Jitter Present at the PSoC 3/5LP Clock Output – KBA90382

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Question: Why does jitter appear at the clock output of a high-frequency clock? How can it be avoided?



Consider the following schematic:

Figure 1. Clock Component Used in the Top Design

In Figure 1, the clock component is configured for 1 MHz and derived from the internal main oscillator (IMO), such as the 24-MHz IMO.
The master clock used in the design is derived from PLL_OUT. Figure 2 shows the clock configuration tree that is used in the design.

Figure 2. Clock Configuration Tree

So, in this project, Master Clock is derived from PLL_OUT, while the clock source Clock_1 is based on the IMO. The PLL is uncorrelated to the IMO-based 1-MHz clock. This asynchronicity makes it indeterminate on the PLL clock cycle that will re-sync the IMO-based divided clock. This causes jitter on the clock output.

To eliminate this jitter, do one of the following:

  1.   Make the clock source of Clock_1 based on PLL_OUT.
  2.   Uncheck the option “Sync with MASTER_CLK” if Clock_1 is based on the IMO as shown in Figure 3.  

       Figure 3. Disable Sync With Master Clock