Erratum 2.2.1 (Unstable LSI when it clocks RTC or CSS on LSE) indicates the following workaround:
If LSI clocks the RTC or when the LSECSSON bit is set, reset the backup domain upon each VDD power up
(when the BORRSTF flag is set). If VBAT is separate from VDD, also restore the RTC configuration, backup
registers and anti-tampering configuration.
Although this works much of the time, sometimes it does not. This seems to be related to exactly what happens on the Vdd rail: if Vdd gets low enough to set the RCC_CSR PORRSTF but not below ~0v6, then the LSI sometimes runs fast (~48.9kHz) on startup, even if the workaround (reset backup power domain) is implemented.
Unfortunately, in the real world, this Vdd scenario happens: when main power glitches, bulk capacitance in most designs will gradually drain and will drain especially slowly once Vdd falls below MOSFET switching thresholds. When power comes back, Vdd may not have gotten all the way to 0V and this issue can manifest despite the workaround.
For reference, the workaround is implemented as follows:
SET_BIT(RCC->APBENR1, RCC_APBENR1_PWREN); // enable clock to pwr control subsystem
// Start LSI oscillator used by RTC and IWDG
SET_BIT(PWR->CR1, PWR_CR1_DBP); // enable access to backup power domain
while(READ_BIT(PWR->CR1, PWR_CR1_DBP) == RESET);
// See Errata 2.2.1: Unstable LSI after main power domain reset
// if reset was due to POR or BOR, reset the backup power domain
if (RCC->CSR & RCC_CSR_PWRRSTF) {
SET_BIT(RCC->BDCR, RCC_BDCR_BDRST); // reset backup power domain
while(READ_BIT(RCC->BDCR, RCC_BDCR_BDRST) == RESET);
CLEAR_BIT(RCC->BDCR, RCC_BDCR_BDRST);
}
// Start LSI and wait until ready
SET_BIT(RCC->CSR, RCC_CSR_LSION); // turn on LSI oscillator
while (!(RCC->CSR & RCC_CSR_LSIRDY)); // wait for LSI ready
CLEAR_BIT(PWR->CR1, PWR_CR1_DBP);
Most of the time, the LSI will start and run at 32kHz, but periodically, it will still run at 48.9kHz and then only a POR will clear the condition.
For designs without an LSE, this is a critical condition. The ugly workaround is to measure the LSI with the HSI and then choose prescalers based on the actual frequency.
