STM32L452 LSE not starting

[SLevi.1]

I have an STM32L452 rigged to use an external clock so we can keep the RTC running. Schematic attached.

X2 is a clock from SiTime, programmed to be 32.768kHz. C36 is a supercap acting as battery backup. I used STMCube to generate the startup code to feed into HAL_RCC_OscConfig. When I debug into this function, the LSE never becomes ready (RCC_BDCR_LSERDY bit always zero), and so the code returns HAL_TIMEOUT.

Do I have the correct circuit? Have I set the clock config correctly? It's the same if I run on a Nucleo board.

/** Initializes the RCC Oscillators according to the specified parameters
* in the RCC_OscInitTypeDef structure.
*/
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI|RCC_OSCILLATORTYPE_LSE;
RCC_OscInitStruct.LSEState = RCC_LSE_ON;
RCC_OscInitStruct.HSIState = RCC_HSI_ON;
RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
RCC_OscInitStruct.PLL.PLLM = 2;
RCC_OscInitStruct.PLL.PLLN = 36;
RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV7;
RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV4;
RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV4;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
{
Error_Handler();
}

[MM..1]

Is RCC PWR enabled ? Read STM32 L4 VBAT internal battery charging - STMicroelectronics Community

[SLevi.1]

Tried setting RCC pwr on, before setting PWR->CR4: Still no joy.

SET_BIT(RCC->APB1ENR1, RCC_APB1ENR1_PWREN);

PWR->CR4 |= PWR_CR4_VBRS_Msk; // use 1.5kOhms, gives 3.02v out
PWR->CR4 |= PWR_CR4_VBE_Msk; // enable output

[AScha.3]

i would do a simple test now: connect a 3v batt, cr2032 typ. , to vbat . (no super-cap)

without any problems from charging etc. you should get rtc working (as everybody -- like me is doing.)

then - if you see this working, try with only the cap again.

[Tesla DeLorean]

Wouldn't put a LED on PC15, it low current, and on the battery/low-power domain

RM suggests some sequencing is involved LSEON needs to be zero to enable LSEBYP.

You should be able to a) get the LSE out of a pin, b) measure the frequency with a TIM internally.

[SLevi.1]

I looked at the clock going into the RCC_OSC_In pin (3), and saw a 32.768 pulse. There only looked to be around 1v peak height. What sort of amplitude should it be? 

[MM..1]

I mean your config is wrong. For external active clock you in wizard ioc need config BYPASS and in config sysclock then is generated 

RCC_OscInitStruct.LSEState = RCC_LSE_BYPASS.

[SLevi.1 · Accepted solution]

Turns out that the wrong clock chip had been fitted - they'd used the low-power variant, so the swing on the clock signal was not sufficient for the processor to detect.

[AAlis.23]

Schematic is wrong???. How is it going to work if you don’t have power on Vbat? You’ve placed a capacitor, and that’s it. You’re missing a diode to 3V3; otherwise, how is the capacitor going to charge?

Maybe that series has the diode integrated into the chip or a different system to provide power from the CPU to the external circuit?

[MM..1]

No diode, but switch internal. Read my forward up...