Clock System Operation
383
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Clock System (CS)
•
For active modes (AM_LDO_VCOREx and AM_DCDC_VCOREx) or LPM0 modes
(LPM0_LDO_VCOREx and LPM0_DCDC_VCOREx)
–
HFXT_EN = 1
–
HFXT is a source for MCLK (SELMx = 5).
–
HFXT is a source for HSMCLK (SELSx = 5).
–
HFXT is a source for SMCLK (SELSx = 5).
•
For active modes AM_LF_VCOREx or LPM0 modes LPM0_LF_VCOREx
–
HFXT is not available and is disabled. HFXT_EN has no effect.
•
For LPM3 or LPM4 or LPM3.5 or LPM4.5 modes
–
HFXT is not available and is disabled. HFXT_EN has no effect.
6.2.3.1
Using HFXT Oscillator After Wakeup From Low-Power Modes
The HFXT oscillator can be used as a clock source before entering LPM3, LPM4, LPM3.5, and LPM4.5
modes. Because the HFXT oscillator is disabled during these low-power modes, the application must
reinitialize the HFXT oscillator appropriately after wakeup. The guidelines for enabling the HFXT oscillator
after the low-power mode wake-up scenarios are:
Enabling HFXT oscillator after LPM3 or LPM4 wakeup:
When the device wakes up from LPM3 or
LPM4 mode, all clock settings (for example, clock selections and dividers) that were programmed before
entry into these low-power modes are retained. The application must restart the HFXT oscillator by
clearing the HFXTIFG flag. This is necessary because the HFXT oscillator start-up time is much longer
than the wake-up latency from LPM3 and LPM4 modes.
Enabling HFXT oscillator after LPM3.5 or LPM4.5 wakeup:
When the device wakes up from LPM3.5 or
LPM4.5 mode, all clock settings (for example, clock selections and dividers) that were programmed before
entry into these low-power modes are lost because of the POR reset. The application must reintialize the
HFXT oscillator completely. This is similar to configuring the HFXT oscillator after device power up.
Special care should be taken in this case to ensure that the LOCKLPM5 bit in the PCMCTL1 register is
cleared before starting the HFXT oscillator.
In both of the scenarios, the application can suppress interrupt generation by disabling the HFXTIE bit
before entering the low-power modes. When the HFXT oscillator is stable, the HFXTIE bit can be
reenabled to check for HFXT fault conditions.
6.2.4 Internal Very-Low-Power Low-Frequency Oscillator (VLO)
The VLO provides a typical frequency of 9.4-kHz (see device-specific data sheet for parameters) without
requiring a crystal. The VLO provides for a low-cost ultra-low-power clock source for applications that do
not require an accurate time base. To conserve power, VLO is powered down when not needed and
enabled only when required.
VLO is enabled under any of the following conditions:
•
For any active mode or LPM0 mode
–
VLO_EN = 1
–
VLO is a source for ACLK (SELAx = 1).
–
VLO is a source for MCLK (SELMx = 1).
–
VLO is a source for HSMCLK (SELSx = 1).
–
VLO is a source for SMCLK (SELSx = 1).
–
VLOCLK is a direct source for any module available in active mode or LPM0 and any VLOCLK
unconditional request active.
•
For LPM3 mode or LPM3.5 mode
–
VLO_EN = 1
–
VLOCLK is a direct source for any module available in LPM3 or LPM3.5.
•
For LPM4.5 mode
–
VLO is off. VLO_EN bit has no effect in this mode.