The complexity of embedded devices increases as today's applications request always more services. However, the power consumption of systems-on-chip has significantly increased due to the high-density integration and the high leakage power of current CMOS transistors. To address these issues, emerging technologies are considered. Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM) is seen as a promising alternative solution to traditional memories thanks to its negligible leakage current, high density, and non-volatility. In this work, we present the design and evaluation of a 128 kB STT-RAM in 28-nm FD-SOI technology with SRAM-like interface for ultra-low power microcontrollers. With 0.9 pJ/bit read in 5 ns and 3 pJ/bit write in 10 ns, this embedded non-volatile memory is suitable for devices that run at frequencies under 100 MHz. Considering low-power application with duty-cycled behaviour, we evaluate the STT-MRAM as a replacement of embedded Flash and SRAM by comparing single and multi-memory architecture scenarios.