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)
29.2.5
Reading or Writing Real-Time Clock Registers
Because the system clock may be asynchronous to the RTC_C clock source, special care must be used
when accessing the real-time clock registers.
The real-time clock registers are updated once per second. To prevent reading any real-time clock register
at the time of an update that could result in an invalid time being read, a keep-out window is provided. The
keep-out window is centered approximately 128/32768 seconds around the update transition. The read-
only RTCRDY bit is reset during the keep-out window period and set outside the keep-out the window
period. Any read of the clock registers while RTCRDY is reset is considered to be potentially invalid, and
the time read should be ignored.
An easy way to safely read the real-time clock registers is to use the RTCRDYIFG interrupt flag. Setting
RTCRDYIE enables the RTCRDYIFG interrupt. When enabled, an interrupt is generated based on the
rising edge of the RTCRDY bit, causing the RTCRDYIFG to be set. At this point, the application has
nearly a complete second to safely read any or all of the real-time clock registers. This synchronization
process prevents reading the time value during transition. The RTCRDYIFG flag is reset automatically
when the interrupt is serviced or can be reset with software.
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.
29.2.6 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 low-frequency oscillator clock 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.