We introduce the Serial-MAC-engine (SMAC-engine), a fully-digital hardware accelerator for inference of quantized DNNs suitable for integration in a heterogeneous System-on-Chip (SoC). The accelerator is completely embedded in the form of a Hardware Processing Engine (HWPE) in the PULPissimo platform, a RISCV-based programmable architecture that targets the computational requirements of IoT applications. The SMAC-engine supports configurable precision for both weights (8/6/4 bits) and activations (8/4 bits), with scalable performance. Results in 65 nm technology demonstrate that the serial-MAC approach enables the accelerator to achieve a maximum throughput of 14.28 GMAC/s, consuming 0.58 pJ/MAC @ 1.0 V.