GD32W51x User Manual
406
Note:
Software must configure the FACTOR_A=0x7F and FACTOR_S=0xFF before enabling
reference detection function (REFEN=1)
Reference detection function does not work in Standby Mode and must not be used with
coarse digital function.
16.3.11.
RTC coarse digital calibration
There are two digital methods can be chose for calibration: coarse digital calibration and
smooth digital calibration. These two types cannot be used together.
Coarse digital calibration can be used to add or mask ck_apre clock cycles at the output of
the asynchronous prescaler.
When COSD=0, 2 ck_apre cycles are added every minute for the first 2xCOSS minutes. The
effect of such configuration will make calendar to be updated sooner.
When COSD=1, 1 ck_apre cycle is removed every minute for the first 2xCOSS minutes. The
effect of such configuration will make calendar to be updated later.
Only in initialization mode can configure coarse calibration and the function starts after
clearing INITM bit. The full calibration window lasts 64 minutes. The first 2xCOSS minutes of
this 64-minute window are take adjust.
About 2PPM resolution is taken for negative calibration and about 4PPM resolution is taken
for positive calibration.
Note:
The calibration can be performed either on LXTAL or HXTAL clock. If FACTOR_A<6,
the calibration may not work correctly.
Example:
FACTOR_A and FACTOR_S are default values. LXTAL is the RTC clock source and
frequency is 32.768 KHz.
During a calibration window (64 minutes), the ck_apre clock frequency is only adjusted in the
first 2xCOSS minutes. If COSS=1, this means only the first 2 minutes of 64 minutes will make
adjustment.
The calibration step therefore has the effect of adding 512 or subtracting 256 oscillator cycles
for each calibration window (64min x 60s/min x 32768cycles/s). In another word, this is
equivalent to +4.069PPM or -2.035PPM per calibration step. Then for one month running, the
minimum calibration step is +10.5 or -5.27 seconds and the maximum calibration step is +5.45
to -2.72 minutes.
16.3.12.
RTC smooth digital calibration
RTC smooth calibration function is a way to calibrate the RTC frequency based on RTC clock
in a configurable period time.
Содержание GD32W515 Series
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