AN3133 datasheet(22/43 Pages) STMICROELECTRONICS

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Useful RTC configuration examples

AN3133

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Doc ID 16956 Rev 5

3.4

Maximum and minimum RTC wakeup period

The wakeup unit clock is configured through the WUCKSEL[2:0] bits of RTC_CR1 register.

Three different configurations are possible:

●

Configuration 1

WUCKSEL[2:0] = 0xxb for short wakeup periods (see Section 3.4.1)

●

Configuration 2

WUCKSEL[2:0] = 10xb for medium wakeup periods (see Section 3.4.2)

●

Configuration 3

WUCKSEL[2:0] = 11xb for long wakeup periods (see Section 3.4.3)

3.4.1

Periodic timebase/wakeup clock configuration 1

Figure 12 shows the prescaler connection to the timebase/wakeup unit and Table 11 gives

the timebase/wakeup clock resolutions corresponding to configuration 1.

Figure 12.

Prescalers connected to the timebase/wakeup unit for configuration 1

The minimum timebase/wakeup resolution is 0.125 µs, and the maximum resolution

31.25 ms. As a result:

●

The minimum timebase/wakeup period is (0x0001 + 1) x 0.125 µs = 0.250 µs.

The timebase/wakeup timer counter WUT[15:0] cannot be set to 0x0000 with

WUCKSEL[2:0]=011b (fRTCCLK/2) because this configuration is prohibited. Refer to the

STM8L05xx, STM8L15xx, STM8L162x, STM8AL31xx and STM8AL3Lxx

microcontroller family reference manual (RM0031) for more details.

●

The maximum timebase/wakeup period is (0xFFFF+ 1) x 31.25 ms = 2048 s.

Table 11.

Timebase/wakeup unit period resolution with clock configuration 1

Clock source

Wakeup period resolution

RTCDIV[2:0] = 111b (div64)

RTCDIV[2:0] = 000b (div1)

WUCKSEL[2:0]

= 000b (div16)

WUCKSEL[2:0]

= 011b (div2)

WUCKSEL[2:0]

= 000b (div16)

WUCKSEL[2:0]

= 011b (div2)

HSE = 1 MHz

1.024 ms

0.128 ms

16 µs

2 µs

HSI = 16 MHz

0.064 ms

0.008 ms

1 µs

0.125 µs

LSE = 32 768 Hz

31.25 ms

3.90625 ms

488.2812 µs

61.0351 µs

LSI = 38 kHz

26.9473 ms

3.368421 ms

421.0526 µs

52.6315 µs

WUCKSEL[2]

ai17268_bis

16-bit wakeup

auto-reload timer

Periodic

wakeup flag

Div by 2, 4, 8, 16

Prescaler

RTCCLK

WUCKSEL[1:0]

RTC_WUTR

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