Roland Borrmann

9S08DZ Crystal oscillator circuit / resistor RF

Discussion created by Roland Borrmann on Jan 7, 2009
Latest reply on Jan 10, 2009 by David Payne
Hello,

generally in a Pierce oscillator configuration (MHz range)  with a quartz crystal connected to the MCU the design sometimes calls for an external 'feedback resistor' in the range of 1 to 10 Meg-Ohms. On some MCUs this resistor is already incorporated (e.g. the MC68HC05 datasheet says 'An internal feedback resistor is provided on-chip between OSC1 and OSC2') but in the 9S08DZ-series this resistor is externally needed - see page 381 in data sheet 'MC9S08DZ60 Series Rev.4'.

If the design also needs the drive level limiting resistor RS up to know I was only familiar with the feedback resistor RF being connected to the XTAL-pins of the MCU but not in parallel with the crystal (as shown in the 9S08DZ data sheet). For me this looks like a drawing error - in other osc circuits I never saw RF connected this way.

Does anyone in this group has some background explanation why this circuit should be correct?
(or be technically equivalent to the version with RF parallel to the MCU pins)

I am well aware that this question only arises if RS is present. In an application where the quartz current needs not to be limited by a series resistor then RS equals zero ohms and RF has direct connection to both MCU xtal pins.

Thanks for any help.

-Roland





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