STV3550 datasheet(91/144 Pages) STMICROELECTRONICS

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7.4

Real Time Clock (RTC)

The Real Time Clock (RTC) provides a set of continuously running counters which can be used,

with suitable software, to provide a clock/calendar function. The counter values can be written to set

the current time/date data. The RTC is clocked by a 32.768 kHz crystal oscillator.

7.4.1

Real Time Clock (RTC)

The RTC contains two counters:

●

30-bit “milliseconds” counter,

●

16-bit “weeks” counter.

This allows large time values to be represented with high accuracy, without the complication of

handling long (greater than 32 bit) integers in C.

Both counters increment at 1.024 kHz.

Because the RTC must run continuously, the counters are not reset by the software or Watchdog

reset. Only a global external chip reset will reset the counters.

7.4.1.1 RTC Counters

Because the “milliseconds” counter increments at 1.024 kHz, the value does not actually represent

milliseconds and must be taken into account by any concerned software applications. The

milliseconds counter is modulo 1 week, or 619,315,200; i.e. 1024 (one second) x 60 (one minute) x

60 (one hour) x 24 (one day) x 7 (one week).

The “weeks” counter is incremented when the milliseconds counter wraps from 619,315,199 to 0.

This is a 16-bit counter. The current value of both counters can be read at any time by the CPU.

7.4.1.2 RTC Alarm

The RTC can provide an interrupt to the CPU when the counter reaches a programmed value and

the function is enabled.

Two dedicated registers, a 30-bit “milliseconds” register and a 16-bit “weeks” register can be

programmed by software.

If the Alarm Enable bit is set and the RTC counter reaches (equals) the reference value, an interrupt

is generated.

The alarm fuction is capable of waking up the MCU even in Eco Standby mode.

Application examples could be the automatic TV switch-off or wake-up functions.

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