RTC (LSE) Running very slow

[gkg]

We are using STM32G030's internal RTC with LSE. The bypass capacitors we have used as shown in the schematic is 22pf (assuming 1.5pf stray capacitance). The crystal we used has Load Capacitance is 12.5pf. This resulted in the RTC being slow, 1 min per day.

We also tried with 17pf & 10pf the clock was still slow.

When we remove the load capacitors the RTC becomes fast.

So we used HAL_RTCEx_SetSmoothCalib function, which improved the results (2 seconds slow) when we tested at our lab. But when we send it to the field the RTC time again becomes slower by around 20sec per day.

Anything else can we do to improve the results?

[waclawek.jan]

I would try to remove FB, and connect the copper area from using a wire directly to the VSS pin on the STM32 (is it pin 5?) or to some via or other ground connection which is very close to that pin.

JW

[STOne-32]

Dear @gkg ,

is that possible to know your programmer software drive level of the LSE , I do recommmend to use the High drive mode and also to check using another crystal with CL=12,5pF not the same brand . I see on your PCB the size is not very important and the MCU is smaller than the Crystal and caps , so it should work using same footprint . It is really very key to have a balanced CL1 and CL2 in range of 18 to 20pF to have good pullability ( PPMs).

Regards,

STOne-32

[gkg]

Sure, thanks will try that..

[gkg]

Sure, let me try this with 17pf capacitor and a different crystal. ok?

Although, I don't think the crystal is the problem, its more to do with the layout I guess....

Nevertheless, let me try this too...

[gkg]

Tried with 17pf and high drive level. There was no change. Still the clock is slow.

[gkg]

Hi,

We tried but no improvement.

[waclawek.jan]

Show (post photo detailed enough to see the exact return path).

JW