Electronics in the Pixel Detector of LHC

The pixel detector is a huge array of CMOS sensors that capture the radiation emitted during the experiments. The pixels are interfaced to chips called front-ends that capture the data. The captured data is then sent over optical links to the processing system called Read-out Detector(RODs) which is a few 100 meters away isolated from radiation.

The front-end is an ASIC and ROD is an FPGA. The communicate using CERN’s Slink protocol.

The ROD sends out a trigger signal whenever it wants the front-end to capture data. The front-end has a buffer that stores all incoming triggers when the no. of triggers are greater than it’s processing time. When the buffer is full, the front-end skips triggers and sends a skipped trigger count to the ROD.

Problem

There is a bug in the ASIC which results in the front-end producing wrong skipped trigger counts. The ROD which is keeping track of the triggers it sent out and triggers it received gets confused as result. This results in a desynchronization. When the desynchronization occurs, all useful data is also discarded since it is not reliable.

Solution

Keep track of the no. of triggers sent to the FE. If the number is greater than the size of the buffer, stop sending triggers. In-short, bypass the skipped trigger mechanism in the front-end and handle it in the ROD before it even goes out.

When a trigger needs to be skipped, an empty frame is created to keep the system in sync.

Implementation

Step 1: Identify Triggers

The incoming control signal is sent serially. Therefore a shift register needs to be implemented to detect the pattern “11110” for a trigger.

Step 2: Keep track of the no. of triggers

For each new trigger, increment a counter. Decrement the counter whenever a module trailer is received from the front-end.

Step 3: Set the threshold

The threshold is configurable by the user, therefore implemented as a register. The user uses a command to write the threshold onto the register.

Step 4: Decide whether trigger is skipped or forwarded

If the value of the counter in step 2 is greater than the threshold in step 3, the trigger is not forwarded. Insert an empty frame for the module header.

Step 5: Update counters

Everytime a trigger is skipped, update a counter so we can monitor the inefficiency of the system.

Header consists of a 32-bit word. There are bits to indicate L1ID, BCID, formatter number.

Trailer consists of a 32-bit word. Some bits in the trailer are set to indicate errors like timeout, dataoverflow etc.