The CDF silicon vertex trigger Page: 3 of 4
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and looser fiducial requirements reduce the effi-
ciency further; the ultimate denominator for SVT
would be all XFT-matched offline silicon tracks
that are useful for physics analysis.
SVT is a system of 150 custom 9U VME boards
containing FPGAs, RAMs, FIFOs, and one ASIC
design. CPUs are used only for initialization and
monitoring. SVT's input comprises 144 optical
fibers, 1 Gbit/s each, and one 0.2 Mbit/s LVDS
cable; its output is one 0.7 Mbit/s LVDS cable.
Three key features allow SVT to carry out in
15 ps a silicon track reconstruction that typically
requires 0(0.1 s) in software: a highly paral-
lel/pipelined architecture, custom VLSI pattern
recognition, and a linear track fit in fast FPGAs.
The silicon detector's modular, symmetric ge-
ometry lends itself to parallel processing. SVT's
first stage, converting a sparsified list of channel
numbers and pulse heights into charge-weighted
hit centroids, processes 12 x 6 x 5 (azimuthal x
longitudinal x radial) silicon planes in 360 iden-
tical FPGAs. The overall structure of SVT re-
flects the detector's 12-fold azimuthal symme-
try. Each 30 azimuthal slice is processed in its
own asynchronous, data-driven pipeline that first
computes hit centroids, then finds coincidences
to form track candidates, then fits the silicon hits
and drift chamber track for each candidate to ex-
tract circle parameters and a goodness of fit.
In SVT's usual configuration, a track candi-
date requires a coincidence of an XFT track and
hits in a specified four (out of five available) sili-
con layers. To define a coincidence, each detector
plane is divided into bins of programmable width,
typically 250-700 pm, and XFT tracks are swum
to the outer radius of the silicon detector and
binned with 3 mm typical width. For each 30
slice, the set of 32K most probable coincidences
("patterns") is computed offline in a Monte Carlo
program and loaded into 256 custom VLSI asso-
ciative memory (AM) chips. For every event, each
binned hit is presented in parallel to the 256 AM
chips, and the hit mask for each of the 128 pat-
terns per chip is accumulated in parallel. When
the last hit has been read, a priority encoder enu-
merates the patterns for which all five layers have
a matching hit. The processing time is thus lin-
ear in the total number of hits in each slice and
linear in the number of matched patterns.
There is no exact linear relationship between
the transverse parameters c, 0, d of a track in a
solenoidal field and the coordinates at which the
track meets a set of flat detector planes: the coor-
dinates are more closely linear in cos $, tan 0, and
S. But for pT > 2 GeV, Idl < 1 mm, 101 < 15 ,
a linear fit biases d by at most a few percent. By
linear regression to Monte Carlo data, we derive
the 3 x 6 coefficients V and 3 intercepts fo relating
p = (c, 0, d) to the vector f of cXFT, OXFT, and
four silicon hits: f = fo + V f x. The same regres-
sion produces coefficients C and intercepts xo,
corresponding to the fit's 3 degrees of freedom,
with which we calculate constraints ; = o+C C
and the usual X2 = 1i12. In the start-of-run down-
load, we precompute f and X for the coordinates
at the edge of each pattern and store them in flash
memory. Using each candidate's pattern ID as a
hint, the fitter board computes corrections to f
and X with respect to the pattern edge, using 8-
bit multiplication in 6 parallel FPGAs, in 250 ns
per fitted track. Tracks passing programmable
goodness-of-fit cuts propagate downstream.
3. SVT DIAGNOSTIC FEATURES
An SVT whose processing time, resolution, or
inefficiency were 20-30% larger would still have
enabled novel physics results at CDF. But an
SVT that could not be commissioned quickly or
operated reliably would have been a failure. Sev-
eral design features of SVT contributed to its
rapid commissioning and reliable operation.
The essence of SVT's component-based archi-
tecture is captured by the SVT cable and the SVT
Merger board. Nearly all SVT internal data-
hit centroids, drift chamber tracks, pattern IDs,
track candidates, and fitted SVT tracks-travel
as LVDS signals on common 26-conductor-pair
cables carrying data bits, a data strobe, a flow-
control signal, and a ground pair. The data are
variable-length packets of 21-bit words, plus end-
packet and end-event bits. Data fan-in and fan-
out are performed inside FPGAs, not on back-
planes, by a universal Merger board that con-
catenates event data for up to four SVT cable
inputs and provides two SVT cable outputs. Ev-
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Ashmanskas, B.; Barchiesi, A. & Bardi, A. The CDF silicon vertex trigger, article, June 23, 2003; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc737597/m1/3/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.