An optional ``shutter'' can be enabled. When the shutter is ``closed'' triggers are vetoed and no triggers are sent. When the shutter is ``open'' triggers can be generated and sent to active DUTs.
The shutter cycle can either be started by an external signal or synchronized by a counter clocked by the system clock.
The external signal, if used, must be connected to one of the trigger inputs. An appropriate threshold should be set and the input used for synchronizing the shutter should not be used in the trigger mask.
Figure \ref{fig:shutter_timing} illustrates the timing of the shutter. Behaviour of the shutter is controlled by the IPBus registers described in table \ref{tab:shutter_registers}.
\begin{table}
\begin{tabular}{lp{\0.6\linewidth}}
Register Name & Function \\\hline
ControlRW & Bit-0 controls if shutter pulses are active. 1 = active. Bit-1 selects external synchronization (bit-1 = 0) or internal sequence (bit-1 = 1) \\
ShutterSelectRW & Selects which input is used to trigger shutter sequence.range = 0-5)\\
InternalShutterPeriodRW & Internal sequence period (when using internal sequence). Units of clock cycles.\\
ShutterOnTimeRW & Time between start of sequence and shutter asserted( t1) \\
ShutterVetoOffTimeRW & time between start of sequence and veto being de-asserted (t2)\\
ShutterOffTimeRW & Time between start of sequence and time at which shutter de-asserted and veto reasserted (t3) \\
\end{tabular}
\caption{IPBus registers controlling behaviour of shutter.}
