STMicroelectronics STM8L15x real-time clock Application note

AN3133
Application note
Using the STM8L15x real-time clock (RTC)
Introduction
A real-time clock (RTC) is a computer clock that keeps track of the current time. Although
RTCs are often used in personal computers, servers and embedded systems, they are also
present in almost any electronic device that requires accurate time keeping. Microcontrollers
supporting RTC can be used for chronometer, alarm clock, watches, small electronic
agendas, and many other devices.
This application note describes the features of the real-time clock (RTC) controller
embedded in the STM8L15x microcontrollers, together with the steps required to configure
the RTC calendar, the alarm, and the periodic wakeup unit.
Some useful configurations are described to allow the user to quickly and adequately
configure the RTC for his application.
Three applicative examples are provided. They explain how to use the calendar, the alarm
and the periodic wakeup unit.
Note:
March 2010
All examples and explanations are based on the STM8L15x firmware library and refer to the
STM8L15x reference manual (RM0031).
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www.st.com
Contents
AN3133
Contents
1
Overview of the STM8L15x RTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
STM8L15x RTC calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
STM8L15x RTC alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3
STM8L15x RTC periodic wakeup unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4
RTC and low-power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5
Signals generated by RTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.6
2
3
4
2/28
1.5.1
RTC_CALIB output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5.2
RTC_ALARM output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
RTC security aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.6.1
RTC Register write protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.6.2
Enter/Exit initialization mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.6.3
Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Programming the STM8L15x RTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1
Initializing the calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2
Programming the alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3
Programming the Auto-wakeup unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Useful RTC configuration examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1
Delivering a 1-Hz signal to the calendar using different clock sources . . 15
3.2
Configuring the alarm behavior using the MSKx bits . . . . . . . . . . . . . . . . 16
3.3
Maximum and minimum RTC_CALIB output frequency . . . . . . . . . . . . . . 17
3.4
Maximum and minimum RTC wakeup period . . . . . . . . . . . . . . . . . . . . . . 17
3.4.1
Periodic timebase/wakeup clock configuration 1 . . . . . . . . . . . . . . . . . . 18
3.4.2
Periodic timebase/wake up clock configuration 2 . . . . . . . . . . . . . . . . . 19
3.4.3
Periodic timebase/wakeup clock configuration 3 . . . . . . . . . . . . . . . . . . 19
3.4.4
Summary of timebase/wakeup period extrema . . . . . . . . . . . . . . . . . . . 20
RTC firmware API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1
Function groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2
Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.1
Example 1: calendar and alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.2
Example 2: wakeup from low power mode . . . . . . . . . . . . . . . . . . . . . . 24
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Contents
4.2.3
5
Example 3: periodic event generation using the wakeup unit . . . . . . . . 25
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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List of tables
AN3133
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
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Low power modes where RTC is actor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Steps to initialize the calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Steps to configure the alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Steps to configure the Auto wake-up unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Calendar clock equal to 1Hz with different clock sources. . . . . . . . . . . . . . . . . . . . . . . . . . 16
Alarm combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
RTC_CALIB output frequency versus clock source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Timebase/wakeup unit period resolution with clock configuration 1 . . . . . . . . . . . . . . . . . . 18
Timebase/wakeup unit period resolution with clock configuration 2 . . . . . . . . . . . . . . . . . . 19
Minimum and maximum timebase/wakeup period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
RTC function groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
STM8L15x RTC calendar fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Example of calendar display on an LCD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
STM8L15x Alarm fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
RTC_CALIB clock sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Alarm flag routed to RTC_ALARM output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Periodic wake up routed to RTC_ALARM pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Prescalers from RTC clock source to calendar unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Prescalers connected to the timebase/wakeup unit for configuration 1 . . . . . . . . . . . . . . . 18
Prescalers connected to the wake up unit for configurations 2 and 3 . . . . . . . . . . . . . . . . 19
Calendar example: main program flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Calendar example: RTC alarm ISR flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Wakeup from low power mode example: main program flowchart . . . . . . . . . . . . . . . . . . . 25
Wakeup from low power mode example: RTC ISR flowchart . . . . . . . . . . . . . . . . . . . . . . . 25
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Overview of the STM8L15x RTC
1
AN3133
Overview of the STM8L15x RTC
The RTC embedded in the STM8L15x provides a full-featured calendar, an alarm, and a
periodic wakeup unit.
1.1
STM8L15x RTC calendar
A calendar keeps track of the time (hours, minutes, seconds) and date (day, week, month,
year). The STM8L15x RTC calendar offers many features to easily configure and display the
calendar data fields:
●
Calendar with seconds, minutes, hours in 12-hour or 24-hour format, day of the week
(day), day of the month (date), month, and year.
