3.2. Local Extended Multiblock Clock
The F-Tile JESD204C IP uses the Extended Multiblock Clock (LEMC) as a common timing reference to support multidevice configuration.
LEMC is an internal clock that aligns the boundaries of the extended multiblocks between lanes. In deterministic latency devices, LEMC aligns the boundaries to an external reference, for example, SYSREF. The use of LEMC is mandatory in Subclass 1 modes but optional in Subclass 0 modes.
The F-Tile JESD204C IP implements LEMC as a counter that increments in link clock counts, and depends on the Multiblocks in an extended multiblock (E) parameter. The extended multiblock is a container of a number of multiblocks.
The E parameter depends on these two factors:
- The parameter must allow an integer of F within an extended multiblock. For example, if F=3, 32 multiblocks contain 256 octets (32x64/8). 256 octets is not divisible by F=3. So, for F=3, the minimum E is 3.
- E must be larger than the maximum possible delay variation across any two lanes of a link.
In Subclass 1 deterministic latency system, SYSREF is distributed to the devices to be aligned in the system. The SYSREF signal resets the internal LEMC clock edge when the sampled SYSREF rising edge transitions from 0 to 1.
The F-Tile JESD204C IP does not use the device clock directly to sample SYSREF because of the source synchronous signaling of SYSREF with respect to the device clock sampling from the clock chip. The IP uses the link clock to sample SYSREF. The PLL that provides the link clock or frame clock must be in normal mode to phase-compensate the link clock to the device clock.
You can program a single or multiple sampling of SYSREF through the F-Tile JESD204C control and status registers.
- A single sampling SYSREF does not detect SYSREF period errors.
- A continuous sampling mode detects SYSREF period errors.
In most converter device systems, disable SYSREF sampling if there are no errors, and begin link operation with a link reinitialization request.