Do i need a series resistor for an external crystal with a microcontroller?

A series resistor, Rs, is added to the circuit if, after all other external components are selected to satisfaction, the crystal is still being overdriven. This can be determined by looking at the OSC2 pin, which is the driven pin, with an oscilloscope. Connecting the probe to the OSC1 pin will load the pin too much and negatively affect performance. Remember that a scope probe adds its own capacitance to the circuit, so this may have to be accounted for in your design (i.e., if the circuit worked best with a C2 of 22 pF and the scope probe was 10 pF, a 33 pF capacitor may actually be called for). The output signal should not be clipping or flattened. Overdriving the crystal can also lead to the circuit jumping to a higher harmonic level, or even, crystal damage.

The OSC2 signal should be a clean sine wave that easily spans the input minimum and maximum of the clock input pin. An easy way to set this is to again test the circuit at the minimum temperature and maximum VDD that the design will be expected to perform in, then look at the output. This should be the maximum amplitude of the clock output. If there is clipping, or the sine wave is distorted near VDD and VSS, increasing load capacitors may cause too much current to flow through the crystal or push the value too far from the manufacturer’s load specification. To adjust the crystal current, add a trimmer potentiometer between the crystal inverter output pin and C2 and adjust it until the sine wave is clean. The crystal will experience the highest drive currents at the low temperature and high VDD extremes.

The trimmer potentiometer should be adjusted at these limits to prevent overdriving. A series resistor, Rs, of the closest standard value can now be inserted in place of the trimmer. If Rs is too high, perhaps more than 20 kΩ, the input will be too isolated from the output, making the clock more susceptible to noise. If you find a value this high is needed to prevent overdriving the crystal, try increasing C2 to compensate or changing the oscillator operating mode. Try to get a combination where Rs is around 10 kΩ or less and load capacitance is not too far from the manufacturer’s specification.

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