The 32.768-kHz oscillator can be used for
the system clock, thus eliminating the need for an additional crystal or oscillator. Alternatively,
the Hibernation module contains a low-frequency oscillator (HIB LFIOSC) which is intended to
provide the system with a real-time clock source and may also provide an accurate source of
Deep-Sleep or Hibernate mode power savings. Note that the HIB LFIOSC is a different clock
source than the LFIOSC. Refer to the Electrical Characteristic Chapter for more information on
frequency range.
June 18, 2014
230
Texas Instruments-Production Data
System Control
