On first reset, ROM code writes the estimated XTAL frequency into
RTC_APB_FREQ_REG (aka STORE5). If the application doesn’t specify exact
XTAL frequency (which is always the case for now), rtc_clk_init will
guess what kind of XTAL is used (26M or 40M), based on the estimated
frequency. Later, detected frequency is written into RTC_XTAL_FREQ_REG
(aka STORE4).
When the application switches clock source to PLL, APB frequency changes
and RTC_APB_FREQ_REG is updated. If the application encounters an RTC
WDT reset, RTC_APB_FREQ_REG will not be updated prior to reset. Once the
application starts up again, it will attempt to auto-detect XTAL
frequency based on RTC_APB_FREQ_REG, which now has value of 80000000.
This will fail, and rtc_clk_xtal_freq_estimate will fall back to the
default value of 26 MHz. Due to an incorrect XTAL frequency, PLL
initialization will also take incorrect path, and PLL will run at a
different frequency. Depending on the application this may cause just
garbage output on UART or a crash (if WiFi is used).
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