Ultra Start 23xx User Manual download pdf (Page 9)

AN3371 Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5 9/45

Figure 5. Prescalers from RTC clock source to calendar unit

Note: The length of the synchronous prescaler depends on the product. For this section, it is

represented on 13 bits.

The formula to calculate ck_spre is:

where:

● RTCCLK can be any clock source: HSE_RTC, LSE or LSI

● PREDIV_A can be 1,2,3,..., or 127

● PREDIV_S can be 0,1,2,..., or 8191

Tabl e 3 shows several ways to obtain the calendar clock (ck_spre) = 1 Hz.

Synchronous 13-bit

prescaler (default=256)

RTC

Clock

PREDIV_A

PREDIV_S

Ck_Spre

Asynchronous

prescaler

Synchronous

prescaler

Calendar unit

MS19527V1

Asynchronous 7-bit

prescaler (default = 128)

Shadow registers

(RTC_TR and

RTC_DR)

Table 3. Calendar clock equal to 1 Hz with different clock sources

RTCCLK

Clock source

Prescalers

ck_spre

PREDIV_A[6:0] PREDIV_S[12:0]

HSE_RTC = 1MHz

124

(div125)

7999

(div8000)

1 Hz

LSE = 32.768 kHz

127

(div128)

255

(div256)

1 Hz

LSI = 32 kHz

(1)

1. For STM32L1xx, LSI = 37 KHz, but LSI accuracy is not suitable for calendar application.

127

(div128)

249

(div250)

1 Hz

LSI = 37 kHz

(2)

2. For STM32F2xx and STM32F4xx, LSI = 32 KHz, but LSI accuracy is not suitable for calendar application.

124

(div125)

295

(div296)

1 Hz

ck_spre

RTCCLK

PREDIV_A 1+()PREDIV_S 1+()×

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 44 45

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