Timing in simulation needs investigating

[xross]

In GTKwave the clock is shown as 100MHz (should be 60MHz)

The TB currently also allow for time slipping, however fixing this doesn't resolve the frequency issue.

[xross]

Added @DaveAtkinsonXMOS since he has been modding xsim

[xross]

My current theory is that xsim's time units are "ticks" (rather than ns) but the TB is interpreting each tick as 1ns.

So at 600MHz each tick is 1.66ns. Multiplying the 10ns period we observe on the clock port by 1.66 gives us 16.66 -60MHz as expected.

This is quite a big bug with the TB.. suggest Pyxsims wait_until() is renamed wait_until_ticks() we can also add wait_until_ns() since we know the core freq in the tb

[xross]

Fixed in #220