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AN3133

Useful RTC configuration examples

Doc ID 16956 Rev 5

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3.4.2

Periodic timebase/wake up clock configuration 2

Figure 13 shows the prescaler connection to the timebase/wakeup unit and Table 12 gives

the timebase/wakeup clock resolutions corresponding to configuration 2 for medium density

products and Table 13 for medium+ and high-density products.

Figure 13.

Prescalers connected to the wake up unit for configurations 2 and 3

The minimum resolution for configuration 2 is 0.125 µs, and the maximum resolution 8192 s.

As a result:

●

The minimum timebase/wakeup period is (0x0000 + 1) x 0.125 µs = 0.125 µs.

●

The maximum timebase/wakeup period is (0xFFFF+ 1) x 8192 s = 536870912 s (more

than 16 years).

Table 12.

Timebase/wakeup unit period resolution with clock configuration 2

Clock source

Wakeup period resolution

RTCDIV[2:0] = div64

PREDIV_A[6:0] = div128

PREDIV_S[12:0] = div8192

RTCDIV[2:0] = div1

PREDIV_A[6:0] = div2(1)

PREDIV_S[12:0] = div1

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

HSE = 1 MHz

67.10 s

2 µs

HSI = 16 MHz

4.19 s

125 ns

LSE = 32 768 Hz

2048 s

61 µs

LSI = 38 kHz

1766.02 s

52.6 µs

Table 13.

Timebase/wakeup unit period resolution with clock configuration 2

Clock source

Wakeup period resolution

RTCDIV[2:0] = div64

PREDIV_A[6:0] = div128

PREDIV_S[14:0] = 32768

RTCDIV[2:0] = div1

PREDIV_A[6:0] = div2(1)

PREDIV_S[14:0] = div1

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

HSE = 1 MHz

268.43 s

1 µs

HSI = 16 MHz

16.77 s

62.5 ns

LSE = 32 768 Hz

8192 s

30.5 µs

LSI = 38 kHz

7064.09 s

26.3 µs

HSE

RTCSEL[3:0]

HSI

LSE

LSI

WUCKSEL[2:0]

ai17268

RTC

prescaler

16-bit wakeup

auto-reload timer

Periodic

wakeup flag

RTCDIV[2:0]

Asynchronous

(PREDIV_A) and

synchronous

(PREDIV_S)

prescalers

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