MVSTT: A Multi-Value Computation-in-Memory based on Spin-Transfer Torque Memories

Analog Computation-in-Memory (CiM) with emerging non-volatile memories leads to significant performance and energy efficiency. Spin-Transfer Torque Magnetic Memory (STT-MRAM) is one of the promising technologies for CiM architectures. Although STT-MRAM has various benefits, it does not have the potential to be used directly in analog multi-value CiM operations due to its limited levels of cell resistance states. In this paper, we propose a novel flexible multi-value design for STT-MRAM (MVSTT) with the potential to be used for multi-value CiM. In the multi-value CiM, we are able to have various 2s resistive state combinations from $s$ selected MTJs, which is not possible in the normal STT-MRAM CiM. The size of the MVSTT can be adjusted at run-time depending on the application's requirements. The benefits of the proposed scheme are quantified in representative applications such as multi-value matrix multiplications, which is the basic computation of Neural Networks applications. For the multi-value matrix multiplication, the energy, and delay gain is up to 9.7 × and 13.3 ×, respectively, to non-CiM matrix-vector-multiplication. Also, for the neural network, the proposed design allows up to a 32 × reduction in the STT-MRAM cells per crossbar to achieve a similar inference accuracy as the binarized neural network.