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Crystal value for LPC546xx USB ISP bootloader

Question asked by guanix on Oct 15, 2017
Latest reply on Oct 30, 2019 by Thorsten Wilmer

On a custom board using LPC54605J512, we would like to use the ISP pins to have the option of invoking the USB1 MSC or DFU bootloader from a blank chip, without any code already loaded into flash.


My question is: Are there any restrictions on the crystal used in order to use a USB bootloader in this way? Does the USB ROM bootloader assume a fixed crystal value, or does it automatically measure the crystal value using the frequency measurement function and configure the USB PLL accordingly?


I can see that the LPC54608 development board uses a 12 MHz crystal, so presumably this value is supported. But I would prefer to use a higher frequency crystal that can be physically smaller, for example 25 MHz.