●
Calendar in BCD format
●
Automatic management of 28-, 29- (leap year), 30-, and 31-day months
●
Daylight saving time adjustment programmable by software
Figure 1.
STM8L15x RTC calendar fields
#!,%.$!2
$!4%
-ONTH
7EEKDATE
DATE 24#?$2
24#?$2
HORH
FORMAT
9EAR
24#?$2
!0-
HH
MM
SS
24#?42
24#?42 24#?42
AI
1. RCT_DRx and RTC_TRx are RTC registers.
In some microcontrollers, the calendar is a kind of software counter (usually 32-bit long)
which represents the number of seconds. Software routines convert the counter value to
hours, minutes, day of the month, day of the week, month and year. These data can be
converted to BCD format (binary coded decimal) and displayed on a standard LCD which is
particularly useful in countries where the hours are displayed in 12-hour format plus an
AM/PM indicator (see Figure 2). Conversion routines uses significant program memory
space and are CPU-time consuming which may be critical in some real-time applications.
When using the STM8L15x RTC calendar, software conversion routines are no more
needed because their functionalities are performed by hardware.
Figure 2.
Example of calendar display on an LCD
11:15:28 PM
WED NOV 18 2009
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1.2
Overview of the STM8L15x RTC
STM8L15x RTC alarm
An alarm can be generated at a given time or/and date programmed by the user.
The STM8L15x RTC provides a rich combination of alarms, and offers many features to
easily configure, and display these alarms:
●
Full programmable alarm: seconds, minutes, hours and date fields can be
independently selected or masked to provide the user a rich combinations of alarms.
●
Ability to exit the device from Active-halt mode when the alarm occurs.
●
The alarm event can be routed to a specific output pad with configurable polarity.
●
Dedicated alarm flag and Interrupt.
Figure 3.
STM8L15x Alarm fields
!,!2
!LARMDATE
$AYOFWEEK
$ATE
$AYOFWEEK
$ATE
-3+
HORH
FORMAT
!0-
!LARMTIME
HH
-3+
24#?!,2-!2
MM
SS
-3+
-3+
24#?!,2-!2 24#?!,2-!2 24#?!,2-!2
AI
1. RTC_ALRMARx are RTC registers.
2. MSKx are bits in the RTC_ALARMx registers. They allow enabling and disabling the RTC_ALARMx fields
used for alarm and calendar comparison. For more details refer to Table 6.
In some microcontrollers, the alarm consist of a register with the same length as the RTC
time counter. When the RTC time counter reaches the value programmed in the alarm
register, a flag is set to indicate that an alarm event occurred.
The STM8L15x RTC alarm can be configured by hardware to generate different types of
alarms. For more details refer to Table 6.
1.3
STM8L15x RTC periodic wakeup unit
Like many low consumption microcontrollers, the STM8L15x provides several low power
modes to reduce the power consumption.
The STM8L15x features a periodic timebase and wakeup unit that can wake up the system
when the STM8L15x operates in low power mode. This unit is a programmable
downcounting auto-reload timer. When this counter reaches zero, a flag and an interrupt (if
enabled) are generated.
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Overview of the STM8L15x RTC
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The wakeup unit features are the following:
●
Programmable downcounting auto-reload timer
1.4
●
Specific flag and interrupt capable to wake up the device from low power modes
●
Wakeup alternate function output which can be routed to RTC_ALARM output (unique
pad for both Alarm and Wakeup events) with configurable polarity.
●
A full set of prescalers to select the desired waiting period.
RTC and low-power consumption
The STM8L15xx RTC is designed to minimize the power consumption. The prescalers used
for the calendar are divided in 2: synchronous and asynchronous.
Increasing the value of the asynchronous prescaler allows to reduce the power
consumption.
The RTC continues working in reset mode and its registers are not reset except by Poweron reset. RTC registers values are not lost after a reset and the calendar keeps the correct
time and date.
After a system reset or a power-on reset the STM8L15x operates in Run mode. In addition,
the device supports five low power modes to achieve the best compromise between low
power consumption, short startup time and available wakeup sources.
The RTC peripheral can be active in the following low power modes:
●
Wait
●
Low power run
●
Low power wait
●
Active-halt
The RTC cannot wake up the device from Low power run and Low power wait mode since
there is no associated event.
The RTC remains active in Low power run, Low power wait and Active-halt mode only if the
clock source is LSI or LSE. If the RTC clock is HSI or HSE, and the HALT instruction is
executed, the RTC is stopped (since the HSI and HSE clocks are stopped in Halt mode) and
cannot wake up the device.
