2.4.2 Implementation Example

Figure 2-5. Processor Configuration

Figure 2-6. Main Oscillator Implementation

Two implementations are possible.

One uses an oscillator, which requires using the Bypass mode of the main crystal oscillator. Prior to bypassing the main crystal oscillator, the XOUT pin must be grounded (using R38, in the above figure), and the external clock frequency provided on the XIN pin must be stable and within the values specified under “Main Crystal Oscillator”, in the section “Electrical Characteristics” of the data sheet.

The other implementation is done with a crystal oscillator, which is more straightforward: one crystal and two capacitors must be grounded.

To select the capacitors, use the following formula:

C_LEXT = 2 x (C_CRYSTAL – C_PARA – C_PCB / 2).

where C_PCB is the single-ended (ground-referenced) parasitic capacitance of the PCB on the XIN and XOUT tracks.

As an example, if the crystal is specified for an 8 pF load, with C_PCB = 1 pF (on XIN and on XOUT):

C_LEXT = 2 x (8 - 1 - 0.5) = 13 pF