The normal Arduino works with 16MHz or 8MHz(3.3V). The 20MHz formware is required for 20MHz.
The bootloaders for Arduino are IDE version (4.4) and fixed version (6.2). github
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Windows\system32>cd "C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot"
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot>omake clean
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot>..\..\..\tools\avr\utils\bin\make OS=windows ENV=arduino clean
rm -rf *.o *.elf *.lst *.map *.sym *.lss *.eep *.srec *.bin *.hex
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot>omake AVR_FREQ=20000000L atmega328
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot>..\..\..\tools\avr\utils\bin\make OS=windows ENV=arduino AVR_FREQ=20000000L atmega328
../../../tools/avr/bin/avr-gcc -g -Wall -Os -fno-inline-small-functions -fno-split-wide-types -mshort-calls -mmcu=atmega328p -DF_CPU=20000000L '-DLED_START_FLASHES=3' '-DBAUD_RATE=115200' -c -o optiboot.o optiboot.c
../../../tools/avr/bin/avr-gcc -g -Wall -Os -fno-inline-small-functions -fno-split-wide-types -mshort-calls -mmcu=atmega328p -DF_CPU=20000000L '-DLED_START_FLASHES=3' '-DBAUD_RATE=115200'
-Wl,--section-start=.text=0x7e00 -Wl,--section-start=.version=0x7ffe -Wl,--relax -Wl,--gc-sections -nostartfiles -nostdlib -o optiboot_atmega328.elf optiboot.o
../../../tools/avr/bin/avr-size optiboot_atmega328.elf
text data bss dec hex filename
500 0 0 500 1f4 optiboot_atmega328.elf
../../../tools/avr/bin/avr-objcopy -j .text -j .data -j .version --set-section-flags .version=alloc,load -O ihex optiboot_atmega328.elf optiboot_atmega328.hex
..\..\..\tools\avr\bin\avr-objdump -h -S optiboot_atmega328.elf > optiboot_atmega328.lst
rm optiboot.o optiboot_atmega328.elf
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot>
For github version, rename optiboot-master\optiboot\bootloaders\optiboot\ to optiboot-6.2 and copy to C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\ and build.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Windows\system32>cd "C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot-6.2"
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot-6.2>omake AVR_FREQ=20000000L atmega328
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot-6.2>..\..\..\tools\avr\utils\bin\make OS=windows ENV=arduino AVR_FREQ=20000000L atmega328
avr-gcc (WinAVR 20081205) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
BAUD RATE CHECK: Desired: 115200, Real: 113636, UBRRL = 21, Error=-1.3%
../../../tools/avr/bin/avr-gcc -g -Wall -Os -fno-split-wide-types -mrelax -mmcu=atmega328p -DF_CPU=20000000L -DBAUD_RATE=115200 -DLED_START_FLASHES=3 -c -o optiboot.o optiboot.c
../../../tools/avr/bin/avr-gcc -g -Wall -Os -fno-split-wide-types -mrelax -mmcu=atmega328p -DF_CPU=20000000L -DBAUD_RATE=115200 -DLED_START_FLASHES=3
-Wl,--section-start=.text=0x7e00 -Wl,--section-start=.version=0x7ffe -Wl,--relax -nostartfiles -nostdlib -o optiboot_atmega328.elf optiboot.o -lc
../../../tools/avr/bin/avr-size optiboot_atmega328.elf
text data bss dec hex filename
474 0 0 474 1da optiboot_atmega328.elf
../../../tools/avr/bin/avr-objcopy -j .text -j .data -j .version --set-section-flags .version=alloc,load -O ihex optiboot_atmega328.elf optiboot_atmega328.hex
..\..\..\tools\avr\bin\avr-objdump -h -S optiboot_atmega328.elf > optiboot_atmega328.lst
rm optiboot.o optiboot_atmega328.elf
C:\Program Files (x86)\Arduino\hardware\arduino\bootloaders\optiboot-6.2>
github.com/Optiboot/optiboot 6.2 2017-04-20 snapshot
Register 20MHz to IDE
# Add to C:\Program Files (x86)\Arduino\hardware\arduino\boards.txt
uno2.name=Arduino Uno 20MHz
uno2.upload.protocol=arduino
uno2.upload.maximum_size=32256
uno2.upload.speed=115200
uno2.bootloader.low_fuses=0xff
uno2.bootloader.high_fuses=0xde
uno2.bootloader.extended_fuses=0x05
uno2.bootloader.path=optiboot
uno2.bootloader.file=optiboot_atmega328_20.hex
uno2.bootloader.unlock_bits=0x3F
uno2.bootloader.lock_bits=0x0F
uno2.build.mcu=atmega328p
uno2.build.f_cpu=20000000L
uno2.build.core=arduino
uno2.build.variant=standard