Texas Instruments MSP430x4xx Family, User Manual

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Real-Time Clock Operation
14-4 Real Time Clock
14.2 Real-Time Clock Operation
The Real-Time Clock module can be configured as a real-time clock with
calendar function or as a 32-bit general-purpose counter with the RTCMODEx
bits.
14.2.1 Counter Mode
Counter mode is selected when RTCMODEx < 11. In this mode, a 32-bit
counter is provided that is directly accessible by software. Switching from
calendar to counter mode resets the count value.
The clock to increment the counter can be sourced from ACLK, SMCLK, or
from the BTCNT2 input clock divided by 128 from the Basic Timer1 module,
selected by the RTCMODEx bits. The counter can be stopped by setting the
RTCHOLD bit.
Four individual 8-bit counters are cascaded to provide the 32-bit counter. This
provides interrupt triggers at 8-bit, 16-bit, 24-bit, and 32-bit overflows. Each
counter RTCNT1 - RTCNT4 is individually accessible and may be read or
written to.
Note: Accessing the RTCNTx registers
When the counter clock is asynchronous to the CPU clock, any read from any
RTCNTx register should occur while the counter is not operating. Otherwise,
the results may be unpredictable. Alternatively, the counter may be read
multiple times while operating, and a majority vote taken in software to
determine the correct reading. Any write to any RTCNTx register takes effect
immediately.