AN3371 datasheet(16/45 Pages) STMICROELECTRONICS

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Overview of the STM32 advanced RTC

AN3371

16/45

Doc ID 018624 Rev 5

Periodic timebase/wakeup configuration for clock configuration 2

Figure 9 shows the prescaler connection to the timebase/wakeup unit and Table 9 gives the

timebase/wakeup clock resolutions corresponding to configuration 2.

Figure 9.

Prescalers connected to the wakeup unit for configurations 2 and 3

When RTCCLK= 32768 Hz, the minimum resolution for configuration 2 is 61.035 µs, and the

maximum resolution is 32s.

As a result:

●

The minimum timebase/wakeup period is (0x0000 + 1) x 61.035 µs = 122.07 µs.

●

The maximum timebase/wakeup period is (0xFFFF+ 1) x 32 s = 131072 s (more than

36 hours).

Periodic timebase/wakeup configuration for clock configuration 3

For this configuration, the resolution is the same as for configuration 2. However, the

timebase/wakeup counter downcounts starting from

0x1FFFF to 0x00000, instead of

0xFFFF to 0x0000 for configuration 2.

When RTCCLK= 32768,

●

The minimum timebase/wakeup period is:

(0x10000 + 1) x 61.035 µs = 250.06 ms

●

The maximum timebase/wakeup period is:

(0x1FFFF+ 1) x 32 s = 4194304 s (more than 48 days).

Table 9.

Timebase/wakeup unit period resolution with clock configuration 2

Clock source

Wakeup period resolution

PREDIV_A[6:0] = div128

PREDIV_S [12:0] = div8192

PREDIV_A[6:0] = div2(1)

PREDIV_S [12:0] = div1

PREDIV_A minimum value is ‘1’ on medium density devices.

LSE = 32 768 Hz

32 s

61.035 µs

MS19530V1

RTC Clock

16-bit wakeup auto-

reload timer

Asynch. 7-bit prescaler

(default=128)

Wakeup

autoreload

timer

(RTC_WUTR)

WUCKSEL[2]

Periodic

wakeup flag

Asynchronous

prescaler

(PREDIV_A)

Synchronous

prescaler

(PREDIV_S)

Synchronous 13-bit

prescaler (default=256)

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