Fully connected neural network (FNN) for letter recognition. (A) Schematic diagram of FNN for letter recognition. The 5 × 5 pixel image of the letters “F,” “D,” and “U.” The grayscale values of the pixels were the inputs of the FNN. (B) Circuit diagram of an m × n array composed of 2T1C DRAM cells. (C) Recognition rate as a function of the training epochs. The inset shows the distribution of the 25 × 9 weights after 20 training epochs. (D) Colormaps of weights before and after training. These five-level weights represent the weight voltages V_w written to the 2T1C DRAM cells.