AN3133 datasheet(21/43 Pages) STMICROELECTRONICS

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AN3133

Useful RTC configuration examples

Doc ID 16956 Rev 5

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3.3

Maximum and minimum RTC_CALIB output frequency

On medium density devices, or when COSEL = 0, the RTC can output the RTCCLK clock

divided by a 6-bit asynchronous prescaler. The divider factor is configured through bits

PREDIV_A[5:0] of the RTC_APRER register.

RTC_CALIB maximum and minimum frequencies are 484.85 kHz and 8 Hz, respectively.

When COSEL = 1, the RTC output frequency is the ck_spre frequency.

0x0

Week day, Hours and seconds don’t care in alarm comparison

0x0

Week day, Hours and minutes don’t care in alarm comparison

0x0

Alarm occurs every second

Table 8.

Alarm combination (continued)

MASKSS

[3:0]

MSK4

MSK3

MSK2

MSK1

Alarm behavior

Table 9.

Mask configurations for setting an alarm every 125 ms (for RTCCLK = 32.768kHz)

Prescaler

Configuration

MSKSS[3:0]

MSK4

MSK3

MSK2

MSK1

Comment

PREDIV_A = 0x7F

PREDIV_S = 0xFF

0x5

Alarm occurs every 25 * 128/32768 s

PREDIV_A = 0x0

PREDIV_S = 0x7FFF

0xC

Alarm occurs every 212 * 1/32768 s

Table 10.

RTC_CALIB output frequency versus clock source

Clock source

RTC_CALIB frequency

Minimum

(RTCDIV[2:0] = 111b and

PREDIV_A[5:0] = 111 111b)

Maximum

(RTCDIV[2:0] = 000b and

PREDIV_A[5:0] = 100 000b(1))

PREDIV_A[5] must be set to ‘1’ to enable the RTC_CALIB output signal generation. If PREDIV_A[5] bit is

reset, no signal is output on RTC_CALIB.

HSE = 1 MHz

244.141 Hz

30,303.030 Hz

HSI = 16 MHz

3.906 kHz

484,848.500 Hz

LSE = 32 768 Hz

8.000 Hz

992.970 Hz

LSI = 38 kHz

9.277 Hz

1,151.515 Hz

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