SPRSP85A April 2024 – September 2024 TMS320F28P550SJ , TMS320F28P559SJ-Q1
PRODMIX
In the crystal mode of operation, a quartz crystal with load capacitors has to be connected to X1 and X2.
This mode of operation is engaged when [XTAL On] = 1, which is achieved by setting XTALCR.OSCOFF = 0 and XTALCR.SE = 0. There is an internal bias resistor for the feedback loop so an external one should not be used. Adding an external bias resistor will create a parallel resistance with the internal Rbias, moving the bias point of operation and possibly leading to clipped waveforms, out-of-specification duty cycle, and reduction in the effective negative resistance.
In this mode of operation, the resultant clock on X1 is passed through a comparator (Comp) to the rest of the chip. The clock on X1 needs to meet the VIH and VIL of the comparator. See the XTAL Oscillator Characteristics table for the VIH and VIL requirements of the comparator.