49.4.1 Crystal Oscillator Characteristics

The following table describes the characteristics for the oscillator when a crystal is connected between XIN and XOUT as shown in Figure 45-5. The user must choose a crystal oscillator where the crystal load capacitance C_L is within the range given in the table. The exact value of C_L can be found in the crystal data sheet. The capacitance of the external capacitors (C_LEXT) can be calculated as follows:

C_LOAD = { ( [C_XIN + C_LEXT] * [C_XOUT + C_LEXT] ) / [C_XIN + C_LEXT + C_LEXT + C_XOUT] } + C_STRAY.

Where,

CSTRAY is the capacitance of the PCB.

For C_XIN ≠ C_XOUT, but with a difference smaller than 4 pF, we can consider C_XIN ≈ C_XOUT ≈ C_Xeff = ( C_XIN + C_XOUT) / 2.

This results in a simplified formula for computing C_LEXT, as follows:

C_LEXT = 2*CL - 2*C_STRAY - C_Xeff