  The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC)
is designed to study a wide range of high energy physics phenomena. It employs
a large all-silicon tracker within a 3.8 T magnetic solenoid, which allows
precise measurements of transverse momentum ($p_\mathrm{T}$) and vertex
position. This tracking detector will be upgraded to coincide with the
operation of the High-Luminosity LHC, which will provide luminosities of up to
$7.5\,\times\,10^{35}\,\mathrm{cm^{-2}s^{-1}}$ to CMS, or 200 collisions per 25
ns bunch crossing. This new tracker must maintain the nominal physics
performance in this more challenging environment. Novel tracking modules that
utilise closely spaced silicon sensors to discriminate on charged particle
$p_\mathrm{T}$ have been developed and allow the selective readout of hits
compatible with tracks of $p_\mathrm{T}\,>\,2-3$ GeV to off-detector trigger
electronics. This would allow the use of tracking information at the Level-1
trigger of the experiment, a requirement to keep the Level-1 triggering rate
below the 750 kHz target, while maintaining physics sensitivity. This paper
presents a concept for an all-FPGA based track finder using a fully
time-multiplexed architecture. Hardware demonstrators have been assembled to
prove the feasibility and capability of such a system. The performance for a
variety of physics scenarios will be presented, as well as the proposed scaling
of the demonstrators to the final system.
