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Konkalat
Konkalat
Visitor II
February 20, 2023
Question

STM32F446 MCU LSE Ready Issue

  • February 20, 2023
  • 1 reply
  • 838 views

Hello, I used STM32F446 MCU. I'm disconnecting the external RTC crystal. Then I turn the device off and on, but the LSERDY bit does not become zero. What could be the reason for this? Thanks.

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1 reply

waclawek.jan
waclawek.jan
Super User
February 20, 2023

LSERDY is set when LSE oscillations are detected, but cleared only when LSE is switched off (by clearing LSEON ). And, given the description in RM (After the LSEON bit is cleared, LSERDY goes low after 6 external low-speed oscillator clock cycles.) I am not sure it LSERDY will be cleared at all if the oscillator is not working when LSEON is cleared, so it may require backup-domain reset to be cleared in that case.

In other words, LSERDY is not the best indicator LSE is running. For that purpose you may want to compare LSE to other clock, e.g. using the LSE->TIM5 interconnection (see TIM5_OR).

Some newer STM32 families have CSS on LSE (LSECSS, see e.g. 'L4), but 'F4 don't.

JW

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