TM4C1230C3PM Datasheet, PDF (206/1103 Pages) Texas Instruments – Tiva TM4C1230C3PM Microcontroller

System Control
5.2.5.2
(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
20-13 on page 1071. The supported crystals are listed in the XTAL bit field in the RCC register
(see page 236).
■ 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 1072 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.
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 206 shows how the various clock sources can be used in a system.
Table 5-3. Clock Source Options
Clock Source
Drive PLL?
Used as SysClk?
Precision Internal Oscillator
Yes
BYPASS = 0, OSCSRC Yes
= 0x1
BYPASS = 1, OSCSRC = 0x1
Precision Internal Oscillator divide by No
-
4 (4 MHz ± 1%)
Yes
BYPASS = 1, OSCSRC = 0x2
Main Oscillator
Yes
BYPASS = 0, OSCSRC Yes
= 0x0
BYPASS = 1, OSCSRC = 0x0
Low-Frequency Internal Oscillator No
-
(LFIOSC)
Yes
BYPASS = 1, OSCSRC = 0x3
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
■ Crystal input selection
Important: Write the RCC register prior to writing the RCC2 register.
When transitioning the system clock configuration to use the MOSC as the fundamental
clock source, the MOSCDIS bit must be set prior to reselecting the MOSC or an undefined
system clock configuration can sporadically occur.
206
June 12, 2014
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