Speaker
Description
For the planned FoCal-E subdetector in ALICE, the ALPIDE pixel modules are currently being assembled and there is a need to qualify them for use. The pixel modules are tested in three stages: isolated test, assembly test and integration test. To do this, a production test system has been adopted from the Bergen proton CT project along with changes based on experiences gained. In particular, the system has been migrated from FreeRTOS to Linux to improve maintainability and operational flexibility, and support for Outer Barrel modes has been added. The test system architecture is outlined and preliminary results are presented.
Summary (500 words)
The ALICE FoCal-E commissioning relies on the successful assembly and qualification of two pixel layers, organised in 22 modules, each module containing 2 x 6 multi-layered flexible cables (strings), with 12 or 15 ALPIDEs on each. The system is composed of 3,888 ALPIDE chips in total, all of which must be qualified. A Production Test Box (PTB) system based on a Xilinx Zynq UltraScale SoC was originally developed for the Bergen proton CT project to test similar ALPIDE-based detector modules. Adapting it to FoCal-E required significant hardware, firmware and software changes. The resulting test design is relevant to other ALPIDE and MAPS-based production test systems.
The FoCal-E PTB is required to test ALPIDEs in Inner Barrel (IB), Outer Barrel master (OBM) and Outer Barrel slave (OBS) modes, and it can test one pixel string at a time in string-test mode or two ALPIDE chips in a cantilever test socket for isolated testing. A transition card connects the FireFly links on the PTB to the string flex ZIF cables for string-test mode. The PTB interfaces with a host PC running test scripts via TCP.
An assembly process has been developed to produce pixel modules and exclude the possibility of mounting defective ALPIDE chips in multi-chip strings. Special interconnection elements (chip-cables) are used to connect the ALPIDE chips to a clamshell test socket on the FoCal-E PTB, and they are tested in isolation. Only the ALPIDEs that pass the isolated test are mounted on multi-chip strings. Three strings are then assembled in parallel on the detector layer backplate, with up to 15 ALPIDEs per string depending on string type. Once fully assembled, the strings are qualified with all ALPIDE chips connected.
The pCT uses ALPIDE only in IB mode and has a different module arrangement from FoCal-E, so a new PTB hardware design with updated string pinouts and Outer Barrel support is required. The primary firmware challenge is the addition of the OBM and OBS modes. This is complicated by the difference in bus speed between IB (1.2 Gbps) and OBM (400 Mbps), which cannot be dynamically changed in the FPGA. The problem is solved by reconfiguring the programmable logic at runtime with FPGA Manager, without restarting the system. Configurable addressing of chips in the test socket has also been developed.
The pCT PTB was based on FreeRTOS with lwIP, originally chosen for fast response to power transients. Since pCT assembly showed that this capability was not critical, and bare-metal maintenance proved difficult, the system has been ported to Linux. SSH access and standard debugging tools (GDB, tcpdump, perf) are now available, while the option to run time-critical software is preserved by the auxiliary Cortex-R5 core.
The test system architecture and experience from development and testing will be presented, together with the per-chip and per-string test time, and the FPGA reconfiguration latency between IB and OB modes.