A highly granular electromagnetic calorimeter prototype based on tungsten absorber and sampling units equipped with silicon pads as sensitive devices for signal collection is under construction. The full prototype will have in total 30 layers and be read out by about 10000 Si cells of $1 \times 1$ cm². A first module consisting of 14 layers and depth of $7.2 X_{0}$ at normal incidence, having in total 3024 channels of 1 cm², was tested recently with $e^{-}$ beam. We describe the prototype and discuss some preliminary testbeam results on its performance with respect to position resolution, response inhomogeneity and transverse containment.