Question: Why FX3 cannot boot from SPI flash/EEPROM, if SPI MISO/MOSI lines are pulled HIGH?
FX3’s bootloader is designed to boot from SPI flash/EEPROM when the address cycle is not the same. FX3’s bootloader uses the floating state of the SPI MISO line to determine the number of address cycles for the flash/EEPROM. If pull-up is present on the MISO line, then the bootloader cannot determine the correct address cycles. That stops the FX3 boot from SPI flash. Please make sure that there is no pull-up on SPI MISO/MOSI lines to allow FX3 to boot from SPI flash/EEPROM.