STM32L0 LPUART LSE Error

[MLam]

Hello everyone,

I'm working on a project with a STM32L072CBT6.

I'm using LPUART (PB10 / PB11) at 9600 bauds to communicate with an other board. For low power consumption, i'm using LSE for my LPUART.

When i check my bauds rate on my oscilloscope, i get the value of 93.3 that represent ~10718 bauds.

I have try to modifiy load capacitor of my resonnator (12pF and 6pF) but no change.

Do you have advice about this error in baud rate ?

 Resonnator configuration  (ABS07-32.768KHZ-6-4-T):

My clock configuration in STM32CubeMX:

My LPUART configuration :

My RCC configuration:

Best regards,

MLam

[AScha.3]

1. check LSE frequ. -> activate MCO and set to LSE ; check mco output ...

2. maybe try crystal drive high , not low.

3. try lower baud, for test 1200 or so.

[waclawek.jan · Accepted solution]

If you measure individual bits, you can only measure integer periods of the 32768Hz LSE clock, i.e. the calculated "baudrate" from individual bits can be for 9600 setting only 32768/3=10922 or 32768/4=8192. LPUART achieves a relatively good 9600 Bauds by dithering, i.e. alternating appropriately between these two bit lengths.

JW

[MLam]

Hello AScha.3,

This my return after your reply:

1) Frequency is correct, Picture below get from MCO output

2) I have try in High LSE drive capability but nothing change.

3) Resultat for various bauds :

9600 bauds configured	10718 bauds mesured
4800 bauds configured	5480 bauds mesured
1200 bauds configured	1210 bauds mesured

[AScha.3]

btw set drive -> high , just if crystal not starting or shows problems (depends on specs of crystal, flux remains on board etc.)

+

did you read, what Jan wrote? 

[MLam]

OK but when i get frequency on my MCO GPIO that seems that my crystal start correctly no ?

Yes i have read what Jan wrote, i'm currently trying to modify UCESM bit.

[AScha.3]

yes, seem working fine.

[MLam]

To conclude,

I have reduce my baud rate to 1200 to get a better match.

In a same time i see that my DMA+LPUART often crash when i try to get my data at initialization.

So i have added a delay for my LPUART Interrupt when i power on my system.