LM3S628 Datasheet, PDF (158/531 Pages) List of Unclassifed Manufacturers – Microcontroller

System Control
5.2.4.7
5.2.5
■ Change to the XTAL value in the RCC register—writes of the same value do not cause a relock.
■ Change in the PLL from Power-Down to Normal mode.
A counter is defined to measure the T_READY requirement. The counter is clocked by the main
oscillator. The range of the main oscillator has been taken into account and the down counter is set
to 0x1200 (that is, ~600 μs at an 8.192 MHz external oscillator clock). Hardware is provided to keep
the PLL from being used as a system clock until the T_READY condition is met after one of the two
changes above. It is the user's responsibility to have a stable clock source (like the main oscillator)
before the RCC register is switched to use the PLL.
If the main PLL is enabled and the system clock is switched to use the PLL in one step, the system
control hardware continues to clock the controller from the oscillator selected by the RCC register
until the main PLL is stable (T_READY time met), after which it changes to the PLL. Software can use
many methods to ensure that the system is clocked from the main PLL, including periodically polling
the PLLLRIS bit in the Raw Interrupt Status (RIS) register, and enabling the PLL Lock interrupt.
Clock Verification Timers
There are three identical clock verification circuits that can be enabled though software. The circuit
checks the faster clock by a slower clock using timers:
■ The main oscillator checks the PLL.
■ The main oscillator checks the internal oscillator.
■ The internal oscillator divided by 64 checks the main oscillator.
If the verification timer function is enabled and a failure is detected, the main clock tree is immediately
switched to a working clock and an interrupt is generated to the controller. Software can then
determine the course of action to take. The actual failure indication and clock switching does not
clear without a write to the CLKVCLR register, an external reset, or a POR reset. The clock
verification timers are controlled by the PLLVER , IOSCVER , and MOSCVER bits in the RCC register.
System Control
For power-savings purposes, the RCGCn , SCGCn , and DCGCn registers control the clock gating
logic for each peripheral or block in the system while the controller is in Run, Sleep, and Deep-Sleep
mode, respectively. The DC1 , DC2 and DC4 registers act as a write mask for the RCGCn , SCGCn,
and DCGCn registers.
There are three levels of operation for the device defined as:
■ Run Mode. In Run mode, the controller actively executes code. Run mode provides normal
operation of the processor and all of the peripherals that are currently enabled by the RCGCn
registers. The system clock can be any of the available clock sources including the PLL.
■ Sleep Mode. In Sleep mode, the clock frequency of the active peripherals is unchanged, but the
processor and the memory subsystem are not clocked and therefore no longer execute code.
Sleep mode is entered by the Cortex-M3 core executing a WFI(Wait for Interrupt)
instruction. Any properly configured interrupt event in the system will bring the processor back
into Run mode. See “Power Management” on page 77 for more details.
Peripherals are clocked that are enabled in the SCGCn register when auto-clock gating is enabled
(see the RCC register) or the RCGCn register when the auto-clock gating is disabled. The system
clock has the same source and frequency as that during Run mode.
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July 14, 2014
Texas Instruments-Production Data