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Functional Description
208
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
System Control
4.1.5.2.3 System Clock (SysClk) Frequency
The SysClk is distributed to the processor and the integrated peripherals after clock gating. The SysClk
frequency is based on the frequency of the clock source and the divisor factor. For example, if the PLL is
not being used and the device is not in deep-sleep mode, then the OSYSDIV bit field in the RSCLKCFG
register is the divisor used to determine the system clock. If the PLL is being used, then PSYSDIV bit field
in the RSCLKCFG register must be programmed as well as the values in the PLLFREQ0 and PLLFREQ1
registers. If the device is in deep-sleep mode, then the DSCLKCFG register can be programmed with the
divisor bit field DSSYSYDIV to modify the clock source frequency.
lists the different system
clock frequency calculations based on the operation mode, clock source, and PLL encoding.
Table 4-4. System Clock Frequency
Clock Mode
USEPLL
(RSCLKCFG)
SYSCLK Value
Divisor Factors Used
Run or sleep
1
f
VCO
/ (P 1)
PSYSDIV bit field in RSCLKCFG; MINT, MDIV
in PLLFREQ0; Q, N bits in PLLFREQ1
Run or sleep
0
f
OSCCLK
/ (O 1)
OSYSDIV bit field in RSCLKCFG
Deep sleep
PLL not enabled in
deep-sleep mode
f
OSCCLK
/ (DS 1)
DSSYSDIV bit field in DSCLKCFG
4.1.5.3
PIOSC Operation
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 (typical) clock. 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. When calibration is complete, the PIOSC
is trimmed using the trimmed value returned in the CT field.
4.1.5.4
MOSC Operation
The MOSC supports the use of crystals with a frequency of 5 to 25 MHz. The 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 lets the user turn off power to the MOSC if no crystal is connected,
which reduces power draw from the MOSC circuit.
4.1.5.4.1 MOSC Verification Circuit
The clock control includes circuitry to ensure that the MOSC is running at the appropriate frequency. The
circuit monitors the MOSC 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 RESC register is set.
2. The system clock is switched from the main oscillator to the PIOSC.