Clock System Operation
385
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Clock System (CS)
NOTE:
When REFO is turned on directly in 128-kHz mode by setting REFO_EN = 1 while
REFOFSEL = 1, the first clock pulse is stretched and its period corresponds to a 32.768-kHz
clock. The subsequent clock pulses are generated correctly at 128-kHz frequency.
NOTE:
If REFO is disabled when entering into LPM3 or LPM4 mode, it is not fully enabled and
stable upon exit from the selected mode because its enable time is much longer than the
wake-up time. If the application requires to keep REFO enabled during these modes, the
REFO_EN bit can be set before entering these modes. This causes REFO to remain
enabled. The REFO_EN bit has no effect in LPM4.5 mode.
6.2.6 Module Oscillator (MODOSC)
The clock system also supports an internal oscillator, MODOSC, that can be used by MCLK, HSMCLK,
SMCLK, and directly by other modules in the system. To conserve power, MODOSC is powered down
when not needed and is enabled only when required.
MODOSC is enabled under any of the following conditions:
•
For active modes (AM_LDO_VCOREx and AM_DCDC_VCOREx) or LPM0 modes
(LPM0_LDO_VCOREx and LPM0_DCDC_VCOREx)
–
MODOSC_EN = 1
–
MODOSC is a source for MCLK (SELMx = 4).
–
MODOSC is a source for HSMCLK (SELSx = 4).
–
MODOSC is a source for SMCLK (SELSx = 4).
–
MODCLK is a direct source for any module available in active mode or LPM0 mode and any
MODCLK unconditional request active.
•
For LPM3 or LPM4 or LPM3.5 or LPM4.5 mode
–
MODOSC is not available and is disabled. MODOSC_EN has no effect.
6.2.7 System Oscillator (SYSOSC)
Certain modules in the system require an embedded oscillator for general-purpose timing, but do not
require the stringent accuracy and start-up requirements of MODOSC. To conserve power, SYSOSC is
powered down when not needed and is enabled only when required.
SYSOSC is used by the system for the following purposes:
•
Memory controllers (flash and SRAM) state machine clock.
•
Fail-safe clock source for HFXT. (see
)
•
Power control manager (PCM) and power supply system (PSS) state machine clock.
•
eUSCI module for clock time-out feature for SMBus support.
6.2.8 Digitally Controlled Oscillator (DCO)
The DCO is an integrated digitally controlled oscillator. The DCO has six ranges that are factory calibrated
at frequency centers for each respective range. Each range is selectable by the DCORSEL bits. Using the
DCOTUNE bits, the application can tune the frequency within a selected range in 0.2% nominal steps to
adjust the DCO frequency up or down from the calibrated center frequency. Each range overlaps with its
neighboring ranges thereby giving a continuous range of frequencies available for use in the application.
The DCO can be used as a source for MCLK, HSMCLK, or SMCLK.
DCO is enabled under any of the following conditions:
•
For active modes (AM_LDO_VCOREx and AM_DCDC_VCOREx) or LPM0 modes
(LPM0_LDO_VCOREx and LPM0_DCDC_VCOREx)
–
DCO_EN = 1
–
DCO is a source for MCLK (SELMx = 3).