JESD204C subclass 1 outlines a method to achieve deterministic latency across the serial link. Multi-device synchronization is not possible when using the PLL/VCO. If two devices achieve the same deterministic latency then they can be considered synchronized. This latency must be achieved from system startup to startup to be deterministic. There are two key requirements to achieve deterministic latency. The first is proper capture of SYSREF. SYSREF resets the LMFC counter in each device to act as a known timing reference.

The second requirement is to choose a proper elastic buffer release point in the receiver. The converter device is the receiver (RX) in the JESD204C link and the logic device is the transmitter (TX). The elastic buffer is the key block for achieving deterministic latency and does so by absorbing variations in the propagation delays of the serialized data as the data travels from the transmitter to the receiver. A proper release point is one that provides sufficient margin against delay variations. An incorrect release point results in a latency variation of one LMFC period, or in some cases result in a buffer overflow that prevents the link from establishing. Only when the multiframe is short can the user get a bad release point and when that happens. The lanes are usually deskewed incorrectly resulting in bad data. Choosing a proper release point requires knowing the average arrival time of data at the elastic buffer, referenced to an LMFC edge, and the total expected delay variation for all devices. With this information the region of invalid release points within the LMFC period can be defined, which stretches from the minimum to maximum delay for all lanes. Essentially, the designer must makes sure that the data for all lanes arrives at all devices after the previous release point occurs and before the next release point occurs. The invalid region can also be found experimentally, see Programming RBD.

Figure 7-54 provides a simplified timing diagram that demonstrates this requirement. In this figure, the data for two transmitters (ADC or logic device) is shown. The second transmitter (TX 2) has a longer routing distance (t_PCB) and results in a longer link delay than the first transmitter (TX 1). First, the invalid region of the LMFC period is marked off as determined by the data arrival times for all devices. Then, the release point is set by using the release buffer delay (RBD) parameter to shift the release point an appropriate number of quad-octet steps from the LMFC edge so that the release point occurs within the valid region of the LMFC cycle. For Figure 7-54, the LMFC edge (RBD = 0) is a good choice for the release point because there is sufficient margin on each side of the valid region.

Figure 7-54 LMFC Valid Region Definition for Elastic Buffer Release Point Selection

The TX and RX LMFC do not necessarily need to be phase aligned, but knowledge of their phase is important for proper elastic buffer release point selection. Also, the elastic buffer release point occurs within every LMFC cycle, but the buffers only release when all lanes have arrived. Therefore, the total link delay can exceed a single LMFC period; see JESD204B multi-device synchronization: Breaking down the requirements for more information.