| ??? 02/27/08 03:34 Read: times Msg Score: +1 +1 Informative |
#151508 - 32 KHz Oscilator, Capacitors and Resistor Responding to: ???'s previous message |
The crystal is only part of the oscillator circuit, and you have not described the rest of it.
Most 32 Hz oscillators are built around an inverter. There needs to be a feedback resistor to provide proper DC bias. If the resistor is missing, or is too small, then the circuit may have a hard time starting. When you touch it, the 60 HZ (or 50Hz) noise on your fingers may be enough to start the oscillator. For low frequencies, it is frequently desirable to have a 300K resistor in series with the output to provide additional phase shift. Do you have this? Most 32 KHz crystals are looking for a total of 32 pF of load. You must take into account the capacitance of your inverter, crystal, and board routing, which can be 5 or 10 pF. As a result, 22 pF capacitors are frequently used in this application. |
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