Re: [Freemint-discuss] nanoseconds delay function

[David G. <dga...@gm...>]

2018-05-18 22:19 GMT+02:00 Roger Burrows <an...@xp...>:
>
> Something similar to this is done in EmuTOS, in case you haven't noticed.  I

I saw the this approach for the nanoseconds delay in the USB TOS
drivers that took the idea from EmuTOS.

> think that, unless you really expect MiNT to run native on modern PCs, trying
> to get a nanosecond-level value is a bit optimistic.
>

Having 1 nsec granularity is very optimistic I know :-), but if my
calculations are correct with a 060 at 66 Mhz you can have between 15
nsec and 20 nsec granularity that is not so bad.

> The EmuTOS approach is to calculate the number of loops for a millisecond delay
> value during EmuTOS initialisation.  Individual drivers multiply (rare) or
> divide it as required during driver initialisation.  If the MiNT mdelay()
> function is accurate, then you could use the same approach.
>

Similar to what MiNT does, it calculates the value "loops per second"
during the system's boot.

> The key thing is to make the actual delay loop itself as repeatable as
> possible.  EmuTOS does a subq.l from a register, which should be relatively
> immune to caching & pipeline variability.
>

The same than MiNT, this was imported from Linux-m68k years ago.