Posted on May 17, 2011 at 11:35Table 32 refers to circuits using a ceramic resonator--as mentioned in my previous post, I am interested in a circuit using a crystal oscillator. The way section 7.5 is written, I would assume that if I subtract the PCB parasitic capacitance and the uC pin capacitance from the C1=C2 value given in Table 31 for a crystal with a specific ESR, I would arrive at the value for the external capacitance for C1 and C2. I've discussed this with an ST FAE and the best answer he could give me was to select the external capacitors as 22pF. I can estimate both the pin capacitance and the PCB stray capacitance to justify the 22pF, but I would have preferred ST to specify a pin capacitance.
In the past year I've had to correct two circuits designed by others who did not follow the manufacturers recommendations. One circuit was overdriven and would often start up at x3 frequency, the other exhibited erratic start up at cold temperatures. Measuring the drive level and calculating the safety factor would have revealed potential problems, but just following the manufacturer's recommendations from the beginning would have save considerable time and expense.