Refer to the low power modes section of the STM8L15x reference manual for more details
about low power modes.
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Overview of the STM8L15x RTC
Table 1.
Low power modes where RTC is actor
Mode
Entry
Oscillator
CPU
Peripherals
status
wake up
Wait mode
WFI/WFE(1)
ON
ON
ON
Internal or external
event, reset
Low power run
mode
Software
sequence
LSI or LSE clock
ON
ON except RTC
and 1 other
peripheral(2)
Software sequence,
reset
Low power wait
mode
Software
sequence
+ WFE
LSI or LSE clock
OFF
ON except RTC
and 1 other
peripheral(2)
Internal or external
event, reset
Active-halt
mode
HALT(3)
Off except LSI
or LSE clock
OFF
OFF except RTC
and possibly LCD
External interrupts,
RTC interrupt, reset
1. There is no event associated to the RTC. As a consequence, the interrupt is served both in WFE and WFI
modes.
2. Only the peripherals which running with the LSI or LSE clock can be enabled.
3. Before executing the HALT instruction, the application must clear all pending peripheral interrupts by
clearing the corresponding interrupt bit in the peripheral configuration register. Otherwise, the HALT
instruction is not executed and program execution continues.
1.5
Signals generated by RTC
The RTC peripheral has 2 outputs:
1.5.1
●
RTC_CALIB: it can be used to generate an external clock.
●
RTC_ALARM: unique output resulting from the multiplexing of the RTC alarm and
wakeup events.
RTC_CALIB output
The RTC_CALIB output is used to generate a variable-frequency square signal. Depending
on the user application, this signal can play the role of a reference clock to calibrate an
external device, or be connected to a buzzer to generate a sound.
The signal frequency is configured through the 6 LSB bits (PREDIV_A [5:0]) of the
Asynchronous prescaler register, RTC_APRER. PREDIV_A[5] must be set to 1 to enable
the RTC_CALIB output signal generation. If PREDIV_A[5] is reset, no signal is output on
RTC_CALIB.
Note:
The RTC_CALIB output is available on PD3 for 28-pin devices and on PD6 for 32- and 48pin devices.
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Overview of the STM8L15x RTC
Figure 4.
AN3133
RTC_CALIB clock sources
24#3%,;=
(3%
(3)
,3%
,3)
02%$)6?!;=
24#$)6;=
24##,+
#,+24#
PRESCALER
02%6)$?!;=
!SYNCHRONOUS
PRESCALER
24#?#!,)"
AI
1. RTCDIV[2:0] and RTCSEL[3:0] are bits of the CLK_CRTCR register.
1.5.2
RTC_ALARM output
The RTC_ALARM output can either be connected to the RTC alarm unit to trigger an
external action, or routed to the RTC wakeup unit to wake up an external device.
Note:
The RTC_ALARM pin is on PB3 for 28-pin devices, on PD7 for 32- and 48-pin devices.
RTC_ALARM output connected to the RTC alarm unit
When the calendar reaches the value pre-programmed in the RTC_ALRMARx registers, the
alarm flag (ALRAF bit in RTC_ISR2 register) is set to 1. If the alarm flag is routed to the
RTC_ALARM output (OSEL[1:0] bits set to 01 in RTC_CR3), this pin is set to VDD or to
GND, depending on the polarity selected. The output toggles when the alarm flag is cleared.
Figure 5.
Alarm flag routed to RTC_ALARM output
!LARM
SSMM((DATE
#ALENDAR !LARMFLAG
$AYDATE HHMMSS
MONTHYEAR FORMAT
24#?!,!2OUTPUT
24#?#2
/3%,;=
AI
RTC_ALARM output connected to the wakeup unit
When the wakeup downcounting timer reaches 0, the wakeup flag is set to 1. If this flag is
selected as source for the RTC_ALARM output (OSEL[1:0] bits set to 11 in RTC_CR3
register), the output will be set depending to the polarity selected and will remain set as long
as the flag is not cleared.
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Overview of the STM8L15x RTC
Figure 6.
Periodic wake up routed to RTC_ALARM pinout
24#?#2/3%,;=
7AKEUPUNIT
!LARMFLAG
BITAUTORELOAD
TIMER
24#?!,!2OUTPUT
0ERIODICWAKEUPFLAG
AI
1.6
RTC security aspects
1.6.1
RTC Register write protection
To protect RTC registers against possible parasitic write accesses after reset, the RTC
registers are automatically locked. they must be unlocked to update the current calendar
time and date.
