ALCT-2001 algorithm description.
Alex Madorsky,
University of Florida/Physics
This document describes the basics of the ALCT-384-2001 algorithm, which is
implemented in XCV600E FPGA. This description is provided only for the reference for
program model writers, and does not include some information (timing, some control
registers, DAQ readout description, etc.).
The algorithm is pipelined. Shown below are the steps of this pipeline:
1. Pulse extenders.
Pulse extenders detect the rising edge on each of the 384 input bits, and start the output
pulse at this moment. The output pulse duration is 6 clocks (150 ns). This is used to allow
any drifting ions created by the charged particle to reach the wires.
2. Pattern detectors
The outputs of the pulse extenders are supplied to the pattern detectors. There are 64 of
them – because there are 64 key wire groups for which the patterns can be found.
Each pattern detector can detect the following patterns:
 Two programmable “collision” patterns (A and B)
 One fixed “accelerator” pattern
The input data for the collision pattern detector are selected as shown below:
n-2 n-1 n
n-1 n
n
n n+1
n n+1 n+2
n n+1 n+2
Layer
Layer
Layer
Layer
Layer
Layer
1
2
3
4
5
6
n in this diagram is the key wire group number, which this particular pattern detector is
searching the patterns for. As you can see, the input bits form the pattern envelope
identical to the pattern used in the ALCT-2000 design. The programming of the two
programmable collision patterns A and B is implemented as a simple masking-out of the
bits, which we do not want to include in the pattern. Shown below is the pseudo-language
formula for pattern detection:
number of layers hit for collision pattern A or
(L1[n-2]& m1) | (L1[n-1]& m2) | (L1[n] & m3)
(L2[n-1]& m4) | (L2[n] & m5)
(L3[n] & m6)
(L4[n] & m7) |
(L5[n] & m9) |
(L6[n] & m12) |
B =
+
+
+
(L4[n+1]& m8)
+
(L5[n+1]& m10) | (L5[n+2]& m11) +
(L6[n+1]& m13) | (L6[n+2]& m14) ;
Lx in this expression means Layer x, [y] means bit y (from the layer), mz means collision
pattern A or B mask bit z. The user can program different masks for collision patterns A
and B.
Accelerator pattern is a vertical pattern which cannot be reprogrammed. The formula for
it looks like this (see “formula for collision pattern diagram” above for explanation):
number of layers hit in the accelerator pattern =
L1[n] + L2[n] + L3[n] + L4[n] + L5[n] + L6[n] ;
Each pattern detector reports the number of the layers hit in collision and accelerator
patterns minus (nph_pattern - 1) value. These outputs are called “quality of the track”
(see explanation below). One collision and one accelerator pattern are reported. patb
output bit shows which collision pattern (A or B) was found.
There are several parameters which user can adjust that change the functionality of the
pattern detector. They are described very well in the ALCT-2000 document, page 55. The
names are:
trig_mode
nph_thresh
nph_pattern
drift_delay
3. Ghost Cancellation Logic (GCL)
The outputs of the pattern detectors are connected to the ghost cancellation logic. This
block is totally new to ALCT-2001 design, it was not present in the ALCT-2000.
Tracks passing through the chamber often get registered by two or more pattern detectors,
especially if the user programmed wide patterns. This way the “ghost” track(s) are
reported along with the actual track. To avoid that, GCL analyzes the pattern detector’s
outputs, and the ghost gets cancelled if:
 There is track in the neighboring key wire group
 This other track has better quality than the ghost
 This other track was detected in the same clock or up to 4 clocks before the ghost
Please note that if the ghost and the actual track have the same quality, nothing is
cancelled, because the algorithm does not know which track is ghost. (We are still
thinking how to improve that)
The cancellation is done separately for collision and accelerator patterns.
4. Best track searcher.
The outputs of the GCL are connected to the best track searcher, which simply selects the
track with the best quality. The algorithm for it is directly borrowed from the ALCT-2000
design.
5. Track promotion.
The best collision and accelerator tracks that are found in the previous step are modified
according to the following configuration variable:
alct_amode
Please see ALCT-2000 document page 56 for details.
6. Mask out the best track.
This step just masks out the best track in the data coming from pattern detectors, so the
next steps can search for the second best track.
7. Second best track search and promotion.
This step repeats steps 4 and 5 for the second best track.
Finally, the following parameters for each of the two best tracks are reported to the output
of the board:




Quality of the track
Key wire group
Pattern B flag (0 means pattern A was found, 1 means pattern B)
Accelerator muon flag (1 means accelerator pattern was found)