LM3S5652 Luminary Micro, Inc, LM3S5652 Datasheet - Page 70

• Manufacturer Part Number: LM3S5652
• Description: Lm3s5652 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

System Control
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The internal system clock (SysClk), is derived from any of the four sources plus two others: the
output of the main internal PLL, and the internal oscillator divided by four (3 MHz ± 30%). The
frequency of the PLL clock reference must be in the range of 3.579545 MHz to 16.384 MHz
(inclusive).
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.
Figure 6-2 on page 71 shows the logic for the main clock tree. The peripheral blocks are driven by
the system clock signal and can be programmatically enabled/disabled. The ADC clock signal is
automatically divided down to 16 MHz for proper ADC operation.
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 through
16.384 MHz (inclusive). If the PLL is not being used, the crystal may be any one of the supported
frequencies between 1 MHz and 16.384 MHz. The single-ended clock source range is from DC
through the specified speed of the device. The supported crystals are listed in the XTAL bit field
in the RCC register (see page 85).
Internal 30-kHz Oscillator: The internal 30-kHz oscillator is similar to the internal oscillator,
except that it 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 main oscillator to be powered down.
External Real-Time Oscillator: The external real-time oscillator provides a low-frequency,
accurate clock reference. It is intended to provide the system with a real-time clock source. The
real-time oscillator is part of the Hibernation Module (“Hibernation Module” on page 128) and may
also provide an accurate source of Deep-Sleep or Hibernate mode power savings.
Preliminary
June 02, 2008