Hello, and welcome to this presentation of the STM32 Real

Hello, and welcome to this presentation of the STM32 Real
Hello, and welcome to this presentation of the STM32 RealTime Clock. It covers the main features of this peripheral,
which is used to provide a very accurate time base.
The RTC peripheral features an ultra-low power calendar
with alarms, which run in all low-power modes.
Additionally, when it is clocked by the low-speed external
oscillator (LSE) at 32.768 kHz, the RTC is functional even
when the main supply is off and when the VBAT domain is
supplied by a backup battery.
The RTC embeds 128 bytes of backup registers, used to
preserve data when the main supply is off. These backup
registers can be used to store secure data, as they are
erased when a tamper event is detected on the tamper pins.
The RTC consumes 620 nA at 1.7 V, including the LSE
power consumption. The hardware calendar is provided in
binary-coded decimal (BCD) format to reduce software load,
particularly when the date and time must be displayed. The
anti-tamper circuitry includes ultra-low-power digital filtering,
avoiding false tamper detections.
The key features of the RTC are:
Seconds, minutes, hours, week day, date, month, and year,
provided in binary-coded decimal format. Sub-seconds are
provided in binary format.
Add or remove one hour on the fly to the calendar, in order
to manage daylight savings.
Two programmable alarms, which can wake up the
microprocessor from all low-power modes.
An embedded auto-reload timer, which can be used to
generate a periodic flag or interrupt with wakeup capability.
The resolution of this timer is programmable.
The calendar can be calibrated thanks to a reference clock
source which is the mains at 50 or 60 Hz.
A digital calibration circuit allowing compensation of the
crystal accuracy, with 0.95 ppm resolution.
A timestamp function to save calendar contents in timestamp
registers, depending on an external event.
128 bytes of backup registers, split into thirty-two 32-bit
backup registers. These registers are preserved in all lowpower modes and in VBAT mode, and are erased when a
tamper detection event occurs on any one of the three
tamper pins. 2 of the 3 tamper pins are available in VBAT [“V”
“BAT”] mode.
Here is the RTC block diagram. The RTC has two clock
sources: the RTC clock (RTCCLK) is used for the RTC
timer counter, and the APB clock is used for RTC register
read and write accesses. The RTC clock can use either
the high-speed external oscillator (HSE), divided by a
programmable factor from 2 to 31, the low-speed
external oscillator (LSE), or the low-speed internal
oscillator (LSI). To be functional in Stop or Standby
mode, the RTC clock must use the LSE or LSI. To be
functional in VBAT mode, the RTC clock must use the
The RTC clock is first divided by a 7-bit programmable
asynchronous prescaler, which provides the ck_apre
clock. Most of the RTC is clocked at the ck_apre
frequency, so, in order to reduce power consumption, it
is recommended to set a high asynchronous division
value. The default value is 128.
Then, a 15-bit programmable synchronous prescaler
provides the ck_spre clock. Ck_spre must be 1 Hz in
order to update the time and date BCD registers in 1second increments. The sub-second register resolution is
defined by the ck_apre frequency. By default, it is 256 Hz.
The SSR register resolution is increased by reducing the
asynchronous prescaler value. The asynchronous
prescaler can also be bypassed; in this case the subsecond register resolution is defined by the RTC clock
The RTC is initialized using a secure method.
The RTC registers are write-protected to avoid any possible
parasitic write accesses. First, the Disable Backup Domain
Protection bit must be set in the Power Controller control
register in order to enable RTC write accesses. Then, a
specific sequence must be written in the RTC write
protection register.
Initialization mode must be entered in order to change the
clock prescaler values or the calendar value.
The RTC calendar keeps running in all low-power modes, in
VBAT mode, and during reset.
Initialization of the Time and Date registers is performed
through their shadow registers, which are in the APB clock
domain. The Sub-second register cannot be initialized.
The calendar Sub-second, Time, and Date registers content
can be read in two different modes.
When the Bypass Shadow Registers control bit is cleared,
the shadow registers are read. The advantage of this mode
is that it guarantees that all three registers are consistent:
when the Time register is read, the Date register is frozen
until it is read. When the Sub-second register is read, the
Time and Date registers are frozen until the Date register is
read. The disadvantage of this mode is that when exiting
Stop, Standby mode, the software must wait for a
synchronization delay to ensure that the shadow registers
are updated with the last calendar register values. This
synchronization delay can be up to two RTC clock periods.
When the Bypass Shadow Registers control bit is set, the
actual calendar registers are read directly. The advantage of
this mode is that there is no need to wait for the
synchronization delay. The disadvantage is that the read
values can be false or not consistent due to synchronization
issues, so they must be read twice and compared with
previous read values to ensure they are correct and
This slide presents the main calendar features.
Daylight savings can be managed by software, with
automatic 1 hour addition or subtraction.
It is possible to synchronize the RTC clock to a remote clock
by adding or subtracting an offset to the Sub-second register
on the fly, with ck_apre clock resolution. This feature is
commonly used in RF applications.
