The LHCb Timing and Fast Control
           ==================================

           Z. Guzik, R. Jacobsson and B. Jost

            To be presented by R. Jacobsson

Abstract
----------------

In this paper we describe the LHCb Timing and Fast Control
(TFC) system. It is different from that of the other LHC
experiments in that it has to support two levels of
high-rate triggers. Furthermore, emphasis has been put on
partitioning and on locating the TFC mastership in one type
of module: the Readout Supervisor. The Readout Supervisor
handles all timing, trigger, and control command
distribution. It generates auto-triggers as well as control
the trigger rates.

Partitioning is handled by a programmable patch panel/switch
introduced in the TTC distribution network between
a pool of Readout Supervisors and the Front-End electronics.

Summary
----------------

The LHCb Timing and Fast Control (TFC) system is in the
prototyping phase. Although the backbone of the timing,
trigger and control distribution network is based on the
CERN RD12 system (TTC), several components are unique to
the LHCb experiment due to the fact that the readout
system is different from that of the other experiments in
several respects. Firstly, the LHCb TFC system has to
handle two levels of high-rate triggers: a Level 0 (L0)
trigger with an accept rate of maximum 1.1 MHz and a
Level 1 (L1) trigger with an accept rate of maximum 60 kHz.
Secondly the TFC architecture focuses on partitioning.
A partition is in LHCb defined as a configurable ensemble
of parts of a sub-detector, an entire sub-detector or a
combination of sub-detectors that can be run in parallel,
independently and with a different timing, trigger and
control configuration than any other partition.
Furthermore, the aim has been to locate the entire TFC
mastership of a partition in a single module: the Readout
Supervisor (RS). The idea is to have a pool of such
Readout Supervisors, in which one is interfaced to the
two central trigger decision units and is used for physics
data taking. The others are fully configurable masters for
testing, debugging and calibrating any partition
stand-alone.

The Readout Supervisors receive the LHC bunch clock via
the LHC machine interface. The RS used for physics data
taking also receives the L0 and L1 triggers from the
central L0 and L1 decision units. One task of the Readout
Supervisor is to distribute these, as well as internally
generated triggers and various synchronous control
commands, to the Front-End electronics via the TTC
network. The L0 decisions are transmitted on channel A of
the TTC system, and the L1 decision as well control
commands share channel B and are transmitted as short
broadcasts. The six user-definable bits of the short
broadcasts allow qualifying the L1 triggers and encoding
different types of control commands. The RS also acts as
a trigger rate controller by converting positive trigger
decisions to negative whenever data congestion occurs
in the system. Fast buffers such as in the L0 Front-End
electronics cannot feed back overflow signals and their
occupancies are therefore emulated in the RS. Slower
parts of the system signal congestion via hardware.

The partitioning is implemented by introducing a
programmable patch panel/switch in the TTC distribution
network between a pool of Readout Supervisors and the
Front-End electronics. The switch can be configured to
define independent distribution networks between the
Readout Supervisors and sets of sub-detector components
(partitions). The Readout Supervisors can thus trigger
and control different partitions independently. In order
to feed back the overflow signals from different
sub-detector components to the appropriate RS, a second
configurable "OR switch" has been devised. It performs
an OR of the signals coming from components belonging to
the same partition.