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Rev. 1.00
401 of 576
January 28, 2022
32-Bit Arm
®
Cortex
®
-M0+ MCU
HT32F54231/HT32F54241/HT32F54243/HT32F54253
18 Real T
ime Clock (RTC)
18 Real T
ime Clock (RTC)
Functional Descriptions
RTC Related Register Reset
The RTC registers can be reset by either a V
DD
Domain power on reset, POR, or by a V
DD
Domain
software reset by setting the PWCURST bit in the PWRCR register. Other reset events have no
effect to clear the RTC registers.
Reading RTC Register
The RTC control logic and the related registers are powered by the V
DD
supply voltage. Therefore,
the RTC circuitry remains operational in the power saving modes where V
CORE
is powered off. Only
the APB bus, which is located in the V
CORE
domain, is interconnected to the RTC circuits located in
the V
DD
domain using level shift circuitry.
Low Speed Clock Configuration
The default RTC clock source, CK_RTC, is derived from the LSI oscillator. The CK_RTC clock
can be derived from either the external 32,768 Hz crystal oscillator, named the LSE oscillator,
or the internal 32 kHz RC oscillator named the LSI oscillator, by setting the RTCSRC bit in the
RTCCR register. A prescaler is provided to divide the CK_RTC by a ratio ranged from 2
0
to 2
15
determined by the RPRE [3:0] field. For instance, setting the prescaler value RPRE [3:0] to 0xF will
generate an exact 1 Hz CK_SECOND clock if the CK_RTC clock frequency is equal to 32,768 Hz.
The LSE oscillator can be enabled by the LSEEN control bit in the RTCCR register respectively.
In addition, the LSE oscillator startup mode can be selected by configuring the LSESM bit in the
RTCCR register. This enables the LSE oscillator to have either a shorter startup time or a lower
power consumption, both of which are traded off depending upon specific application requirements.
An example of the startup time and the power consumption for different startup modes are shown
in the accompanying table for reference.
Table 43. LSE Startup Mode Operating Current and Startup Time
Startup Mode
LSESM Setting
in the RTCCR Register
Operating Current
Startup Time
Normal startup
0
2.0 μA
Above 500 ms
Fast startup
1
3.5 μA
Below 300 ms
@ V
DD
= 3.3 V and LSE clock = 32,768 Hz; these values are only for reference, actual values are
dependent on the specification of the external 32.768 kHz crystal.