PCIe400 generic readout board qualification test

The PCIe400 is a generic board for high-throughput data acquisition systems in high energy physics experiments. Its purpose is to interface up to 48 bidirectional links, supporting custom protocols at 1 to 26 Gbit/s, to modern commercial back-end links providing 400 Gbit/s bandwidth. It also targets clock distribution with phase determinism below 10 ps peak-to-peak. It has been designed for LHCb LS3 enhancement upgrade with experimental features to prepare LHCb Upgrade II, foreseeing an aggregated throughput of 200 Tbit/s. However, its versatility allows it to be used in several experimental environments. The board embeds Altera’s flagship Agilex 7 M-series FPGA with a PCIe Gen 5 interface and an experimental QSFP112 serial interface. We present the results of qualification tests performed on prototype boards and the challenges encountered to meet specifications. Section 1 describes board-level validation, including power-up behavior and peripheral access. Section 2 focuses on high-bandwidth interface qualification through BER measurements. Finally, Section 3 investigates phase determinism in Agilex transceivers, a key requirement for precise clock distribution.

💡 Research Summary

The paper presents a comprehensive qualification campaign for the PCIe400, a generic high‑throughput read‑out board developed for the LHCb LS3 enhancement and the upcoming Upgrade II, but also intended for broader use in high‑energy physics experiments. The board integrates an Intel (formerly Altera) Agilex 7 M‑series FPGA with 1.3 M ALMs, 32 GB of HBM2e memory, up to 48 bidirectional custom links (1–26 Gb/s each), a PCIe Gen 5 x16 interface, and an experimental QSFP112 400 Gb/s Ethernet interface. The authors structure the qualification into three main sections: power‑up and peripheral validation, high‑bandwidth link integrity, and phase‑deterministic clock distribution.

In the power‑up study, 23 voltage rails were measured with a custom coaxial probe and DC‑blocking capacitor. All rails met the ±0.5 % voltage accuracy and ripple specifications, and the idle FPGA consumed 24 W. Initial power‑up times exceeded the 100 ms requirement due to a fixed 100 ms delay in the hot‑swap controller and a schematic error that prevented a DC‑DC converter from starting reliably. After correcting the schematic, the power‑up time was reduced to 134 ms; the authors note that replacing the hot‑swap controller with a zero‑delay version would provide a comfortable 60 % margin. Peripheral access was exercised via PCIe and USB/JTAG, exposing an Avalon memory‑mapped bus. A test framework built on pytest and Polars executed more than 400 compliance checks in under a minute, demonstrating robust software‑hardware integration suitable for production testing.

The high‑bandwidth interface qualification focused on Bit Error Rate (BER) measurements. PCIe performance was evaluated on a Gen 4 server in a 2 × 8 bifurcated mode, achieving a BER below 10⁻¹⁵ at 95 % confidence on both lanes, confirming correct enumeration and DMA transfers. For the 4 × 100 Gb/s QSFP112 link, the authors used PRBS‑31 patterns and an RS(544,514) forward error correction scheme. Seven hours of continuous testing yielded no uncorrected errors and at most two corrected symbols per codeword, corresponding to a BER better than 1 × 10⁻¹⁵. These results validate the integrity of the 50 mm PCB trace between the FPGA and the QSFP112 connector and the effectiveness of the on‑board 400 GbE hard IP.

Phase determinism, a critical requirement for precise clock distribution, was investigated using four 10.24 Gb/s serial channels on an Agilex development kit. The authors employed a digital dual‑mixer time‑difference (DDMTD) circuit with 1 ps resolution to measure the recovered clock (rx_clkout) after serial loopback. A deliberate one‑bit shift in the transmitted pattern produced an exact one‑unit‑interval (UI) phase shift in the recovered clock, confirming deterministic receiver behavior after synchronization. Successive transmitter resets caused quantized phase jumps of ±3 UI (≈300 ps), and statistical analysis of 500 receiver synchronizations without resets revealed a bimodal distribution separated by ~20 ps, attributed to the receiver’s clock data recovery circuit locking into two stable states. By selecting the most probable lock position and applying a ±10 ps filter window, the authors reduced the peak‑to‑peak phase variation across four channels to 6–8 ps over 500 resets spanning 12 hours, thereby meeting the sub‑10 ps phase‑determinism target.

The paper concludes that the PCIe400 prototype meets its primary specifications in power‑up behavior, peripheral accessibility, high‑speed link integrity, and phase‑deterministic clock distribution. Remaining work includes extensive testing of the PCIe Gen 5 interface, temperature‑dependent phase stability studies, and validation across different firmware compilations. Once these additional campaigns are completed, the PCIe400 will be ready for deployment in the high‑data‑rate environments of LHCb Upgrade II and other future particle‑physics experiments.