I have a habit of converting optical mice for Amiga use. In case the mouse uses a suitably old controller (many PS/2 era optical mice), I just tap the quadrature signals directly from the mouse. If it uses an unsuitable controller, I use a PIC with PS2M firmware to make a small adapter. On the Amiga, both kinds work fine, but when I try to rewire it for the ST, the mouse pointer starts moving erratically upon rapid movements — otherwise, it works fine.
Is this due to the ST using too small a counter for mouse movements — causing overflow on big movements — or is there a suitable fix I've overlooked?