PLL unlocks hard to deal with

[darrylmasson]

This blurs the line between a hardware problem redax isn't supposed to deal with, and a software solution that redax could be responsible for.

If something wonky happens with a digitizer's PLL input, the board's behavior deviates from normal. The board will reacquire its PLL lock after a second or so, but it's now at a different phase in the clock cycle. Resetting the PLL lock of one board invalidates the PLL lock of all the boards down the clock chain.

Boards perform the PLL lock procedure when they boot, so traditionally when one needs to reset the PLL one reboots the crate. This is tedious and requires manual intervention. As redax lacks the inter-process communication necessary to access sequential digitizers mounted in a VME crate, an alternate solution would be to have a crate controller in each crate that instructs each digitizer to reset its PLL lock. The arming sequence then would begin with each crate controller going down the clock chain and resetting all boards.