LM3S317 Luminary Micro, Inc, LM3S317 Datasheet - Page 54

• Manufacturer Part Number: LM3S317
• Description: Lm3s317 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

System Control
Figure 6-2. Main Clock Tree
6.1.4.2
6.1.4.3
6.1.4.4
54
OSC1
OSC2
a. These are bit fields within the Run-Mode Clock Configuration (RCC) register.
PLL Frequency Configuration
The user does not have direct control over the PLL frequency, but is required to match the external
crystal used to an internal PLL-Crystal table. This table is used to create the best fit for PLL
parameters to the crystal chosen. Not all crystals result in the PLL operating at exactly 200 MHz,
though the frequency is within
Translation (PLLCFG) register (see page 83).
Table 6-4 on page 82 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 (see page 78). Any time the XTAL field changes, a read of the
internal table is performed to get the correct value. Table 6-4 on page 82 describes the available
crystal choices and default programming values.
PLL Modes
The PLL has two modes of operation: Normal and Power-Down
The modes are programmed using the RCC register fields as shown in Table 6-4 on page 82.
PLL Operation
If the PLL configuration is changed, the PLL output is not stable for a period of time (PLL
T
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
1-8 MHz
Internal
15 MHz
READY
Main
Osc
Osc
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 (see page 78)—writes of the same value do not
cause a relock.
Change in the PLL from Power-Down to Normal mode.
=0.5 ms) and during this time, the PLL is not usable as a clock reference.
÷4
OSCSRC
μs
a
±
at a 8.192-MHz external oscillator clock). Hardware is provided to
1%. The result of the lookup is kept in the XTAL to PLL
Preliminary
PWRDN
(200 MHz
XTAL
OEN
output )
PLL
READY
a
a
a
requirement. The counter is clocked by the main
BYPASS
a
(16.667 MHz output )
PWMDIV
SYSDIV
Constant
Divide
a
a
USEPWMDIV
USESYSDIV
February 6, 2007
a
a
System Clock
PWM Clock
ADC Clock