For security issues in portable applications such as smart card, various proposed techniques can be applied to harden the ALU against fault attacks. Among others, time redundancy is a good candidate to offer a low hardware cost. The main disadvantage of this scheme is high extra time due to the recomputation. However, this impact can be considerably reduced by exploiting idle hardware in the ALU. In this paper, we will show that applying this scheme in a simple processor-based smart card can reduce down to 30%-20% of extra time and requires a reasonable hardware overhead about 20.4%.