A two-level fanout system for the CDF silicon vertex tracker Page: 2 of 4
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(Table I). The EP bit specifies if the same data packet oc-
cupies only one or more words. Its value is set to 1 for the
XFT tracks since each packet (track) occupies one single
word. The EE bit marks the end of data corresponding
to one event. The Hold and Data Strobe are used for the
communication protocol among boards. The DS signal is
driven by the source board and indicates when the data
become valid and can be latched by the destination board.
The Hold signal is driven by the destination board. When
the destination board does not keep up with the incom-
ing data rate, the Hold signal is set active and the source
responds by suspending the data flow, which is resumed
as soon as the Hold goes out. This protocol is uniform
among all the SVT boards. The word with the EE bit set
to 1 is called End Event word (EE word), its data field
contains no information related to tracks but it is used to
keep track of possible errors occurred during data trans-
mission or processing and to record global information on
SVT DATA WORD FORMAT
24 23 22 21 20...0
HD DS EE EP Data field
The data field of the XFT word contains 12 bits spec-
ifying the azimuthal angle, 6 the curvature and 1 respec-
tively the charge, the isolation and the pseudorapidity of
the track. The data field in the End Event word contains
the Event Tag (8 bits), the Parity (PA) of the event (1 bit),
the Error Flags (8 bits), the L1 Trigger information (LiT)
generated by the Fanout system (2 bits) and the Level 2
Buffer number (L2B) assigned to the event by the Trig-
ger Supervisor (2 bits) . The format of the End Event
word data field is reported in Table II, where the additional
EP, EE (both set to 1), DS and HD signals are omitted.
The meaning of the information recorded in the End Event
word relevant to the Fanout system will be discussed in Sec.
END EVENT DATA FIELD FORMAT
20...19 18...17 16...9 8 7...0
L2B LiT Error Flags PA Event Tag
Two streams from the Global Level 1 trigger carry the
64 Level 1 trigger decisions to the Fanout A board. This
information is transmitted using one word per stream per
event. The format of data is the same in both streams
and uses a 33 bit data field with a DS signal to indicate
when data become valid. No End Packet, End Event and
Hold signal are present. The data field contains 32 trigger
decisions (the 32 least significant bits) and 1 bit of the
Level 2 Buffer number (Table 3). The combination of the
two L2B bits from the two streams must match the L2B
number recorded in the EE word of the XFT data.
GLOBAL LEVEL 1 TRIGGER WORD FORMAT
33 32 31...0
DS L2B L1 Trigger decisions
For each XFT track received in input, the Fanout sys-
tem outputs a two word packet through the B boards to
the proper SVT slices, depending on the azimuthal angle
of the track. The first word of the packet is used by the
AM system while the second one by the Track Fitter. The
format of the two output words is the standard SVT data
format. In the word used by the AM system only 15 out of
the available 21 bits of the data field are actually used: 12
bits contain the SuperStrip number, 3 bits the Layer num-
ber (set to 5) associated to the XFT tracks. The remaining
6 bits of the data field are spare. The word used by the
Track Fitter is a copy of the XFT input word. Since in this
case one data packet is logically made of two words, the EP
bit is set to 0 in the first word and to 1 in the second one.
The end of the output event is marked by the EE word,
which is sent simultaneously to all the SVT slices. All the
12 End Event words are identical except for the Parity bit
which can change from stream to stream because different
streams transmit different data sets (Sec. II-B).
A. Fanout A board
The Fanout A board performs the entire data handling
within the Fanout system. It receives and processes all the
input streams and constructs the output data packets as
well as the output End Event word (Sec. II-A.1). The B
boards perform simply the fanout function.
XFT tracks and L1 trigger bits are received asynchro-
nously and are stored on three sets (1 for the XFT tracks
and 2 for the L1 Trigger bits) of two 4K x 18 bit FiFos each.
The set of FiFos receiving XFT tracks provides an Almost
Full signal that is sent back to the source meaning Hold.
If the Fanout A does not keep up with the incoming data
rate, the FiFos become Almost Full and the Hold signal is
asserted. The signal is active while the amount of unread
data in the FiFos remains above the Almost Full threshold.
Although this communication protocol is uniform among
all the SVT modules, no handshake is implemented with
the Global Level 1 trigger. This is allowed by the fact that
the Level 1 trigger information is received at a much lower
rate (one word per stream per event) than the XFT tracks.
The Fanout A board transmits data to the B board
through a customized P3 backplane. The data stream con-
tains one standard SVT format word (21 bit data field plus
EP, EE, DS and HD bits), 12 "enable" bits, used by the B
boards to select which SVT slices (one or more) must re-
ceive the word, and 2 clock lines routed on the backplane to
have each B board receiving one line (Sec. IV). When the
Data Strobe bit is active, the B boards receive one word
for each clock cycle, validated at the positive edge. The
Hold line is driven and is common to both the B boards.
It is implemented as a wired-or chain. When the Hold line
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al., A. Bardi et. A two-level fanout system for the CDF silicon vertex tracker, article, November 2, 2001; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc718395/m1/2/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.