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fabio239955_stm1_stmicro_com
fabio239955_stm1_stmicro_com
Associate
February 11, 2011
Question

32.768kHz clock doesn't start sometimes!

  • February 11, 2011
  • 17 replies
  • 5205 views
Posted on February 11, 2011 at 18:08

32.768kHz clock doesn't start sometimes!

#lse-rtc #lse-low-temperature
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17 replies

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emalund
emalund
Associate III
October 25, 2011
Posted on October 25, 2011 at 17:49

These circuits are hugely sensitive to capacitive loading, and resistance, as well as the physical characteristics of the PCB, traces, crystal, and soldering.

 

......

 

layout, layout, layout

 

 

and the response is:

I am purchasing the proper crytal( 6pF), hopfully this should solved the problem.

BTW I have never heard of a 6pf crystal

Erik
santi
santi
Visitor II
October 25, 2011
Posted on October 25, 2011 at 18:20

Dear Erik,

I am reffering to 32.768KHz CL=6pF.

emalund
emalund
Associate III
October 25, 2011
Posted on October 25, 2011 at 18:48

OK, let's try this way

are the traces to the xtal and the burden caps  less than 1cm ?

are the traces to the Vdd decoupling caps less than 1cm ?

is every Vdd pin decoupled?

does at least one of the Vdd pins have a tantalum in addition to the ceramic

santi
santi
Visitor II
October 25, 2011
Posted on October 25, 2011 at 19:10

Dear Erik,

Yes, I have.

1  yes, the traces to the xtal and the burden caps are less than 1cm. ~8mm. 

2  yes, the traces to the Vdd decoupling caps less than 1cm . ~5mm.

3  yes, the every Vdd pin is decoupled with 0.1uF ceramic.

4  yes, there is one 22uF tantalum  at Vdd.

My PCB already fit these but still no luck.

thank you

nguercetti9
nguercetti9
Visitor II
February 20, 2012
Posted on February 20, 2012 at 09:30

I have a similar problem: at low temperature My LSE start whitn some seconds of delay, aboaut 12s at -20°C.

This delay is very shot at +20°C, 3seconds.

My cristal is ESR=40K, CL = 6p Co= 1.2p and CL1=CL2=6.8p, layout very similar to KAIL evaluation board.

Any Ideas?

fiim
fiim
Associate III
April 19, 2013
Posted on April 19, 2013 at 21:12

For the stm32L15x problem solved:  From the pinOut32kHz to the crystal must be resistor 1kOm.

zzdz2
zzdz2
Associate
April 20, 2013
Posted on April 20, 2013 at 09:30

It may be interesting to measure LSE transconductance of the chips that fail to start RTC.

Maybe it's out of specs, who knows?

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