LM3S8971 Luminary Micro, Inc, LM3S8971 Datasheet - Page 69

• Manufacturer Part Number: LM3S8971
• Description: Lm3s8971 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

6.1.4.3
6.1.4.4
6.1.4.5
6.1.5
July 26, 2008
Main PLL Frequency Configuration
The main PLL is disabled by default during power-on reset and is enabled later by software if
required. Software specifies the output divisor to set the system clock frequency, and enables the
main PLL to drive the output.
If the main oscillator provides the clock reference to the main PLL, the translation provided by
hardware and used to program the PLL is available for software in the XTAL to PLL Translation
(PLLCFG) register (see page 85). The internal translation provides a translation within ± 1% of the
targeted PLL VCO frequency.
The Crystal Value field (XTAL) on page 81 describes the available crystal choices and default
programming of the PLLCFG register. The crystal number is written into the XTAL field of the
Run-Mode Clock Configuration (RCC) register. Any time the XTAL field changes, the new settings
are translated and the internal PLL settings are updated.
PLL Modes
The PLL has two modes of operation: Normal and Power-Down
■
■
The modes are programmed using the RCC/RCC2 register fields (see page 81 and page 86).
PLL Operation
If a PLL configuration is changed, the PLL output frequency is unstable until it reconverges (relocks)
to the new setting. The time between the configuration change and relock is T
23-7 on page 561). During the relock time, the affected PLL is not usable as a clock reference.
The PLL is changed by one of the following:
■
■
A counter is defined to measure the T
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
changes above. It is the user's responsibility to have a stable clock source (like the main oscillator)
before the RCC/RCC2 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/RCC2
register until the main PLL is stable (T
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.
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.
Normal: The PLL multiplies the input clock reference and drives the output.
Power-Down: Most of the PLL internal circuitry is disabled and the PLL does not drive the output.
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.
Preliminary
READY
READY
requirement. The counter is clocked by the main
time met), after which it changes to the PLL. Software
READY
condition is met after one of the two
LM3S8971 Microcontroller
READY
(see Table
69