I have been looking and search bootloaders for AVR out there. And there are a number of them, e.g., https://www.avrfreaks.net/index.p... . I have also scanned through the Bootloader FAQ posted by AVRFreaks.net . But, I can't seem to find a definite answer of having a bootloader that be possible to use the maximum clock frequency an MCU offers.
In my case, I have the ATMEGA644, which can have a max clock freq of 20Mhz. There are serial and USB bootloaders, but they require clock frequencies well below the 20Mhz, especially USB booloaders, which seem to require 12Mhz.
So, how would one a bootloader that works at a clock speed below an MCU's max, then have the MCU's application run at max speed? Surely, I can't mess with the fuses. Huge delays in the bootloader, that is, waste cycles?
By the way, suggestion of bootloaders are appreciated.