LM3S5737 Datasheet, PDF (174/833 Pages) Texas Instruments – Stellaris® LM3S5737 Microcontroller

System Control
5.2.4
5.2.5
5.2.5.1
main oscillator verification failure because the necessary code can be removed from the general
reset handler, speeding up reset processing. The detection circuit is enabled using the CVAL bit in
the Main Oscillator Control (MOSCCTL) register. The main oscillator verification error is indicated
in the main oscillator fail status bit (MOSCFAIL) bit in the Reset Cause (RESC) register. The main
oscillator verification circuit action is described in more detail in “Clock Control” on page 174.
Power Control
The Stellaris^® microcontroller provides an integrated LDO regulator that is used to provide power
to the majority of the controller's internal logic. For power reduction, the LDO regulator provides
software a mechanism to adjust the regulated value, in small increments (VSTEP), over the range
of 2.25 V to 2.75 V (inclusive)—or 2.5 V ± 10%. The adjustment is made by changing the value of
the VADJ field in the LDO Power Control (LDOPCTL) register.
Note: On the printed circuit board, use the LDO output as the source of VDD25 input. Do not use
an external regulator to supply the voltage to VDD25. In addition, the LDO requires decoupling
capacitors. See “On-Chip Low Drop-Out (LDO) Regulator Characteristics” on page 769.
VDDA must be supplied with 3.3 V, or the microcontroller does not function properly. VDDA
is the supply for all of the analog circuitry on the device, including the clock circuitry.
Clock Control
System control determines the control of clocks in this part.
Fundamental Clock Sources
There are multiple clock sources for use in the device:
■ Internal Oscillator (IOSC). The internal oscillator is an on-chip clock source. It does not require
the use of any external components. The frequency of the internal oscillator is 12 MHz ± 30%.
Applications that do not depend on accurate clock sources may use this clock source to reduce
system cost. The internal oscillator is the clock source the device uses during and following POR.
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.
■ 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 192). Note that the MOSC must have a clock source for the USB
PLL.
■ 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 (see “Hibernation Module” on page 236) and
may also provide an accurate source of Deep-Sleep or Hibernate mode power savings.
174
November 17, 2011
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