LM3S6G11 Datasheet, PDF (176/827 Pages) Texas Instruments – Stellaris® LM3S6G11 Microcontroller

System Control
OBSOLETE: TI has discontinued production of this device.
5.2.5.2
applications that require an accurate 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.
■ 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 must be one of the supported frequencies between 3.579545 MHz to
16.384 MHz (inclusive). If the PLL is not being used, the crystal may be any one of the supported
frequencies between 1 MHz to 16.384 MHz. The single-ended clock source range is from DC
through the specified speed of the microcontroller. The supported crystals are listed in the XTAL
bit field in the RCC register (see page 198).
■ Internal 30-kHz Oscillator. The internal 30-kHz oscillator provides an operational frequency of
30 kHz ± 50%. It is intended for use during Deep-Sleep power-saving modes. This power-savings
mode benefits from reduced internal switching and also allows the MOSC to be powered down.
■ Hibernation Module Clock Source. The Hibernation module can be clocked in one of two ways.
The first way is a 4.194304-MHz crystal connected to the XOSC0 and XOSC1 pins. This clock
signal is divided by 128 internally to produce the 32.768-kHz clock reference. The second way
is 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. The
Hibernation module clock source 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.
The internal system clock (SysClk), is derived from any of the above sources plus two others: the
output of the main internal PLL and the precision internal oscillator divided by four (4 MHz ± 1%).
The frequency of the PLL clock reference must be in the range of 3.579545 MHz to 16.384 MHz
(inclusive). Table 5-4 on page 176 shows how the various clock sources can be used in a system.
Table 5-4. Clock Source Options
Clock Source
Drive PLL?
Precision Internal Oscillator
Yes
BYPASS = 0,
OSCSRC = 0x1
Precision Internal Oscillator divide by 4 No
-
(4 MHz ± 1%)
Main Oscillator
Yes
BYPASS = 0,
OSCSRC = 0x0
Internal 30-kHz Oscillator
No
-
Hibernation Module 32.768-kHz
No
-
Oscillator
Hibernation Module 4.194304-MHz No
-
Crystal
Used as SysClk?
Yes
BYPASS = 1, OSCSRC = 0x1
Yes
BYPASS = 1, OSCSRC = 0x2
Yes
BYPASS = 1, OSCSRC = 0x0
Yes
BYPASS = 1, OSCSRC = 0x3
Yes
BYPASS = 1, OSCSRC2 = 0x7
No
-
Clock Configuration
The Run-Mode Clock Configuration (RCC) and Run-Mode Clock Configuration 2 (RCC2)
registers provide control for the system clock. The RCC2 register is provided to extend fields that
offer additional encodings over the RCC register. When used, the RCC2 register field values are
used by the logic over the corresponding field in the RCC register. In particular, RCC2 provides for
176
July 24, 2012
Texas Instruments-Production Data