Not enough space to place the orbit trailer safely
After investigation with P5 KCU1500 connected to uGMT, the following potential issue has been discovered:
- before creating the step where the orbit trailer is generated, there are occasionally only 17 frames of space between the end of the orbit and the start of the following one. Until now, however, everything is ok
- after the trailer is applied, its 17 frames fill completely the gap between one orbit and the other, in particular they fill the gap in the valid bit
- this means that there is not valid bit transition between one orbit and the other
- so the fifo filler module is not able to distinguish the two orbits and they count as one
- this explains why in scdaq we see sometimes packets with more orbits than specified and so why we see files with more than 4096 orbits
ILA capture attached as example