2.7 Oscillator Value Conditions on Device Start-up

If the PLL of the target device is enabled and configured for the device start-up oscillator, the maximum oscillator source frequency must be limited to a certain frequency (see Oscillator with High-Frequency PLL) to comply with device PLL start-up conditions. This means that if the external oscillator frequency is outside of this range, the application must start up in FRC mode first. The default PLL settings after a POR, with an oscillator frequency outside of this range, will violate the device operating speed.

Once the device powers up, the application firmware can initialize the PLL SFRs, CLKDIV and PLLFBD to suitable values, and then perform a clock switch to the Oscillator + PLL clock source. Note that clock switching must be enabled in the device Configuration Word.