6.2.1 12MHz Crystal

The calculation, selection, and verification of the external load capacitors C317 and C318 for the on-board 12MHz crystal was done wrong in design phase of the SAM4E Xplained Pro evaluation kit. The capacitors mounted on the board are 6.8pF, while they should have been 18pF. The difference in capacitance will cause the frequency generated by the crystal to be ~500Hz (~44ppm) too high.

Fix/Workaround:: Replace C317 and C318 with 18pF capacitors.

Load capacitance calculation

C_crystal = 20pF (crystal datasheet),

C_L = 9.5pF (SAM4E datasheet), C_PCB = 2.0pF (estimate)

C = 2 x ( C_crystal - C_L - C_PCB )

C = 2 x ( 20pF - 9.5pF - 2.0pF )

C = 17pF

Design selection:

C = 18pF

The selected 12MHz crystal FQ5032B-12-C-C-C-200-1 has a load of 20pF, this is outside the recommended range of 12.5pF to 17.5pF in the ATSAM4E16E datasheet. Using a crystal with loads outside the recommended range can cause instability when operating the device close to maximum/minimum parameters for supply voltage and temperature.

Fix/Workaround:: None, any new design should select a crystal within the limits specified in the datasheet.