Writing to the RTC registers is enabled by programming a key into the Write protection
register, RTC_WPR.
The following steps are required to unlock the RTC register write protection:
1.
Write 0xCA into the RTC_WPR register.
2.
Write 0x53 into the RTC_WPR register.
Writing a wrong key automatically reactivates the RTC register write access protection.
1.6.2
Enter/Exit initialization mode
The RTC can operate in two modes:
●
Initialization mode where the counters are stopped.
●
Free-running mode where the counters are running.
The calendar cannot be updated while the counters are running. The RTC must
consequently be switched to initialization mode before updating the time and date.
When operating in this mode, the counters are stopped. They start counting from the new
value when the RTC enters free-running mode.
The INIT bit of the RTC_ISR1 register allows to switch from one mode to another, while the
INITF bit can be used to check the RTC current mode.
The RTC must be in initialization mode to program the time and date registers (RTC_TRx
and RTC_DRx) and the prescaler registers (RTC_SPRERx and RTC_APRER). This is done
by setting the INIT bit and waiting until RTC_ISR1_INITF flag is set.
To return to the free-running mode and restart counting, the RTC must exit initialization
mode. This is done by resetting the INIT bit.
Only a power-on reset can reset the calendar. A system reset does not affected it but resets
the shadow registers which are read by the application. They will be updated again when
the RSF bit is set. After a system reset, the application can check the INITS status flag in
RTC_ISR1 to verify if the calendar is already initialized. This flag is reset when the calendar
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Overview of the STM8L15x RTC
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year field is set to 0x00 (power-on reset value), meaning that the calendar must be
initialized.
1.6.3
Synchronization
When the application reads the calendar, it actually accesses shadow registers which
contain a copy of the real calendar time and date clocked by the RTCCLK clock. To make
sure that the shadow registers are updated with the current calendar value, the application
must check that the RSF bit is set in the RTC_ISR1 register. This bit is set by hardware each
time the calendar time and date shadow registers are updated, that is when the RTCCLK
clock is synchronized with the system clock SYSCLK. The application software must clear
the RSF bit after reading the calendar registers.
When the system is woken up from Active-halt mode (SYSCLK was off), the application
must first clear the RSF bit, and then wait until it is set again before reading the calendar
registers. This ensures that the value read by the application is the current calendar value,
and not the value before entering Active-halt mode.
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Programming the STM8L15x RTC
2
Programming the STM8L15x RTC
2.1
Initializing the calendar
The steps described in Table 2 are required to configure correctly the calendar time and
date.
Table 2.
Step
What to do
1
Enter initialization phase mode
Set INIT bit to 1 in
RTC_ISR1 register
2
Wait for the confirmation of the
initialization mode (clock
synchronization).
Poll INITF bit of in
It takes around 2 RTCCLK
RTC_ISR1 until it is set clock cycles.
3
Program the 3 prescaler
registers if needed.
RTC_APRER and
RTC_SPRERx
4
Load time and date values in
the shadow registers
Set RTC_TRx and
RTC_DRx registers
5
Configure the time format (12h
or 24h)
Set FMT bit in
RTC_CR1 register
Exit initialization mode.
Clear the INIT bit in
RTC_ISR1 register
6
2.2
Steps to initialize the calendar
How to do it
Comments
The calendar counter is
stopped to allow update.
The current calendar counter is
then automatically loaded and
the counting restarts after 4
RTCCLK clock cycles.
Programming the alarm
The steps described in Table 3 are required to configure the alarm.
Table 3.
Step
Steps to configure the alarm
What to do
How to do it
Comments
Clear ALRAE in
RTC_CR2.
1
Disable the alarm
2
Check that the RTC_ALRMARx Poll ALRAWF until it is
registers can be accessed.
set in RTC_ISR1.
3
Configure the alarm.
Configure
RTC_ALRMARx
registers.
4
Enable the alarm again
Set ALRAE in
RTC_CR2.
It takes around 2 RTCCLK
clock cycles (clock
synchronization).
The alarm hour format be the
same as the RTC Calendar in
RTC_ALARM3(1).
1. As an example, if the alarm is configured to occurs at 3:00:00 PM, the alarm will not occur even if the
calendar time is 15:00:00, because the RTC calendar is 24-hour format and the alarm is 12-hour format.
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Programming the STM8L15x RTC
2.3
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Programming the Auto-wakeup unit
The steps described in Table 4 are required to configure the Auto-wakeup unit.
Table 4.
Step
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Steps to configure the Auto wake-up unit
What to do
How to do it
1
Disable the wakeup timer.
