RTC_C Operation
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SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Real-Time Clock C (RTC_C)
29.2.9 Real-Time Clock Operation in LPM3.5 Low-Power Mode
The regulator of the Power Management Module (PMM) is disabled when the device enters LPM3.5,
which causes most of the RTC_C configuration registers to be lost; only the counters and calibration
registers are retained.
shows which registers are retained in LPM3.5. Also the configuration of
the interrupt enables is stored so that the configured interrupts can cause a wakeup upon exit from
LPM3.5. Interrupt flags that are set before entering LPM3.5 are cleared upon entering LPM3.5 (Note: this
can only happen if the corresponding interrupt is not enabled). The interrupt flags RTCTEVIFG, RTCAIFG,
RT1PSIFG, and RTCOFIFG can be used as RTC_C wakeup interrupt sources. Any interrupt event that
occurs during LPM3.5 is stored in the corresponding flags, but only enabled interrupts can wake up the
device. After restoring the configuration registers (and clearing LOCKLPM5), the interrupts can be
serviced as usual.
The detailed flow is as follows:
1. Set all I/Os to general-purpose I/Os and configure as needed. Optionally, configure input interrupt pins
for wakeup. Configure RTC_C interrupts for wake-up (set RTCTEVIE, RTCAIE, RT1PSIE, or
RTCOFIE. If the alarm interrupt is also used as wake-up event, the alarm registers must be configured
as needed).
2. Enter LPM3.5 with LPM3.5 entry sequence:
bic #RTCHOLD, &RTCCTL13
bis # REGOFF, &PMMCTL0
bis #LPM4, SR
3. LOCKLPM5 is automatically set by hardware upon entering LPM3.5, the core voltage regulator is
disabled, and all clocks are disabled except for the 32-kHz crystal oscillator clock as the RTC_C is
enabled with RTCHOLD = 0.
4. An LPM3.5 wake-up event like an edge on a wake-up input pin or an RTC_C interrupt event starts the
BOR entry sequence and the core voltage regulator. All peripheral registers are set to their default
conditions. The I/O pin state and the interrupt configuration for the RTC_C remain locked.
5. The device can be configured. The I/O configuration and the RTC_C interrupt configuration that was
not retained during LPM3.5 should be restored to the values that they had before entering LPM3.5.
Then the LOCKLPM5 bit can be cleared, which releases the I/O pin conditions and the RTC_C
interrupt configuration. Registers that are retained during LPM3.5 should not be altered before
LOCKLPM5 is cleared.
6. After enabling I/O and RTC_C interrupts, the interrupt that caused the wake-up can be serviced.
If the RTC_C is enabled (RTCHOLD = 0), the 32-kHz oscillator remains active during LPM3.5. The fault
detection also remains functional. If a fault occurs during LPM3.5 and the RTCOFIE was set before
entering LPM3.5, a wake-up event is issued.