Clock System Operation
384
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Clock System (CS)
NOTE:
If VLO is disabled when entering into LPM3 or LPM4 modes, 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 VLO enabled during these modes, the VLO_EN bit
can be set before entering these modes. This causes VLO to remain enabled. The VLO_EN
bit has no effect in LPM4.5 mode.
6.2.5 Internal Low-Power Low-Frequency Oscillator (REFO)
The REFO provides a typical frequency of 32.768 kHz (see device-specific data sheet for parameters)
without requiring a crystal. The REFO provides for a low-cost ultra low-power clock source for applications
that do not require a crystal accurate time base, but more accuracy than what can be achieved with the
VLO. To conserve power, REFO is powered down when not needed and enabled only when required.
REFO is enabled under any of the following conditions:
•
For any active mode or LPM0 mode
–
REFO_EN = 1
–
REFO is a source for ACLK (SELAx = 2).
–
REFO is a source for BCLK (SELB = 1).
–
REFO is a source for MCLK (SELMx = 2).
–
REFO is a source for HSMCLK (SELSx = 2).
–
REFO is a source for SMCLK (SELSx = 2).
–
REFOCLK is a direct source for any module available in active mode or LPM0 mode and any
REFOCLK unconditional request active.
–
LFXTCLK is a source for ACLK (SELAx = 0) and LFXTIFG is set due to crystal oscillator fault.
–
LFXTCLK is a source for BCLK (SELB = 0) and LFXTIFG is set due to crystal oscillator fault.
–
LFXTCLK is a source for MCLK (SELMx = 0) and LFXTIFG is set due to crystal oscillator fault.
–
LFXTCLK is a source for HSMCLK (SELSx = 0) and LFXTIFG is set due to crystal oscillator fault.
–
LFXTCLK is a source for SMCLK (SELSx = 0) and LFXTIFG is set due to crystal oscillator fault.
–
HFXTCLK is a source for MCLK (SELMx = 5) (to support HFXTCLK fail-safe operation).
–
HFXTCLK is a source for HSMCLK (SELSx = 5) (to support HFXTCLK fail-safe operation).
–
HFXTCLK is a source for SMCLK (SELSx = 5) (to support HFXTCLK fail-safe operation).
•
For LPM3 mode or LPM3.5 mode
–
REFO_EN = 1
–
REFO is a source for BCLK (SELB = 1) and a BCLK request from any backup domain peripheral is
active.
–
REFOCLK is a direct source for any module available in LPM3 or LPM3.5.
•
For LPM4.5 mode
–
REFO is off. REFO_EN bit has no effect in this mode.
REFO supports two frequencies of operation: nominally 32.768 kHz (default) and 128 kHz. The frequency
is selected by the REFOFSEL bit. Setting REFOFSEL = 1 is useful as a low-power clock source for low-
frequency active modes (AM_LF_VCOREx) and low-frequency LPM0 modes (LPM0_LF_VCOREx).
When
REFO is the source for BCLK, REFO always provides a 32.768-kHz source to BCLK, irrespective of
the REFOFSEL setting.
REFO is turned on (if not already on) whenever HFXT oscillator is enabled for use on any of the clocks
(MCLK, SMCLK, or HSMCLK). This is done because REFO is used to check the stability of the HFXTCLK
and to control generation of HFXTIFG.