Clear WUTE in
RTC_CR2.
2
Make sure the access to
Wakeup auto-reload counter
and to the WUCKSEL[2:0] bits
is allowed.
Poll WUTWF until it is
set in RTC_ISR1.
3
Program the value into the
wakeup timer.
Set RTC_WUTRL and
RTC_WUTRH.
4
Program
Select the desired clock source. WUCKSEL[2:0] bits in
RTC_CR1.
5
Enable the wakeup timer again.
Comments
It takes around 2 RTCCLK clock
cycles.
see Section 3.4: Maximum and
minimum RTC wakeup period
Set WUTE in RTC_CR2 The wakeup timer restarts
register.
down-counting.
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3
Useful RTC configuration examples
Useful RTC configuration examples
This section explains how to configure the STM8L15x 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
whatever the clock source.
Figure 7.
Prescalers from RTC clock source to calendar unit
24#3%,;=
(3%
(3)
,3%
,3)
24#$)6;=
24#
0RESCALER
02%$)6?!;=
!SYNCHRONOUS
0RESCALER
#ALENDAR
UNIT
02%$)6?3;=
3YNCHRONOUS PRESCALER
CK?SPRE
AI
1. RTCDIV[2:0] and RTCSEL[3:0] are bits of the CLK_CRTCR register.
The formula to calculate ck_spre is:
CLKSrc
ck_spre = ------------------------------------------------------------------------------------------------------------------------------------RTCDIV [ 2:0 ]
× ( PREVID_A + 1 ) × ( PREVID_S + 1 )
2
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
Table 5 shows several possibilities to obtain ck_spre = 1 Hz.
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Useful RTC configuration examples
Table 5.
AN3133
Calendar clock equal to 1Hz with different clock sources
Prescalers
Clock source
3.2
ck_spre
RTCDIV[2:0]
PREDIV_A[6:0]
PREDIV_S[12:0]
HSE = 1 MHz
6
(div64)
124
(div125)
124
(div125)
1 Hz
HSI = 16 MHz
6
(div64)
124
(div125)
1999
(div2000)
1 Hz
LSE = 32.768 kHz
0
(div1)
127
(div128)
255
(div256)
1 Hz
LSI = 38 kHz
0
(div1)
124
(div125)
303
(div304)
1 Hz
Configuring the alarm behavior using the MSKx bits
The alarm behavior can be configured through the MSKx bits (x=1, 2, 3, 4) bits of the
RTC_ALRMARx registers.
Table 6 shows all the possible alarm settings. As an example, to configure the alarm time to
23:15:07 on Monday, MSKx bits must be set to 0000b.
Table 6.
Alarm combination
MSK4 MSK3 MSK2
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MSK1
Alarm behavior
0
0
0
0
All fields are used in alarm comparison:
Alarm occurs at 23:15:07, each Monday.
0
0
0
1
Seconds don’t care in alarm comparison
The alarm occurs every second of 23:15, each Monday.
0
0
1
0
Minutes don’t care in alarm comparison
The alarm occurs at the 7th second of every minute of 23:XX,
each Monday.
0
0
1
1
Minutes and seconds don’t care in alarm comparison
0
1
0
0
Hours don’t care in alarm comparison
0
1
0
1
Hours and seconds don’t care in alarm comparison
0
1
1
0
Hours and minutes don’t care in alarm comparison
0
1
1
1
Hours, minutes and seconds don’t care in alarm
comparison:
The alarm is set every second, each Monday, during the whole
day.
1
0
0
0
Week day (or date, if selected) don’t care in alarm
comparison:
Alarm occurs all days at 23:15:07.
1
0
0
1
Week day and seconds don’t care in alarm comparison
1
0
1
0
Week day and minutes don’t care in alarm comparison
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Useful RTC configuration examples
Table 6.
Alarm combination (continued)
MSK4 MSK3 MSK2
3.3
MSK1
Alarm behavior
1
0
1
1
Week day, minutes and seconds don’t care in alarm
comparison
1
1
0
0
Week day and Hours don’t care in alarm comparison
1
1
0
1
Week day, Hours and seconds don’t care in alarm
comparison
1
1
1
0
Week day, Hours and minutes don’t care in alarm
comparison
1
1
1
1
Alarm occurs continuously what ever is the calendar time
Maximum and minimum RTC_CALIB output frequency
The RTC can output the RTCCLK clock divided by a 6-bit asynchronous prescaler. The
divider factor is configured through PREDIV_A[5:0] to the RTC_APRER register.
RTC_CALIB maximum and minimum frequencies are 484.85 kHz and 8 Hz, respectively.
