AN3133 datasheet(14/43 Pages) STMICROELECTRONICS

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Real-time clock overview

AN3133

14/43

Doc ID 16956 Rev 5

Note:

The RTC_CALIB output is available on PD3 for 28-pin devices and on PD6 for 32- and 48-

pin devices.

Figure 8.

RTC_CALIB clock sources

RTCDIV[2:0] and RTCSEL[3:0] are bits of the CLK_CRTCR register.

1.7.2

RTC_ALARM output

The RTC_ALARM output can either be connected to the RTC alarm unit to trigger an

external action, or routed to the RTC wakeup unit to wake up an external device.

Note:

The RTC_ALARM pin is on PB3 for 28-pin devices, on PD7 for 32- and 48-pin devices.

RTC_ALARM output connected to the RTC alarm unit

When the calendar reaches the value pre-programmed in the RTC_ALRMARx registers, the

alarm flag (ALRAF bit in RTC_ISR2 register) is set to ‘1’. If the alarm flag is routed to the

RTC_ALARM output (OSEL[1:0] bits set to ‘01’ in RTC_CR3), this pin is set to VDD or to

GND, depending on the polarity selected. The output toggles when the alarm flag is cleared

Figure 9.

Alarm flag routed to RTC_ALARM output

Ai17264a

HSE

HSI

LSE

LSI

RTCCLK

CLKRTC

Prescaler

Asynchronous

Prescaler

PREDIV_A[5:0]

RTCDIV[2:0]

RTC_CR3 COE

RTCSEL[3:0]

RTC_CALIB

RTC_CR3

COSEL

512 Hz output

1 Hz output

ai17265

Alarm flag

RTC_ALARM

output

Calendar

Day/date/

month/year

hh:mm:ss

(12/24 format)

Alarm

ss, mm, HH/date

RTC_CR3/

OSEL[1:0]

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