AN3133 datasheet(9/43 Pages) STMICROELECTRONICS

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AN3133

Real-time clock overview

Doc ID 16956 Rev 5

9/43

The alarm consists of a register with the same length as the RTC time counter. When the

RTC time counter reaches the value programmed in the alarm register, a flag is set to

indicate that an alarm event occurred.

The STM8L/STM8AL RTC alarm can be configured by hardware to generate different types

of alarms. For more details refer to Table 8.

1.3

RTC periodic wakeup unit

Like many low consumption microcontrollers, STM8L/STM8AL microcontrollers provide

several low power modes to reduce power consumption.

STM8L/STM8AL microcontrollers feature a periodic timebase and wakeup unit that can

wake up the system when the device operates in low power mode. This unit is a

programmable downcounting auto-reload timer. When this counter reaches zero, a flag and

an interrupt (if enabled) are generated.

The wakeup unit has the following features:

●

Programmable downcounting auto-reload timer

●

Specific flag and interrupt capable of waking up the device from low power modes

●

Wakeup alternate function output which can be routed to RTC_ALARM output (unique

pad for both Alarm and Wakeup events) with configurable polarity

●

A full set of prescalers to select the desired waiting period

1.4

RTC smooth digital calibration

The RTC clock frequency can be digitally calibrated by a series of small adjustments by

adding or subtracting RTC clock cycles.

The RTC calibration block is designed to compensate the accuracy of typical crystal

oscillators.

Crystal accuracy is highly dependant on:

●

Temperature

●

Crystal aging

Crystal accuracy is typically ±35 ppm at 25° C (see Figure 4) which corresponds to

±1.5 min. per month.

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