AN3133

Useful RTC configuration examples

Doc ID 16956 Rev 5

19/43

3

Useful RTC configuration examples

This section explains how to configure the RTC and provides examples of configurations.

All the values provided in this section correspond to an HSE clock frequency of 1 MHz.

However the HSE frequency can be up to 16 MHz.

3.1

Delivering a 1-Hz signal to the calendar using different clock

sources

The RTC features several prescalers that allow delivering a 1-Hz clock to the calendar unit,

regardless of the clock source.

Figure 11.

Prescalers from RTC clock source to calendar unit

1.

RTCDIV[2:0] and RTCSEL[3:0] are bits of the CLK_CRTCR register.

The formula to calculate ck_spre is:

ck_spre

CLKSrc

2

RTCDIV 2:0



PREDIV_A

1

+





PREDIV_S

1

+





--------------------------------------------------------------------------------------------------------------------------------------

=

where:

CLKSrc can be any clock source: HSE, HSI, LSE or LSI

RTCDIV[2:0] can be 0,1,2,..., or 6

PREDIV_A can be 1,2,3,..., or 127

PREDIV_S can be 0,1,2,..., 8191 (for medium-density products)

PREDIV_S can be 0,1,2,..., 32767 (for medium+ and high-density products)

Table 7 shows several possibilities to obtain ck_spre = 1 Hz.

Asynchronous

Prescaler

HSE

Synchronous

Prescaler

Calendar

unit

RTCSEL[3:0]

HSI

LSE

LSI

RTC

Prescaler

PREDIV_S [14:0]

ai17267

RTCDIV [2:0]

PREDIV_A [6:0]

ck_spre