1.3.4. Deterministic Latency (Subclass 1)
Figure below shows a block diagram of the deterministic latency test setup. A SYSREF generator provides a periodic SYSREF pulse for both the AD6676 and JESD204B IP Core. The SYSREF generator is running in link clock domain and the period of SYSREF pulse is configured to the desired multiframe size. The SYSREF pulse restarts the LMFC counter and realigns it to the LMFC boundary.
Figure 4. Deterministic Latency Test Setup Block Diagram
Figure 5. Deterministic Latency Measurement Timing Diagram
With the setup above, three test cases were defined to prove deterministic latency. By default, the JESD204B IP Core detects a single SYSREF pulse. The SYSREF single-shot mode is enabled on the AD6676 for this deterministic measurement.
| Test Case |
Objective |
Description |
Passing Criteria |
|---|---|---|---|
| DL.1 |
Check the FPGA SYSREF single detection. |
Check that the FPGA detects the first rising edge of SYSREF pulse. Read the status of sysref_singledet (bit[2]) identifier in the syncn_sysref_ctrl register at address 0x54. |
The value of sysref_singledet identifier should be zero. |
| DL.2 |
Check the SYSREF capture. |
Check that the FPGA and ADC capture SYSREF correctly and restart the LMF counter for every reset and power cycle. Read the value of rbd_count (bit[10:3]) identifier in rx_status0 register at address 0x80. |
If the SYSREF is captured correctly and the LMF counter restarts, for every reset and power cycle, the rbd_count value should only vary by two integers due to word alignment. |
| DL.3 |
Check the latency from start of SYNC~ deassertion to the first user data output. |
Check that the latency is fixed for every FPGA and ADC reset and power cycle. Record the number of link clocks count from the start of SYNC~ deassertion to the first user data output, which is the assertion of jesd204_rx_link_valid signal. The deterministic latency measurement block has a counter to measure the link clock count. |
Consistent latency from the start of SYNC~ deassertion to the assertion of jesd204_rx_link_valid signal. |