Table 7.
RTC_CALIB output frequency versus clock source
RTC_CALIB frequency
Clock source
Maximum
Minimum
(RTCDIV[2:0] = 111b and
(RTCDIV[2:0] = 000b and
PREDIV_A[5:0] = 111 111b)
PREDIV_A[5:0] = 100 000b(1))
HSE = 1 MHz
244.141 Hz
30,303.03 Hz
HSI = 16 MHz
3.906 kHz
484,848.5 Hz
LSE = 32 768 Hz
8 Hz
992.97 Hz
LSI = 38 kHz
9.277 Hz
1,151.515 Hz
1. PREDIV_A[5] must be set to 1 to enable the RTC_CALIB output signal generation. If PREDIV_A[5] bit is
reset, no signal is output on RTC_CALIB.
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)
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3.4.1
AN3133
Periodic timebase/wakeup clock configuration 1
Figure 8 shows the prescaler connection to the timebase/wakeup unit and Table 8 gives the
timebase/wakeup clock resolutions corresponding to configuration 1.
Figure 8.
Prescalers connected to the timebase/wakeup unit for configuration 1
24#3%,;=
(3%
(3)
,3%
,3)
24#$)6;=
24#
PRESCALER
75#+3%,;=
!SYNCHRONOUS
PRESCALER
BITWAKEUP
AUTORELOADTIMER
0ERIODIC
WAKEUPFLAG
AI
Table 8.
Timebase/wakeup unit period resolution with clock configuration 1
Wakeup period resolution
Clock source
RTCDIV[2:0] = 111b (div64)
RTCDIV[2:0] = 000b (div1)
WUCKSEL[2:0]
WUCKSEL[2:0]
WUCKSEL[2:0]
WUCKSEL[2:0]
= 000b (div16)
= 011b (div2)
= 000b (div16)
= 011b (div2)
HSE = 1 MHz
1.024 ms
0.128 ms
16 µs
2 µs
HSI = 16 MHz
0.064 ms
0.008 ms
1 µs
0.125 µs
LSE = 32 768 Hz
31.25 ms
3.90625 ms
488.2812 µs
61.0351 µs
LSI = 38 kHz
26.9473 ms
3.368421 ms
421.0526 µs
52.6315 µs
The minimum timebase/wakeup resolution is 0.125 µs, and the maximum resolution
31.25 ms. As a result:
●
The minimum timebase/wakeup period is (0x0001 + 1) x 0.125 µs = 0.250 µs.
The timebase/wakeup timer counter WUT[15:0] cannot be set to 0x0000 with
WUCKSEL[2:0]=011b (fRTCCLK/2) because this configuration is forbidden. Refer to the
STM8L15x reference manual for more details.
●
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The maximum timebase/wakeup period is (0xFFFF+ 1) x 31.25 ms = 2048 s.
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3.4.2
Useful RTC configuration examples
Periodic timebase/wake up clock configuration 2
Figure 9 shows the prescaler connection to the timebase/wakeup unit and Table 9 gives the
timebase/wakeup clock resolutions corresponding to configuration 2.
Figure 9.
Prescalers connected to the wake up unit for configurations 2 and 3
24#3%,;=
(3%
(3)
,3%
,3)
75#+3%,;=
24#$)6;=
24#
PRESCALER
!SYNCHRONOUS
PRESCALER
BITWAKEUP
AUTORELOADTIMER
0ERIODIC
WAKEUPFLAG
AI
Table 9.
Timebase/wakeup unit period resolution with clock configuration 2
Wakeup period resolution
Clock source
RTCDIV[2:0] = div64
RTCDIV[2:0] = div1
RTCDIV_A[6:0] = div128
RTCDIV_A[6:0] = div2(1)
RTCDIV_S[12:0] = div8192
RTCDIV_S[12:0] = div1
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
1. PREDIV_A minimum value is 1.
The minimum resolution for configuration 2 is 0.125 µs, and the maximum resolution 2048 s.
As a result:
3.4.3
●
The minimum timebase/wakeup period is (0x0000 + 1) x 0.125 µs = 0.125 µs.
●
The maximum timebase/wakeup period is (0xFFFF+ 1) x 2048 s = 134217728 s (more
than 4 years).
Periodic timebase/wakeup clock configuration 3
For this configuration, the resolution is the same as for configuration 2. However the
timebase/wakeup counter downcounts starting from 0x1FFFF to 0x00000, instead of
0xFFFF to 0x0000 for configuration 2.
As a result:
●
The minimum timebase/wakeup period is (0x10000 + 1) x 0.125 µs = 8.192125 ms.