A reference clock, mains at 50 or 60 Hz, can be used to
enhance long-term calendar precision. The reference clock
is automatically detected and used to update the calendar
when it is present. When the reference clock is not available,
the LSE clock is automatically used to update the calendar.
This feature is not available in Standby and VBAT modes.
A timestamp function is available: the calendar values, Subsecond, Time, and Date registers are saved in timestamp
registers when an event occurs on the timestamp I/O. A
timestamp event can also occur when a switch to VBAT
The digital calibration is used to compensate crystal inaccuracy
and accuracy variations with temperature and aging.
It consists in masking or adding a programmable number of RTC
clock cycles, fairly well distributed in a configurable window. The
calibration value can be changed on the fly, depending on
detected temperature changes for instance. A 1 Hz calibration
output signal is provided to measure the crystal frequency before
and after applying the calibration value.
The accuracy shown here is the resolution of the digital
calibration. The calibration window size is configurable, between
8, 16, and 32 seconds. For a 32 s calibration window, the
accuracy is plus or minus 0.48 ppm. The total correction range is
from -480 to 480 ppm. The accuracy resolution scales with the
calibration window size. Final accuracy in the application will
depend on the crystal parameter precision, temperature
detection precision, how often the software calibration procedure
is launched, etc.
In order to reach the precision of the calibration window, the
measurement window must be a multiple of the calibration
The RTC embeds two flexible alarms, based on comparison
with the calendar value. The alarm flags are set if the
calendar sub-seconds, seconds, minutes, hours or date
match the value programmed in the alarm registers.
The alarms events can wake up the device from all lowpower modes.
The alarms event can also be routed to the specific output
pin RTC_OUT, with configurable polarity.
The calendar alarm sub-second, seconds, minutes, hours or
date fields can be independently masked or not masked for
the comparison. When the masks are used, periodic alarms
are generated.
In addition to the calendar and alarms, another 16-bit autoreload counter can generate periodic events with wakeup
from low-power modes capability. This counter cannot be
Depending on the software configuration, the wakeup timer
clock can be the RTC clock divided by 2, 4, 8 or 16, or the
output of the synchronous prescaler. With the divided RTC
clock, the wakeup period can be from 122 microseconds to
32 seconds when the RTC clock frequency is 32.768 kHz.
The resolution is down to 61 microseconds in this case. With
the ck_spre clock, the wakeup period can be from 1 second
to 36 hours when the ck_spre clock is at 1 Hz.
The RTC embeds ultra-low-power tamper detection circuitry.
The purpose is to detect physical tampering in a secure
application, and to automatically erase sensitive data in case
of intrusion.
3 tamper pins and events are supported, and are functional
in all low-power modes. Two of these 3 pins are functional in
VBAT mode.
The detection can be edge- or level-triggered, and the active
edge or level is configurable for each event.
Backup register contents are erased when a tamper event is
detected. In addition, a tamper event prohibits any read
access to the backup SRAM.
A tamper event can generate a timestamp event.
The tamper detection circuit includes an ultra-low power
digital filter. The internal I/O pull-up can be used to detect the
anti-tamper switch state.
The I/O pull-up is applied only during the pre-charging pulse
in order to avoid any consumption if the tamper pin is at a
low level. The pre-charging pulse duration is configurable to
support different capacitance values, and can be 1, 2, 4 or 8
RTC clock cycles. The pin level is sampled at the end of the
pre-charging pulse.
A filter can be applied to the tamper pins. It consists of
detecting a given number of consecutive identical events
before issuing an interrupt to wake up the device. This
number is configurable and can be 1, 2, 4 or 8 events, at a
programmable sampling rate from 1 to 128 Hz.
This figure illustrates tamper detection using the internal pullup. The internal pull-up can be applied for 1, 2, 4 or 8 cycles.
If the switch is opened, the level is pulled-up by the resistor.
If the switch is closed, the level remains low.
The input voltage is sampled at the end of the pre-charge
Several RTC events can generate an interrupt. All interrupts
can wake the microprocessor up from all low-power modes.
The Alarm A interrupt is set when the calendar value
matches the Alarm A value.
Similarly, the Alarm B interrupt is set when the calendar
value matches the Alarm B value.
The wakeup timer interrupt is set when the wakeup auto
reload timer reaches zero.
The timestamp interrupt is set when a timestamp event
The tamper 1, 2 and 3 interrupts are set when a tamper
event is detected respectively on the RTC_TAMP1 ,
The RTC peripheral is active in all low-power modes and the
RTC interrupts cause the device to exit the low-power mode.
In Stop and Standby modes, only the LSE or LSI clocks can
be used to clock the RTC. Note that only the LSE is
functional in VBAT mode.
A bit is available in the MCU Debug interface, in order to
stop the RTC counter when the core is halted for debugging.
This is a list of peripherals related to the real-time clock.
Please refer to these peripheral trainings for more
information if needed.
- Reset and clock control
- Power control
- Extended interrupt controller
Thank you.
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