ST AN2821 Application note

AN2821

Application note

Clock/calendar implementation on the STM32F10xxx microcontroller RTC

The internal RTC (real-time clock) of the 32-bit, ARM® Cortex™-M3-based medium-density STM32F10xxx microcontroller is an independent timer. It provides a set of continuously running counters, which, using suitable software, can be used to implement a clock/calendar function. The RTC and backup registers are supplied through a switch that draws power from either the VDD supply (when present) or the backup battery. Thus the RTC runs even when the main supply is switched off.

This application note gives an example of how to implement the clock/calendar functionality using the medium-density STM32F10xxx microcontroller in low-power and standard applications. The firmware example also performs leap year counting and tamper time stamping. It provides a low-power mode demonstration and a basic RTC calibration routine to compensate for crystal and temperature variations.

The STM3210B-EVAL board supports the firmware example described in this application note without any modification to its hardware.

Figure 1. STM3210B-EVAL board

April 2009

Doc ID 14949 Rev 2

1/28

www.st.com

Contents	AN2821

Contents

1	Overview of the medium-density STM32F10xxx backup domain . . . . .		6
	1.1	Main backup domain features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.2	Main RTC features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
2	Configuring the RTC registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8
3	Clock/calendar functionality features . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
	3.1	Clock/calendar basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9

3.1.1Implementing the clock function on the medium-density STM32F10xxx . 9

3.1.2Implementing calendar function on the medium-density STM32F10xxx . 9

3.1.3 Summer time correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Clock source selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Low-power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4.1 Stop mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.4.2 Standby mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.5 Tamper detection and time stamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.5.1 TAMPER-RTC pin functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5.2 Tamper timestamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5.3 Battery tamper timestamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4	Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
5	Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
6	Running the demonstrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	6.1 Clock/calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18

6.1.1 Setting the time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1.2 Setting the alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1.3 Setting the date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1.4 Summer time correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.2 Low-power demonstration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.2.1 Stop mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.2.2 Standby mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2/28	Doc ID 14949 Rev 2

AN2821	Contents

6.3 Timestamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6.3.1 Tamper events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.3.2 Battery removal and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6.4 Clock calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.5 Clock source selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

		6.5.1	50 Hz time base selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
		6.5.2	60 Hz time base selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
Appendix A		Calibration table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		23
Appendix B		Additional notes on calibration and clock accuracy . . . . . . . . . . .		25
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			27

Doc ID 14949 Rev 2

3/28

List of tables	AN2821

List of tables

Table 1. Calibration lookup table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 2. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4/28	Doc ID 14949 Rev 2

AN2821	List of figures

List of figures

Figure 1. STM3210B-EVAL board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. RTC block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. Time system flowchart(1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 4. Calendar algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 5. Leap year flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Calibration process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 7. Stop mode entry sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 8. Standby mode entry sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 9. STM3210B-EVAL board description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 10. RTC time base connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 11. Clock source selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Doc ID 14949 Rev 2

5/28

Overview of the medium-density STM32F10xxx backup domain	AN2821

1Overview of the medium-density STM32F10xxx backup domain

Figure 2 shows the RTC block diagram.

Figure 2. RTC block diagram

		HSE OSC
		LSI RC	1/128
	RTC alarm	LSE OSC/ EXT CLOCK
		RTCSel
		[1:0]
	=	RTC prescaler
		frtc
	RTC counter	RTC divider
		Backup registers
	RTC control register

Alarm	Overflow Second
IT	IT	IT

ai15437

1.1Main backup domain features

The backup domain of the medium-density STM32F10xxx:

●includes ten 16-bit backup data registers, backup control/status registers and RTC registers.

●is powered by VBAT when the VDD main power is switched off.

●is not reset due to system reset, power reset or when the microcontroller wakes up from the Standby mode.

●is protected from parasitic write access after reset (access to the backup domain is disabled after reset)

●consists of two main units:

–an APB1 interface used to interface with the APB1 bus. The backup domain registers are accessible through the APB1 bus in read/write access mode.

–an RTC core which is clocked by the RTC clock.

6/28	Doc ID 14949 Rev 2

AN2821	Overview of the medium-density STM32F10xxx backup domain

1.2Main RTC features

The RTC:

●has three clock sources:

–LSE (low-speed external) clock that can also be bypassed by an external clock

–LSI (low-speed internal) RC oscillator

–HSE (high-speed external) clock divided by 128.

