I never said the recommanded Fuse is a 20MHz.
I said the maximum possible Clock Frequency is 20MHz.
To burn in Fuses via SPI you always need a clock source!
It doesn't matter with one is used, but the choosen with fuses must!!! be present.
When burning Fuses via ISP to "external Crystal" and there is no external crystal present, will stop any function of ISP.
You can't get access to the Chip until the fused clock source is present.
With parallel programming it is possible to programm such a chip without additional Clock source.
But for that the Chip must be taken out of the Circuit and puted into a special Programmer.
In the Fuses were only 4 Types of Crystals be choosen.
0,4 up to 0,9MHz
0,9 up to 3MHz
3 up to 8MHz
greater 8MHz
The full swing types should only be choosen, when the Clock Oscillator of the Controller shoud drive additional devices.
The SUT is the Start Up time of the Controller and has only effect at the startup of the Controller.
I nearly always use the slowest startup time to avoid problems with slow rising Power supplies.
The Fuses setting must fit to the used crystal. The Controller runs with the speed of the crystal.
The Datasheet say's:
Operating Voltage:
– 1.8 - 5.5V for ATmega88V
– 2.7 - 5.5V for ATmega88
• Speed Grade:
– ATmega88V: 0 - 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V
– ATmega88: 0 - 10 MHz @ 2.7 - 5.5V, 0 - 20 MHz @ 4.5 - 5.5V
All the other Information you ask, is included into the Datasheet too. But Nobody want's to read that Today
Link:
https://www.electro-tech-online.com/custompdfs/2012/03/doc8025-1.pdf or
https://www.electro-tech-online.com/custompdfs/2012/03/doc8271.pdf )
Ok you're right the Clock Generation and Fuse Setting is described there a little bit complicated.
But with Fuse Calculator, or Settings at the programmer of AVR Studio 4 makes that quite easy.
What speed grade you need, appends of your application.
Software signal generation or complex mathematics need high speed grades.
For Battery supplied solution you should better use lower speed grades and the Power Down Modes to save energy. Maximum possible frequency appends of the Controller Type ( A, V, P ) too. Look a little bit up.
Note - Higher frequencies draw more current.