Low Speed External Crystal Oscillator - Lse; Low Speed Internal Rc Oscillator - Lsi; Clock Ready Flag; System Clock (Ck_Sys) Selection - Holtek HT32F50231 User Manual

32-Bit Arm ® Cortex ® -M0+ MCU
HT32F50231/HT32F50241
Low Speed External Crystal Oscillator – LSE
The low speed external crystal or ceramic resonator oscillator with 32.768 kHz frequency produces a
low power but highly accurate clock source for the circuits of Real-Time-Clock peripheral, Watchdog
Timer or system clock. The associated hardware configuration is shown in the following figure. The
crystal or ceramic resonator must be placed across the two LSE pins (X32KIN / X32KOUT) and the
external capacitors are necessary to make it oscillate properly. The LSE oscillator can be switched
on or off by using the LSEEN bit in the RTC Control Register (RTCCR). The LSERDY flag in the
Global Clock Status Register (GCSR) will indicate if the LSE clock is stable.

Figure 16. External Crystal, Ceramic and Resonators for LSE

Low Speed Internal RC Oscillator – LSI
The low speed internal RC oscillator with frequency of about 32 kHz produces a low power
clock source for the circuits of Real-Time-Clock peripheral, Watchdog Timer or system clock.
The LSI is also a clock source of low cost because no external component is needed to make it
oscillates. The accuracy of the frequency of the low speed internal RC oscillator LSI is shown as
the corresponding data sheet. The LSIRDY flag in the Global Clock Status Register (GCSR) will
indicate if the LSI clock is stable.

Clock Ready Flag

CKCU provides clock ready flags for HSI, HSE, LSI and LSE to confirm those clocks are stable
before using them as system clock source or other purpose. Software can check specific clock is
ready or not by polling separate clock ready status bits in GCSR register.

System Clock (CK_SYS) Selection

After the system reset occurs, the default system clock source CK_SYS will be the high speed
internal RC oscillator HSI. The CK_SYS may come from the HSI, HSE, LSI and LSE output clock
and it can be switched from one clock source to another via the System Clock Switch bits, SW, in
the Global Clock Control Register GCCR. The system will still run under the original clock until
the destination clock gets ready. The corresponding clock ready status bits in the Global Clock
Status Register GCSR will indicate whether the selected clock is ready to use or not. The CKCU
also contains the clock source status bits in the Clock Source Status Register CKST to indicate
which clock is currently used as the system clock. More details about function of clock enable are
described in below.
Rev. 1.00
X32KIN
32.768 kHz
C
L1
75 of 486
X32KOUT
C
L2
July 31, 2018