●
The maximum timebase/wakeup period is (0x1FFFF+ 1) x 2048 s = 268435456 s
(more than 8 years and a half)
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3.4.4
AN3133
Summary of timebase/wakeup period extrema
The minimum and maximum period values, according on the configuration, are listed in the
following table.
Table 10.
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Minimum and maximum timebase/wakeup period
Configuration
Minimum period
Maximum period
1
0.250 µs
2048 s
2
0.125 µs
More than 4 years
3
8.192125 ms
More than 8 years and a half
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RTC firmware API
4
RTC firmware API
4.1
Function groups
The STM8L15x RTC driver can be divided into 8 function groups related to the
functionalities embedded in the RTC peripheral.
●
RTC initialization and configuration functions
●
RTC time and date functions
●
RTC alarm functions
●
RTC wakeup functions
●
RTC daylight saving functions
●
RTC output pin configuration function
●
RTC calibration output pin function
●
RTC flags and interrupt functions
Table 11.
RTC function groups
Group
Function name
1
2
Function Description
RTC_DeInit
Deinitialize the RTC registers to their default reset values.
RTC_Init
Initialize the RTC registers according to the specified
parameters in RTC_InitStruct <Hour format,
Asynchronous predivisor, Asynchronous predivisor>.
RTC_StructInit
Fill each RTC_InitStruct member with its default value.
RTC_EnterInitMode
Enter the RTC Initialization mode.
RTC_ExitInitMode
Exit the RTC Initialization mode.
RTC_WriteProtectionCmd
Enable or disable the RTC registers write protection.
RTC_WaitForSynchro
Wait until the RTC Time and Date registers (RTC_TRx
and RTC_DRx) are synchronized.
RTC_RatioCmd
Configure the RTC ratio.
RTC_SetTime
Set the RTC current time < RTC hours, RTC minutes,
RTC seconds, RTC 12-hour clock period (AM/PM)>.
RTC_SetDate
Set the RTC current date. < Calendar weekday, Calendar
Month, Calendar date, Calendar year>.
RTC_GetTime
Get the RTC current time.
RTC_GetDate
Get the RTC current date.
RTC_SetAlarm
Set the RTC alarm configuration. < Alarm time fields,
Alarm masks, Alarm date/Weekday selection, Alarm
Date/Weekday value>.
RTC_GetAlarm
Get the RTC alarm configuration.
RTC_AlarmCmd
Enable or disable the RTC alarm.
3
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RTC firmware API
Table 11.
Group
AN3133
RTC function groups (continued)
Function name
Function Description
RTC_WakeUpClockConfig
Configure the RTC wakeup clock source.
RTC_SetWakeUpCounter
Set the RTC Wakeup counters.
RTC_GetWakeUpCounter
Return the RTC Wakeup timer counter value.
RTC_WakeUpCmd
Enable or Disable the RTC Wakeup timer.
RTC_DayLightSavingConfig
Add or subtract one hour from the current time depending
on the daylight saving parameter.
RTC_GetStoreOperation
Return the daylight saving stored operation.
6
RTC_OutputConfig
Configure the RTC output for the output pin.
7
RTC_CalibOutputCmd
Enable or disable connection of the RTCCLK/
PREDIV_A[6:0] clock to be output through the relative pin.
RTC_ITConfig
Enable or disable the specified RTC interrupts.
RTC_GetFlagStatus
Check whether the specified RTC flag is set or not.
RTC_ClearFlag
Clear the RTC pending flags.
RTC_GetITStatus
Check whether the specified RTC interrupt has occurred
or not.
RTC_ClearITPendingBit
Clear the RTC interrupt pending bits.
4
5
8
4.2
Application examples
The STM8L15x RTC firmware is provided with a set of examples to quickly become familiar
with the firmware library.
The present section provides three examples:
The first one shows how to configure and display calendar and alarm settings.
●
The second example shows how to configure wakeup unit in low power mode.
●
●
4.2.1
The third example shows how to generate wakeup events at a frequency higher than
1 Hz.
Example 1: calendar and alarm
This example provides a short description of how to use the RTC peripheral calendar
features: seconds, minutes, hours (12 or 24 format), day, date, month, and year.
This example is delivered within the STML8L15x firmware library available from
http://wwwst.com/stm8l (Documents and files for STM8L family). It is located at
STM8l15x_StdPeriph_Lib.zip\Project\STM8L15x_StdPeriph_Examples\RTC\RTC_Calenda\.
This example runs on the STM8L1526-EVAL. It allows configuring the RTC calendar and
alarm and displaying their values in real-time using the LCD and joystick.
