  The Mu2e electromagnetic calorimeter is made of two disks of un-doped
parallelepiped CsI crystals readout by SiPM. There are 674 crystals in one disk
and each crystal is readout by an array of two SiPM. The readout electronics is
composed of two types of modules: 1) the front-end module hosts the shaping
amplifier and the high voltage linear regulator; since one front-end module is
interfaced to one SiPM, a total of 2696 modules are needed for the entire
calorimeter; 2) a waveform digitizer provides a further level of amplification
and digitizes the SiPM signal at the sampling frequency of $200\
\text{M}\text{Hz}$ with 12-bits ADC resolution; since one board digitizes the
data received from 20 SiPMs, a total of 136 boards are needed. The readout
system operational conditions are hostile: ionization dose of $20\
\text{krads}$, neutron flux of $10^{12}\ \mathrm{n}(1\
\text{MeVeq})/\text{cm}^2$, magnetic field of $1\ \text{T}$ and in vacuum level
of $10^{-4}\ \text{Torr}$. A description of the readout system and
qualification tests is reported.
