Here's an arcane bug for you:
When powering down, the Spartan 7 3.3V I/O drivers will
output a roughly 800us wide glitch as 3.3V falls below around
1.7V, if the 0.95V core voltage comes down first (which it does
because we have an active discharge on 0.95V, but not on 3.3V).
In this case, it's just wide and high enough to cause the
BOOST mode line to engage, which takes the BOOST output
and ties it to VBUS, which is just enough for the system to
think you're powering it back on again. Which is not what
you want when powering down.
The ultimate fix was to put a heftier pull-down on the BOOST
line so that it can't trip the BOOST output into the VBUS.
Another option could be more active discharge FETs, but that
would require a full board spin and we are already at certification,
so we'll stick with the stronger pull-down on BOOST. It seems to
be a robust fix in the worst corner cases we can find.
The downside is that when powering an external USB device,
you are burning an extra 30mW of power overcoming the pull-down,
but typically, any external USB devices plugged into Precursor
will burn far in excess of that, so the delta should be negligible.
Use deadlines for sleep, rather than calculating the amount of remaining
time.
This keeps time consistent, because it means we don't have to
recalculate all sleeps whenever one expires.
It also allows us to take advantage of the hardware which will fire an
interrupt if we schedule a wakeup in the past.
Signed-off-by: Sean Cross <sean@xobs.io>
Servers will, by default, get randomly-assigned SIDs. Certain servers
will want to have well-known SIDs, such as the log server or the
nameserver. These will use a separate call in order to perform this
priveledged operation.
A subsequent patch will add `CreateServer`.
Signed-off-by: Sean Cross <sean@xobs.io>
This is responsible for preemption in the system. Move it to the timer
server since it seems to make more sense to have there.
Signed-off-by: Sean Cross <sean@xobs.io>
Previously UTRA didn't have an interrupt, so we just made one up. Use
the correct interrupt to get things working on hardware.
Signed-off-by: Sean Cross <sean@xobs.io>