●has a 20-bit programmable prescaler that can be programmed to generate the RTC time base. The prescaler load register contains the period counting value of the prescaler.

●has three interrupt sources:

–second

–overflow

–alarm (also connected to EXTI Line 17 for auto-wakeup from Stop mode)

●its clock can be output on the TAMPER-RTC pin for calibration purposes.

	Please refer to Figure 2 for an illustration of the medium-density STM32F10xxx RTC block
	diagram.
	All of the above mentioned features of the medium-density STM32F10xxx backup domain
	can be used to develop a clock/calendar application equivalent to dedicated real-time
	clocks.
Note:	Be careful when choosing the 32.768 kHz crystal for your application. It is strongly
	recommended to use a crystal with a load capacitance less than or equal to 7 pF. Never use
	a crystal with a load capacitance of 12.5 pF.

Doc ID 14949 Rev 2

7/28

Configuring the RTC registers	AN2821

2 Configuring the RTC registers

On startup, follow the steps below to configure the RTC to generate an interrupt every second:

●Enable the APB1 backup domain and power interface clocks by writing the BKPEN and PWREN bits to ‘1’ in the RCC_APB1ENR register

●Enable access to backup domain by writing the DBP bit to ‘1’ in the PWR_CR register

●Enable the LSE clock by writing the LSEON bit to ‘1’ (also write LSEBYP to “1” when the external clock has to be bypassed)

●Poll the LSERDY flag in the RCC_BDCR register until the LSE clock is ready (if the external crystal is used as the clock source).

●Select LSE as the RTC clock source by writing ‘01’ to the RTCSEL bits in the RCC_BDCR register.

●Enable the RTC clock by setting the RTCEN bit in the RCC_BDCR register

●Poll the RSF bit in the RTC_CRL register until the RTC registers are synchronized (if a 50/60 Hz external clock source is used this step may take up to a minute to complete)

●Poll the RTOFF bit in the RTC_CRL register until the last operation on the RTC registers is over

●Enable the RTC second global interrupt by setting the SECIE bit in the RTC_CRH register

●Wait for the last task to complete

●Set the RTC prescaler value using the following formula: fTRCLK = fRTCCLK/(PRL[19:0]+1), where:

–fRTCCLK is the input clock frequency

–fTRCLK is the time base generated from the prescaler block

For example, if an external 32.768 kHz (32 kHz) crystal oscillator is used, set the prescaler to 32767. If an external 50 Hz supply is used set the prescaler value to 49.

8/28	Doc ID 14949 Rev 2

AN2821	Clock/calendar functionality features

3 Clock/calendar functionality features

3.1Clock/calendar basics

A real-time clock keeps track of the time (hours, minutes, seconds) and date (day, month, year). It should also take into account leap years. A leap year is a year where an extra day is added to the calendar in order to synchronize it to the seasons. Since the tropical year is 365.242190 days long, a leap day must be added roughly once every 4 years (4 × 0.242190

= 0.968760). Thus, every four years, the month of February counts 29 days instead of 28.

3.1.1Implementing the clock function on the medium-density STM32F10xxx

The used registers are RTC registers: prescaler register, counter register and alarm register.

The medium-density STM32F10xxx RTC peripheral consists of a chain of programmable counters consisting of two main units:

●the RTC prescaler block generates the RTC time base. Depending on the clock period applied to the LSE input, this prescaler can be adjusted to generate a time base (RTC_CLK) of 1 second by writing to the prescaler load register.

●the 32-bit programmable counter can be initialized to the current system time. The system time is incremented at the RTC_CLK rate and compared with the alarm register value. When it reaches the alarm register value, an interrupt is generated.

The current system time can be inferred from the value in the counter register by following the steps shown in Figure 3 (“/” implies division, “%” implies modulus)

Figure 3. Time system flowchart(1)

Read RTC counter value (TimeVar)

Hours = TimeVar

3600

Minutes = TimeVar % 3600

60

Seconds = (TimeVar % 3600) % 60

ai15438

1. % is for modulo.

3.1.2Implementing calendar function on the medium-density STM32F10xxx

The used registers are the 16-bit backup data registers.

When the 32-bit counter register value reaches 86399, this means that one day has elapsed, and that the date has to be updated. The counter value is then reset. Whenever the system date is updated, the new date is stored into the 16-bit backup data registers so that

Doc ID 14949 Rev 2

9/28