RTC_C Operation
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SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Real-Time Clock (RTC_C)
NOTE:
Reading or writing real-time clock registers
When the counter clock is asynchronous to the CPU clock, any read from any RTCSEC,
RTCMIN, RTCHOUR, RTCDOW, RTCDAY, RTCMON, RTCYEARL, or RTCYEARH register
while the RTCRDY is reset may result in invalid data being read. To safely read the counting
registers, either polling of the RTCRDY bit or the synchronization procedure previously
described can be used. Alternatively, the counter register can be read multiple times while
operating, and a majority vote taken in software to determine the correct reading. Reading
the RT0PS and RT1PS can only be handled by reading the registers multiple times and a
majority vote taken in software to determine the correct reading.
Any write to any counting register takes effect immediately. However, the clock is stopped
during the write. In addition, RT0PS and RT1PS registers are reset. This could result in
losing up to 1 second during a write. Writing of data outside the legal ranges or invalid time
stamp combinations results in unpredictable behavior.
20.2.6 Real-Time Clock Interrupts
At least six sources for interrupts are available, namely RT0PSIFG, RT1PSIFG, RTCRDYIFG,
RTCTEVIFG, RTCAIFG, and RTCOFIFG. These flags are prioritized and combined to source a single
interrupt vector. The interrupt vector register (RTCIV) is used to determine which flag requested an
interrupt.
The highest-priority enabled interrupt generates a number in the RTCIV register (see register description).
This number can be evaluated or added to the program counter (PC) to automatically enter the
appropriate software routine. Disabled RTC interrupts do not affect the RTCIV value.
Writes into RTCIV register clear all pending interrupt conditions. Reads from RTCIV register clear the
highest priority pending interrupt condition. If another interrupt flag is set, another interrupt is immediately
generated after servicing the initial interrupt. In addition, all flags can be cleared by software.
The user-programmable alarm event sources the real-time clock interrupt, RTCAIFG. Setting RTCAIE
enables the interrupt. In addition to the user-programmable alarm, the RTC_C module provides for an
interval alarm that sources real-time clock interrupt, RTCTEVIFG. The interval alarm can be selected to
cause an alarm event when RTCMIN changed or RTCHOUR changed, every day at midnight (00:00:00)
or every day at noon (12:00:00). The event is selectable with the RTCTEV bits. Setting the RTCTEVIE bit
enables the interrupt.
The RTCRDY bit sources the real-time clock interrupt, RTCRDYIFG, and is useful in synchronizing the
read of time registers with the system clock. Setting the RTCRDYIE bit enables the interrupt.
RT0PSIFG can be used to generate interrupt intervals selectable by the RT0IP bits. RT0PS is sourced
with BCLK at 32768 Hz, so intervals of 16384 Hz, 8192 Hz, 4096 Hz, 2048 Hz, 1024 Hz, 512 Hz, 256 Hz,
or 128 Hz are possible. Setting the RT0PSIE bit enables the interrupt.
RT1PSIFG can be used to generate interrupt intervals selectable by the RT1IP bits. RT1PS is sourced
with the output of RT0PS, which is 128 Hz (32768/256 Hz). Therefore, intervals of 64 Hz, 32 Hz, 16 Hz,
8 Hz, 4 Hz, 2 Hz, 1 Hz, or 0.5 Hz are possible. Setting the RT1PSIE bit enables the interrupt.
NOTE:
Changing RT0IP or RT1IP
Changing the settings of the interrupt interval bits RT0IP or RT1IP while the corresponding
prescaler is running or is stopped in a non-zero state can result in setting the corresponding
interrupt flags.
The RTCOFIFG bit flags the failure of the 32-kHz crystal oscillator connected to the BCLK line if a fault
happens to this oscillator while the design is in the Low-Power modes. When the oscillator faults out, the
failsafe is activated and the failsafe clock for BCLK is supplied to the RTC. This failsafe mechanism is
effective in both active and low-power modes. Refer to the
Clock System (CS)
chapter for the details. The
main purpose of the RTCOFIFG flag is to wake up the CPU from low-power mode of operation, because
the fault bit corresponding to the 32-kHz oscillator is not available for CPU interrupt in low-power modes if
an oscillator failure occurs.