Using the RTC for Real Time Tracking

The RTC peripheral can be used to keep track of real time. The period is configurable, so it is not necessary to generate an interrupt every second; this enables a decrease in power consumption. An overflow needs to be generated only when the current time is required e.g. for timestamp generation. The period can be changed from anywhere between one second to more than 18 hours.

Note: Ideally, this method requires that an accurate oscillator is connected to the TOSC pins and the necessary oscillator design considerations have been adhered to. The significant error is introduced when the oscillator used as the RTC clock source has low accuracy. For the purposes of demonstration, the available oscillators on the Xplained Pro and Mini kits are used. However, in practice, these may not be accurate enough.

The RTC may be configured as follows:

Note: Because the RTC and the CPU run asynchronously, the STATUS register of the RTC may be used to verify synchronization of register settings before moving on.

The required oscillator may be enabled in the CLKCTRL module and selected as the RTC clock source (ideally a highly accurate 32.768 kHz oscillator)
The RTC may be allowed to run in standby
The prescaler may be set to an appropriate value, to give a count increment once per second (e.g. for a 32.768 kHz oscillator, choose DIV32768)
The period may be set to the desired overflow interval
The RTC Interrupt flags may be cleared and the overflow interrupt enabled
Finally, the RTC may be enabled to start counting

Additionally, an ISR may be defined for the RTC overflow interrupt vector (RTC_CNT_vect), to complete the time update and any other required periodic tasks such as a sensor measurement and timestamp string generation. Two subroutines are used to update the time. They use a time struct as defined below.

typedef struct{
	uint8_t second;
	uint8_t minute;
	uint8_t hour;
	uint8_t date;
	uint8_t month;
	uint16_t year;
}time;

The 'update time' subroutine increments the 'second' by the defined number of seconds in the period, and checks for the necessity to increment the higher value members, i.e. 'minute' is incremented if 'second' is larger than or equal to 60, and so on. The complete functionality of the subroutine is depicted in the following figure.

Figure 1. Update Time Subroutine

The 'not a leap year?' subroutine is used by the 'update time' subroutine when checking whether 'month' needs to be incremented during February. It returns true if it is not a leap year, i.e. February has 28 days, and false if it is a leap year, i.e. February has 29 days. Its calculations can be seen in the figure below.

Figure 2. Leap Year Check Subroutine