To support deterministic latency with non-power of two octets per frame
(F=3,6) the interface width towards the transport layer must be resized
to match integer multiple of frames.
e.g Input datapath width = 4; Output datpath width = 6;
for F=3 one beat contains 2 frames
for F=6 one beat contains 1 frame
The width change is realized with a gearbox.
Due the interface width change the single clock domain core is split
in two clock domains.
- Link clock : lane rate / 40 for input datapath width of 4 octets 8b10b
- lane rate / 20 for input datapath width of 8 octets 8b10b
- lane rate / 66 for input datapath width of 8 octets 64b66b
- Device clock : Link clock * input data path width / output datapath width
Interface to transport layer and SYSREF handling is moved to device clock domain.
The configuration interface reflects the dual clock domain.
If Input and Output datapath width matches, the gearbox is no longer
required, a single clock can be connected to both clocks.
In case when the SYSREF is connected to an FPGA IO which has a limitation
on the IOB register IN_FF clock line and the required ref clock is high
we can't use the IOB registers.
e.g. the max clock rate on zcu102 HP IO FF is 312MHz but ref clock is 375MHz;
If IOB is used in this case a pulse width violation is reported.
This change makes the IOB placement selectable in such case or
for targets which don't require class 1 operation.
Laszlo Nagy