After power-up, the default date and time are displayed on the LCD. The user can then
modify the date, time and alarm using the joystick buttons.
When an alarm occurs, a message is displayed on the LCD, and the LEDs toggle for a
second.
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RTC firmware API
The flowcharts of this example are presented in Figure 10 and Figure 11.
Figure 10. Calendar example: main program flowchart
34!24 -AIN0ROGRAM
%NAB LE 24#CLOCK , 3%
)NITTHE#ALENDARANDTHE
ALARMANDENABLETHE!LARM
INTERRUPT
)NIT THE ,#$ AND CLEAR IT
#ONFIG)/SFOR,EDS+EYAND
*OYSTICK"UTTONSAND
%NABLE%84))NTERRUPTS
+EY,EFT?*OY2IGHT?*OY
$ISABLEINTERRUPTS
$ISPLAY4IMEAND$ATEON
THE ,#$
%NABLE)NTERRUPTS
&ALSE
!LARM ?/CCURRED 4RUE $ISPLAY4IME!LARMMESSAGE
ONTHE,#$AND4OGGLE,EDS
!LARM /CCU RR ED & ALSE
AI
1. These steps are added to have a secure display on the LCD (which uses SPI connections).
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RTC firmware API
AN3133
Figure 11. Calendar example: RTC alarm ISR flowchart
24#)NTERRUPT2OUTINE
!LARM /CCURRED 4RUE
#LEAR!LARMINTERRUPTPENDINGBIT
2ETURNTOTHE-AINPROGRAM
AI
4.2.2
Example 2: wakeup from low power mode
This example explains how to use the STM8L15x LCD embedded controller to drive the
LCD glass mounted on STM8L1526-EVAL board and how to exit the MCU from Active-halt
mode using the wakeup unit.
This example is delivered within the STML8L15x firmware library available from
http://wwwst.com/stm8l (Documents and files for STM8L family). It is located at
STM8l15x_StdPeriph_Lib.zip\Project\STM8L15x_StdPeriph_Examples\LCD\
LCD_SegmentsDrive\.
This example performs the following actions:
1.
The first step consists in displaying in scrolling mode an ‘STM8L’ string on LCD display.
2.
The ‘LP MODE’ string is then displayed, and the MCU enters Active-halt mode.
3.
After 20 seconds the MCU is woken up from Active-halt mode by the RTC wakeup
event and continue processing.
4.
These steps are executed in a infinite loop.
The flowcharts of this example are presented in Figure 12 and Figure 13.
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RTC firmware API
Figure 12. Wakeup from low power mode example: main program flowchart
34!24-AIN0ROGRAM )NIT ,#$ 'LASS
%NAB LE24#CLOCK
)NIT24#WAKEUPUNITCLOCK
COUNTERTICKSECOND
%NABLE7AKEUPUNITINTERRUPT
$ISPLAYTHE @34-, STRINGON
,#$GLASSINSCROLLINGMODE
%NABLETHE7AKEUPUNIT
3ET24#WAKEUPCOUNTERTO
TICKSS
$ISPLAY @,0-/$%STRINGON,#$
GLASSAND%NTER!CTIVEHALTMODE
$ISABLETHEAKEUPUNIT
AI
Figure 13. Wakeup from low power mode example: RTC ISR flowchart
24#)NTERRUPT2OUTINE
#LEARWAKEUP
INTERRUPTPENDINGBIT
2ETURNTOTHE-AINPROGRAM AI
4.2.3
Example 3: periodic event generation using the wakeup unit
This example explains how to configure the RTC periodic wakeup unit event to toggle LEDs
each 500 ms.
The LSE clock is the RTCCLK source (default RTC clock) and the wakeup timer clock
selection is configured to RTCCLK/16 (WUCKSEL[2:0]= 000b) to obtains a wakeup period
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RTC firmware API
AN3133
resolution of 488.28125 µs. The wakeup unit 16-bit timer downcounter is loaded with 1023
to generate a periodic event every 500 ms (see equation below).
Periodic event = (Timer_DownCounter +1) x Timer_resolution = 1024 step = 500 ms
The code for this example is part of the present application note firmware package. It is
available under STM8L15x_AN3133_FW_V1.0.0\Project\RTC_PeriodicWakeup500ms.
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5
Revision history
Revision history
Table 12.
Document revision history
Date
Revision
Changes
02-Feb-2010
1
Initial release
08-Mar-2010
2
Updated PREDIV_A[5:0] for maximum RTC_CALIB frequency in
Table 7: RTC_CALIB output frequency versus clock source.
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AN3133
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