5.2.5.3
Precision Internal Oscillator Operation (PIOSC)
The microcontroller powers up with the PIOSC running. If another clock source is desired, the PIOSC
must remain enabled because it is used for internal functions. The PIOSC can only be disabled
during Deep-Sleep mode. It can be powered down by setting the
PIOSCPD
bit in the
DSCLKCFG
register.
The PIOSC generates a 16-MHz clock with a ±F
PIOSC
accuracy (see Table 27-19 on page 1837). 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 the following 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.
■ Automatic calibration using the enable 32.768-kHz oscillator from the Hibernation module: Set
the
CAL
bit in the
PIOSCCAL
register; the results of the calibration are shown in the
RESULT
field in the
Precision Internal Oscillator Statistic (PIOSCSTAT)
register. After calibration is
complete, the PIOSC is trimmed using the trimmed value returned in the
CT
field.
5.2.5.4
Main Oscillator (MOSC)
The main oscillator supports the use of crystals from 5 to 25 MHz. The system control's
RSCLKCFG
register can be configured to specify the MOSC as the system clock or as the PLL input source.
The MOSC can be selected as the oscillator source by programming the
OSCRC
bit in the
RSCLKCFG
register. The
NOXTAL
bit in the
MOSCCTL
register allows the user to turn off power to the MOSC
if no crystal is connected reducing power draw from the MOSC circuit.
Main Oscillator Verification Circuit
The clock control includes circuitry to ensure that the main oscillator is running at the appropriate
frequency. The circuit monitors the main oscillator frequency and signals if the frequency is outside
of the allowable band of attached crystals.
The detection circuit is enabled using the
CVAL
bit in the
Main Oscillator Control (MOSCCTL)
register. If this circuit is enabled and detects an error, and if the
MOSCIM
bit in the
MOSCCTL
register
is clear, then the following sequence is performed by the hardware:
1.
The
MOSCFAIL
bit in the
Reset Cause (RESC)
register is set.
2.
The system clock is switched from the main oscillator to the PIOSC.
3.
An internal system reset is initiated.
4.
Reset is deasserted and the processor is directed to the NMI handler during the reset sequence.
5.2.5.5
PLL
The PLL has two modes of operation: Normal and Power-Down
■ Normal: The PLL oscillates based on the values in the
PLLFREQ0
and
PLLFREQ1
registers
and drives the output.
June 18, 2014
236
Texas Instruments-Production Data
System Control