In the bistable chiral splay nematic (BCSN) mode, the splay and pi twist states are used for the two stable states. With a d/p ratio of 0.25, truly bistable characteristics of BCSN mode can be obtained. However, we make the splay state more stable than the 71 twist state with a d/p ratio of 0.2 for the effective transition from the 717 twist state to the splay state. Memory time of pi twist state is limited because the pi twist state is likely to be replaced by the splay state from the pixel boundary. We have theoretically and experimentally investigated how the pixel boundary affects memory time of the pi twist state with a subpixel model.