STM32G484 RTC LSE fails to ready, stuck waiting LSERDY, LSI works fine

[lzy]

Hi,

I'm using STM32G484. I need RTC to keep time after power loss, so I use external 32.768kHz LSE crystal for RTC. The program gets stuck in the waiting loop: while (READ_BIT(RCC->BDCR, RCC_BDCR_LSERDY) != 0U); It never gets LSERDY flag set.

If I switch RTC clock source to LSI, everything works normally. RTC can keep the last time value after power cycle.

Hardware info:

- 32.768kHz crystal connected to LSE_IN / LSE_OUT

-The LSE circuit uses a 32.768 kHz crystal AV25000023, with 5.6 pF load capacitors on both PC14 and PC15. The HSE is a 24 MHz crystal NX2016SA-24MHZ, which works fine. LSE fails to start and the LSERDY flag never gets set, while RTC works correctly with LSI.

-HAL library, STM32G484 - LSE is enabled in RCC BDCR register before waiting LSERDY - No debugger interference when cold power-on

Questions:

1. What possible causes make LSE fail to start on STM32G484?

2. Is there any special errata or limitation for LSE on G4 series?

3. Any recommended debug method or register check? Thanks.

[EXUE.2]

Hi, firstly, if the AV25000023 LSE crystal works after power on, you will monitor the crystal wave by oscilloscope, did you check it?

[TDK]

AV25000023

I can’t find a datasheet for this.

Some possibilities:

• Part is not soldered correctly, or otherwise miswired.
• Wrong or insufficient load capacitors.
• Bad code

The code to get LSE up and running is pretty basic. Doubt there are any issues there.

[mƎALLEm]

Hello ​@lzy ,

By doing a search on AV25000023 crystal part number, that doesn’t seem to be a 32kHz crystal but rather a 25MHz (made by TXC corporation). Are you sure?

You need to share the datasheet of that crystal.

You need first to check if that crystal is compatible with STM32G484 and the suitable drive level:

And AN2867  “Guidelines for oscillator design on STM8AF/AL/S and ST and STM32 MCUs/MPUs”

[lzy]

Thanks for your reply. I have checked the datasheet, it seems our hardware enginneer used a wrong crystal which is not 32khz so that LSE is timeout at the begginning.