LM3S5C31 Datasheet, PDF (199/1220 Pages) Texas Instruments – Stellaris® LM3S5C31 Microcontroller

Stellaris^® LM3S5C31 Microcontroller
5.2.6.4
5.3
the microcontroller to Run mode from one of the sleep modes; the sleep modes are entered on
request from the code. Deep-Sleep mode is entered by first setting the SLEEPDEEP bit in the System
Control (SYSCTRL) register (see page 142) and then executing a WFI instruction. Any properly
configured interrupt event in the system brings the processor back into Run mode. See “Power
Management” on page 100 for more details.
The Cortex-M3 processor core and the memory subsystem are not clocked in Deep-Sleep mode.
Peripherals are clocked that are enabled in the DCGCn registers when auto-clock gating is enabled
(see the RCC register) or the RCGCn registers when auto-clock gating is disabled. The system
clock source is specified in the DSLPCLKCFG register. When the DSLPCLKCFG register is used,
the internal oscillator source is powered up, if necessary, and other clocks are powered down. If
the PLL is running at the time of the WFI instruction, hardware powers the PLL down and overrides
the SYSDIV field of the active RCC/RCC2 register, to be determined by the DSDIVORIDE setting
in the DSLPCLKCFG register, up to /16 or /64 respectively. When the Deep-Sleep exit event occurs,
hardware brings the system clock back to the source and frequency it had at the onset of Deep-Sleep
mode before enabling the clocks that had been stopped during the Deep-Sleep duration. If the
PIOSC is used as the PLL reference clock source, it may continue to provide the clock during
Deep-Sleep. See page 225.
Hibernate Mode
In this mode, the power supplies are turned off to the main part of the microcontroller and only the
Hibernation module's circuitry is active. An external wake event or RTC event is required to bring
the microcontroller back to Run mode. The Cortex-M3 processor and peripherals outside of the
Hibernation module see a normal "power on" sequence and the processor starts running code.
Software can determine if the microcontroller has been restarted from Hibernate mode by inspecting
the Hibernation module registers. For more information on the operation of Hibernate mode, see
“Hibernation Module” on page 285.
Initialization and Configuration
The PLL is configured using direct register writes to the RCC/RCC2 register. If the RCC2 register
is being used, the USERCC2 bit must be set and the appropriate RCC2 bit/field is used. The steps
required to successfully change the PLL-based system clock are:
1. Bypass the PLL and system clock divider by setting the BYPASS bit and clearing the USESYS
bit in the RCC register, thereby configuring the microcontroller to run off a "raw" clock source
and allowing for the new PLL configuration to be validated before switching the system clock
to the PLL.
2. Select the crystal value (XTAL) and oscillator source (OSCSRC), and clear the PWRDN bit in
RCC/RCC2. Setting the XTAL field automatically pulls valid PLL configuration data for the
appropriate crystal, and clearing the PWRDN bit powers and enables the PLL and its output.
3. Select the desired system divider (SYSDIV) in RCC/RCC2 and set the USESYS bit in RCC. The
SYSDIV field determines the system frequency for the microcontroller.
4. Wait for the PLL to lock by polling the PLLLRIS bit in the Raw Interrupt Status (RIS) register.
5. Enable use of the PLL by clearing the BYPASS bit in RCC/RCC2.
January 23, 2012
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