Wait routine for System-5

[roland]

For the Atom we have a way to generate a short delay by checking the vertical sync; it's the routine at #FE66 and it's used by the WAIT statement. Now I need a similar routine for a System-5 computer. Is there a way to check the VSynch of the 6845? I haven't found it in the datasheet how to check this. Or is there another reliable time based wait routine, preferably without using a VIA timer?