5.3 Reference Clock

The IS20xxS SoC is composed of an integrated crystal oscillation function that uses a 16 MHz external crystal and two specified loading capacitors to provide a high quality system reference timer source. Use this feature to remove the initial tolerance frequency errors, which are associated with the crystal and its equivalent loading capacitance in the mass production. Achieve the frequency trim by adjusting the crystal loading capacitance through the on-chip trim capacitors (Ctrim).

The value of the trimming capacitance is 200 fF (200x10-15 F) per LSB at the 5-bit word, and the overall adjustable clock frequency is ±40 kHz. The following figure illustrates the crystal connection of the IS20xxS SoC with two capacitors.

Figure 5-1. Crystal Connection
Note:
  1. Ctrim = 200 fF * (1 to 31); Cint = 3 pF.
  2. CL = [(CL1 x CL2) / (CL1 + CL2)] + (Ctrim / 2) + Cint (set trim value as 16, Ctrim = 3.2 pF).
  3. For a 16 MHz crystal where CL = 9 pF, the CL1 = CL2 = 9.1 pF).
  4. For the CL selection, refer to the data sheet of the crystal.