  Waveform feature is one of the requirements for the FRIB LLRF controllers. It
is desired that the LLRF con-trollers store the internal data (e.g. the
amplitude and phase information of forward/reverse/cavity signals) for at least
one second of sampled data at the RF feedback control loop rate (around 1.25
MHz). One use case is to freeze the data buffer when an interlock event happens
and read out the fast data to diagnose the problem. An-other use case is to
monitor a set of signals at a decimated rate (user settable) while the data
buffer is still running, like using an oscilloscope. The detailed
implementation will be discussed in the paper, including writing data into the
DDR memory through the native interface, reading out the data through the bus
interface, etc.
