(Un)official schematics, wikipedia, and multiple other sources give a 28.6 MHz frequency for SH-2 CPUs. But when i measure MCLK2 (which is the same as MCLK3, which goes directly to both processors) with a scope, it gives me 26.88 clock in bios at time/date entry screen. Haven't checked this in games yet. The question is, is it possible to change the main clock via software? Or is my scope mistaken? I want to multiply this clock for my own purporces, but if it's changeable, there might be undesired side effects... Nevermind, i've alredy found the answer in the sega's docs. The freq differs indeed, but it depends on videomode (NTSC/PAL) and video lines number (320/352) only. Actually it's kind of a strange thing to do such a dependency, saturn was never built with too much economy in mind... Maybe syncing cpu with VDPs was a problem? Well, i'll have to use SCSP clock instead, that one seems to be steady 22.5792MHz.
The reason the speed changes with the video mode is because the VDP Dot Clock is derived from the same clock source (clock / 4). I'm not sure why they wanted it like that, but the 28mhz speeds were quoted as far back as 1993 when the machine was still said to have a NEC V60. So the speed (and the resolution) was most likely chosen early on. I think they were designed to work in sync from the start. But they later expanded the hardware so much, that the system ended up with like 5 clocks in 3 independent subsystems.
