20.1.3.4 Clocks During Boot ROM Code Execution

An important point is that the clocks during boot code execution must be limited to 48 MHz not 72 MHz. This is because the device does not know the temperature environment and the clocks are limited to 48 MHz at > 85°C. Therefore, it is recommended that the boot code initialize the DFLL48M to provide 48 MHz source clock and MCLK derive system clocks from the DFLL48M. The PLL is not used.