10-Nov-2005

1
Thoughts on Data Transmission US-ATLAS Upgrade
RD Meeting UCSC 10-Nov-2005 A. A. Grillo SCIPP
UCSC
2
Old Guidelines for the Present SCT DAT

• Avoid shared parallel write busses where one
  failed IC tends to take out the whole bus, i.e.
  use serial data paths.
• Provide redundant data transmission paths onto
  and off of the detector.
• Employ optical transmission for all high speed
  communication between the detector unit and the
  off detector electronics.
• Saves some services volume (fibre vs. wire).
• Minimizes interference between different detector
  units making scale up based upon test results of
  a small subset more viable.
• Provide a mechanism to bypass a dead chip.
• All electrical communication between ICs or
  between the detector units must be via low
  amplitude fully differential signals.
• Minimizes electrical noise.

3
Present SCT Data Transmission System
Apologies for the extremely old picture.
Two data links, 1 clk, 1 comper module
VCSELs not LEDs Ignore patch panel and fibre
lengths
Redundantclk com
4
Interconnects on Each Module
One Detector Module with Twelve ICs
2 serial outputs
2 serial outputs
Clk Com not shown but bussed in parallel to
all ICs. Likewise for redundant Clk Com.
N normalB bypass
5
Transmission Data Rates Multiplexing

• The natural clock frequency of the readout ICs is
  the beam crossing frequency or perhaps x2 the
  beam crossing frequency.
• Now at BC Frequency 40 MHz
• Could become 67 MHz, 80 MHz or even 100 MHz
• Optical transmission now typically operates at
  GHz rates
• If such rates are attainable with rad-hard
  components, fibre count will be reduced.
• Operating the optical transmission at GHz rates
  implies multiplexing
• Difficult to operate electrical transmission at
  those rates
• Multiplex several serial chains with several ICs
  in each chain (figure).
• The number of chains and the number of ICs in
  each chain will depend upon the details of the
  detector unit (e.g. stave).
• Higher rates for each chain will add flexibility
  for layout options.
• Each chain may require phase adjustment.

6
Possible Multiplexed Architecture
7
The Present Bypassing Scheme

• The present bypass scheme has worked well but
  some difficulties
• The layout of the hybrid circuit has added
  complexity with the addition of four extra traces
  for each IC. The differences in capacitive load
  of the normal and bypass traces required slightly
  different designs for the two drivers/receivers
  in order to keep power to a minimum.
• The connections to bypass an IC at the edge of
  the hybrid required very long traces. These long
  traces required a much more powerful differential
  driver (i.e. more power consumed) and these
  signals became the function limiting the
  operational speed of the IC.
• Some of the redundant paths so complicated the
  topology of the hybrid layout that one was not
  implemented precluding the bypassing of one IC.

8
A Simpler Bypassing Scheme
Define two directions for the data flow (e.g.
left and right) If an IC fails, the data flow
direction of ICs are arranged such that all to
the left of the dead IC send data to the left and
all to the right of the dead IC send data to the
right (figure). Both left and right drivers
drive only short path to adjacent IC Add third
driver (powered only when needed) to drive long
line to multiplexer or displaced section of
chain. (Equivalent of master driver on present
IC.) Number of drivers and hybrid traces are not
reduced but trace routing is simpler and drivers
are more efficiently matched to loads.
9
Bypassing a Dead IC
10
Configuration Read Back

• The present SCT readout IC, the ABCD, does not
  allow for read-back of its configuration
  registers.
• Tests for single event upset in a particle beam
  indicate that we should not have a problem if we
  periodically reload the registers.
• The higher fluences of the upgraded ATLAS and the
  smaller geometries of the IC structures could
  create a worse problem.
• Cannot tell in situ what the real error rate is
  the re-load rate must be set to the worst case
  estimate.
• Upgrade IC should provide for read back of
  configuration registers.
• To minimize fibres, must use same output as for
  data.
• Protocol developed for the SCT readout provided
  for such configuration read back but not
  implemented because of space required for extra
  circuitry. Need to re-examine the protocol to
  see if it can be improved, then implement in the
  new IC design.

11
Command Protocol

• Present command protocol developed to allow
  triggers, short commands and long configuration
  commands using the same input stream.
• A primary 3-bit identifying sequence was chosen
  in order to allow consecutive triggers to appear
  with only a gap of two beam crossing periods.
  (An ATLAS trigger requirement)
• Later relaxed to a longer gap. The Pixel IC made
  use of a longer bit stream making it more immune
  to bit flips in the command sequence.
• Keep longer pattern as Pixels (if still allowed
  by trigger) or increase data rate to allow longer
  sequence as long as SEU rate acceptable.
• If beam crossing rate increases, the command data
  rate must increase proportionally.

12
Encoding of Clock and Command Signals

• Clock and Commands carried on one fibre with
  biphase-mark encoding
• Signal decoded on module into separate LVDS
  signals
• Decoding could be in same IC as multiplexer
• Would allow path for configuring multiplexer
• Possible to multiplex more than one command
  stream on the same fibre.
• Reduce time for frequent re-configuration (if
  necessary)
• But higher data rates will aggravate SEU
  problems
• Need separate electrical drivers for each chain
  of ICs
• Isolate failures
• Possible need for phase adjustments for each
  chain
• Could be advantage to maintain encoding in
  electrical chains and decode at each IC (figure).
• Reduce number of drivers and dual traces

13
Keeping Clk/Com Encoded in E-Streams
14
Redundant Clock Command Signals

• Important to maintain redundant clock and command
  signals.
• Present scheme of sharing clk/com fibre of
  adjacent module as redundant stream creates some
  problems.
• Electrically couples adjacent modules destroying
  isolation
• Electrical connection between modules is
  mechanically awkward.
• Preferable to have two clk/com fibres per
  detector unit to maintain electrical isolation.