Halas ...

reason for porting was needing faster clock speed 818 can go at 20Megs 628 only 4 !
nor the 628 nor 818 have 20 Mhz INTERNAL available ...

sooo, you just have to use an EXTERNAL XTAL in HS mode ...
no need to choose an exotic number and try to port your program for it ... as the 628/04 easily can run @ 20 Mhz EXTERNAL ...

... Alain