I am using the ATMEGA32U4 chip with the default bootloader. The chip comes with the CKDIV8 fuse bit enabled by default. I want to run the chip at 8MHz. As I am aware, I can simply disable this fuse and use an 8MHz external clock source. But writing fuse bits is not possible via the bootloader and requires the ISP interface. But is there any way to achieve this without using the ISP? For instance, can I use a 64MHz external clock source, so that I get my required 8MHz after the CKDIV8 operation?