STM32F101C8 Oscillator Questions

[jwormsley]

Posted on November 02, 2010 at 17:17

STM32F101C8 Oscillator Questions

[jwormsley]

Posted on May 17, 2011 at 14:13

Indeed, that's good information, too bad it wasn't something I saw before starting two years ago, or pointed out as a must read in any of the other literature I did find.

I assume the older eval boards crystal didn't need the Rext resistor, but I am still not clear as to why the older eval board had the Rf resistor, while the new ones do not.  Does leaving it there cause any problems or should I remove it from new boards?

[John F.]

Posted on May 17, 2011 at 14:13

See,

http://www.st.com/stonline/products/literature/an/15287.pdf

[Tesla DeLorean]

Posted on May 17, 2011 at 14:13

I assume the older eval boards crystal didn't need the Rext resistor, but I am still not clear as to why the older eval board had the Rf resistor, while the new ones do not.  Does leaving it there cause any problems or should I remove it from new boards?

Chances are they changed crystal vendor, or picked crystal with slightly different characteristics. You should pick component values that match *your* crystal choice. Leaving the pads there gives you the option to change crystal (supply, cost, etc), if the crystal can take the drive current from the inverter just place a 0 Ohm part. The resistor is there to reduce the drive current into the crystal. This is one area where picking components off someone else's schematic can get you into trouble. You can end up with a circuit that won't oscillate or start properly/reliably, or over-drive the part causing damage/heating/premature-aging.