LM3S9B81 Datasheet, PDF (108/1155 Pages) Texas Instruments – Stellaris® LM3S9B81 Microcontroller

System Control
6.2.5.3
6.2.5.4
this situation, the divisor is equivalent to the (SYSDIV2 encoding with SYSDIV2LSB appended) plus
one. Table 6-7 shows the frequency choices when DIV400 is set. When the DIV400 bit is clear,
SYSDIV2LSB is ignored, and the system clock frequency is determined as shown in Table
6-6 on page 107.
Table 6-7. Examples of Possible System Clock Frequencies with DIV400=1
SYSDIV2 SYSDIV2LSB
Divisor
Frequency (BYPASS2=0)^a
StellarisWare Parameter^b
0x00
reserved
/2
reserved
-
0x01
0
/3
reserved
-
1
/4
reserved
-
0x02
0
/5
80 MHz
SYSCTL_SYSDIV_2_5
1
/6
66.67 MHz
SYSCTL_SYSDIV_3
0x03
0
/7
reserved
-
1
/8
50 MHz
SYSCTL_SYSDIV_4
0x04
0
/9
44.44 MHz
SYSCTL_SYSDIV_4_5
1
/10
40 MHz
SYSCTL_SYSDIV_5
...
...
...
...
...
0x3F
0
/127
3.15 MHz
SYSCTL_SYSDIV_63_5
1
/128
3.125 MHz
SYSCTL_SYSDIV_64
a. Note that DIV400 and SYSDIV2LSB are only valid when BYPASS2=0.
b. This parameter is used in functions such as SysCtlClockSet() in the Stellaris Peripheral Driver Library.
Precision Internal Oscillator Operation (PIOSC)
The microcontroller powers up with the PIOSC running. If another clock source is desired, the PIOSC
can be powered down by setting the IOSCDIS bit in the RCC register.
The PIOSC generates a 16 MHz clock with a ±1% accuracy at room temperatures. Across the
extended temperature range, the accuracy is ±3%. At the factory, the PIOSC is set to 16 MHz at
room temperature, however, the frequency can be trimmed for other voltage or temperature conditions
using software in one of two ways:
■ Default calibration: clear the UTEN bit and set the UPDATE bit in the Precision Internal Oscillator
Calibration (PIOSCCAL) register.
■ User-defined calibration: The user can program the UT value to adjust the PIOSC frequency. As
the UT value increases, the generated period increases. To commit a new UT value, first set the
UTEN bit, then program the UT field, and then set the UPDATE bit. The adjustment finishes within
a few clock periods and is glitch free.
Crystal Configuration for the Main Oscillator (MOSC)
The main oscillator supports the use of a select number of crystals. If the main oscillator is used by
the PLL as a reference clock, the supported range of crystals is 3.579545 to 16.384 MHz, otherwise,
the range of supported crystals is 1 to 16.384 MHz.
The XTAL bit in the RCC register (see page 125) describes the available crystal choices and default
programming values.
Software configures the RCC register XTAL field with the crystal number. If the PLL is used in the
design, the XTAL field value is internally translated to the PLL settings.
108
June 29, 2010
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