3.2 Negative Resistance Test and Safety Factor

The negative resistance test finds the margin between the crystal amplifier load used in your application and the maximum load. At max load, the amplifier will choke, and the oscillations will stop. This point is called the oscillator allowance (OA). Find the oscillator allowance by temporarily adding a variable series resistor between the amplifier output (XTAL2/TOSC2) lead and the crystal, as shown in Figure 3-6. Increase the series resistor until the crystal stops oscillating. The oscillator allowance will then be the sum of this series resistance, R_MAX, and the ESR. Using a potentiometer with a range of at least ESR < R_POT < 5 ESR is recommended.

Finding a correct R_MAX value can be a bit tricky because no exact oscillator allowance point exists. Before the oscillator stops, you may observe a gradual frequency reduction, and there may also be a start-stop hysteresis. After the oscillator stops, you will need to reduce the R_MAX value by 10-50 kΩ before oscillations resume. A power cycling must be performed each time after the variable resistor is increased. R_MAX will then be the resistor value where the oscillator does not start after a power cycling. Note that the start-up times will be quite long at the oscillator allowance point, so be patient.

Using a high-quality potentiometer with low parasitic capacitance is recommended (e.g., an SMD potentiometer suitable for RF) to yield the most accurate results. However, if you can achieve good oscillator allowance/R_MAX with a cheap potentiometer, you will be safe.

When finding the maximum series resistance, you can find the safety factor from Equation 3-2. Various MCU and crystal vendors operate with different safety factor recommendations. The safety factor adds a margin for any negative effects of the different variables such as oscillator amplifier gain, change due to the power supply and temperature variations, process variations, and load capacitance. The 32.768 kHz oscillator amplifier on AVR microcontrollers is temperature and power compensated. So by having these variables more or less constant, we can reduce the requirements for the safety factor compared to other MCU/IC manufacturers. The safety factor recommendations are listed in Table 3-1.