TM4C123GH6PM Datasheet, PDF (220/1409 Pages) Texas Instruments – Tiva Microcontroller

System Control
5.2.5.2
used, the crystal value must be one of the supported frequencies between 5 MHz to 25 MHz
(inclusive). 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 as specified in Table
24-13 on page 1374. The supported crystals are listed in the XTAL bit field in the RCC register
(see page 254). Note that the MOSC provides the clock source for the USB PLL and must be
connected to a crystal or an oscillator.
■ Low-Frequency Internal Oscillator (LFIOSC). The low-frequency internal oscillator is intended
for use during Deep-Sleep power-saving modes. The frequency can have wide variations; refer
to “Low-Frequency Internal Oscillator (LFIOSC) Specifications” on page 1375 for more details. This
power-savings mode benefits from reduced internal switching and also allows the MOSC to be
powered down. In addition, the PIOSC can be powered down while in Deep-Sleep mode.
■ Hibernation Module Clock Source. The Hibernation module is 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. 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 5 MHz to 25 MHz (inclusive).
Table 5-3 on page 220 shows how the various clock sources can be used in a system.
Table 5-3. Clock Source Options
Clock Source
Drive PLL?
Precision Internal Oscillator
Yes
BYPASS = 0,
OSCSRC = 0x1
Precision Internal Oscillator divide by No
-
4 (4 MHz ± 1%)
Main Oscillator
Yes
BYPASS = 0,
OSCSRC = 0x0
Low-Frequency Internal Oscillator No
-
(LFIOSC)
Hibernation Module 32.768-kHz
No
-
Oscillator
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
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
a larger assortment of clock configuration options. These registers control the following clock
functionality:
■ Source of clocks in sleep and deep-sleep modes
■ System clock derived from PLL or other clock source
■ Enabling/disabling of oscillators and PLL
■ Clock divisors
220
June 12, 2014
Texas Instruments-Production Data