Crystal for BNO085

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phlppx
 
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Crystal for BNO085

Post by phlppx »

Hello,
The Orientation IMU module (Product 4754 https://www.adafruit.com/product/4754) uses an external crystal for the BNO085.
I'm interested which 32.768kH crystal specifically is used as this is not written in the schematics on GitHub https://github.com/adafruit/Adafruit-BN ... ree/master.
According to this guide (https://xdevs.com/doc/CEVA/BNO080-BNO08 ... -Guide.pdf) it should have a load capacitance of 22pF and a tolerance of 50ppm.

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jps2000
 
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Re: Crystal for BNO085

Post by jps2000 »

In fact it is better tu use a 32kHz crystal cut for a smaller capacity.
Then it starts better as the capacitors act as a load for the oscillator circuit.
Note that as load both capacitors plus the pin capacitance act in series.
so when you mount 22pf you have to add about 4pf for the pin and wires resulting in 26pF
This is then a load of 13pF for the capacitor.
On the other hand, if the frequency is not that precise - which is important for a real time clock of course- this does not really matter for the IMU performance.
However if the xtal is not starting or ceasing the internal CPU ( a M0 core ) does not work.
This may happen if you solder a chip by hand and some residual flux is trapped between chip body and PCB.
You can not wash it away !!!
This effect took me some weeks of inverstigation until I found this cause why sometimes the BNO did not start or stopped operation.
So NEVER let the chip swim in place as you might do with others
Concluding: If you find a crystal with 7pf or 9pf ballast these are superior to others.

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phlppx
 
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Re: Crystal for BNO085

Post by phlppx »

I didn't quite understand why I should use a 32kHz crystal instead of a 32.768kH one as stated in the datasheet.

I also found this formula to calculate the caps https://community.nxp.com/t5/LPC-FAQs/H ... m-p/464589

If my CL is 12.5pF and Cstray is 4pf then my caps should be 17pf ( 17pF=2(12.5pF-4pf) ).
If my CL is 9pF and Cstray is 4pf then my caps should be 10pf ( 10pF=2(9pF-4pf) ).

Why are the crystals with less CL superior?

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jps2000
 
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Re: Crystal for BNO085

Post by jps2000 »

of course you should use a 32.768khZ. I do not think you will find easily another .
This frequency is 2^15 so with a 15 stage binary divider you get 1 second. Every watch has such a crystal.
A smaller capacitor means less load for the oscillator. A capacitor dampens the oscillation. So starting behaviour and power consumption is better.
More detailed infos you find in the attachments.
However as mentioned above I do not see the frequency critial for the performance of the BNO
MSP430 32khz crystal oscillators.pdf
(411.1 KiB) Downloaded 8 times
Attachments
AVR4100_ Atmel.pdf
(902.2 KiB) Downloaded 3 times

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