The ALICE silicon pixel detector (SPD) constitutes the two innermost layers of the
ALICE inner tracker system which contains 10 million pixels organized in 120
detector modules called half-staves. Each half-stave contains one multi chip
readout module and two detector ladders with each 5 pixel chips bump-bonded to one
sensor. The ALICE SPD off-detector electronics controls, configures and reads out
the detector via bidirectional optical links. The front-end data streams are
processed in 20 readout modules (Router), based on FPGAs, each carrying three 2-
channel link-receiver daughter cards. The processed data are sent to the ALICE-DAQ
system on the ALICE detector link (DDL) for permanent storage. The SPD control,
configuration and data monitoring are performed using the VME interface of the
Each of the two link-receiver channels has three optical fibre links; two links for
the clock and the serial trigger, control and configuration data and one 800 Mbit/s
G-link compatible link to receive data from the detector.
The Routers receive the trigger control signals from the ALICE Central Trigger
Processor (CTP) through the on-board TTCrx chip and forward the trigger commands to
the pixel detector. Upon reception of the L1 trigger signal the pixel data are
copied into multi-event buffers on the pixel chips. After reception of a positive
L2 decision data are sent from the detector to the link receivers. On the link
receiver the pixel data stream is de-serialized and stored in a buffer-FIFO before
data are zero suppressed, encoded, re-formatted and written to a dual port memory.
Once the link receiver has processed the data the router processor is informed to
read the data from the link receiver dual port memory.
Each Router sequentially reads one event from each of the link receiver channels in
order to merge data from the 6 channels and label them with trigger and status
information to build one router sub event. The sub events of each of the routers
are sent to the ALICE-DAQ system through the ALICE detector link (DDL). The read
out data stream can also be copied to a dual port memory, where it is accessible
for data monitoring and analysis via the VME-interface.
The data access for the SPD control and configuration is performed via the router
VME-interface. The router converts the data to JTAG compatible commands which are
sent to the detector on the optical links with a maximum data rate of 5 Mbit/s.
The full SPD read out chain was successfully operated with two half staves during a
beam test in November 2004 where detector modules of all ITS systems (silicon pixel
(SPD), silicon drift (SDD) and silicon strip detectors (SSD)) where installed. The
functionality and the interface of the off-detector electronics to the ALICE TTC,
the DAQ, a simplified version of the DCS system and the on-detector electronics
were verified in a combined run with SSD and SDD during 11 hours where some 5 000
000 events were recorded without errors.
The SPD detector system including the off-detector electronics is currently pre-
installed in the CERN DSF where the full detector system is integrated and
commissioned before being moved to the underground area.