On Mon, Feb 23, 2009 at 12:25 PM, Simon Owen <[email protected]> wrote: > Andrew Collier wrote: >> I think the only question it doesn't answer is what happens to the >> 112 t-state difference between the number of t-states measured with >> the screen off (119696) and the number that would be expected from >> the timing characteristics of the screen display (119808). > > Doesn't that just mean the real SAM framerate is ~50.08Hz instead of > 50Hz? That's a difference of 6 seconds per day, so unlikely to be > noticed by most programs.
I don't think so, as such - I wasn't assuming the framerate was exactly 50Hz, rather I was calculating using the geometry of the display. That is to say, the Sam's screen should consist of 312 lines, (standard PAL50 would have 625 lines in 1/25 of a second, but the Sam misses one out because it doesn't interlace) and we can measure that each line takes 384 t-states to display. That would mean there should be 384*312=119808 t-states between frame interrupts, but according to that article, if you measure the number of instructions executed during a frame, then even with the screen off it turns out to be slightly fewer than expected. Andrew
