Description styles of fault-tolerant finite state machines for unmanned aerial vehicles

Abstract

The subject matter of this article is finite state machines (FSMs), which are used as control devices in unmanned aerial vehicles (UAVs). The goal of this study is to develop description styles for fault-tolerant FSMs in hardware description languages (HDLs) that prevent failures in the state register and in the input vector of the FSM. The tasks to be solved are as follows: development of description methods for FSM transitions from illegal states in case of failure in the state register, as well as for FSM transitions from each state in case of failure in the input vector; determination of FSM output vector values in case of the above failures; development of description styles for fault-tolerant FSMs; and investigation of the efficiency of the proposed description styles for fault-tolerant FSMs. The methods used are: the theory of finite state machines, state encoding methods of FSMs, description styles of FSMs, and Verilog hardware description language. The following results were obtained: two styles of describing fault-tolerant FSMs have been developed, safe0 and safe1, which do not increase the area and do not decrease the performance of FSMs, and in some cases allow the area to be reduced (for some examples by a factor of 4.8) and increase the performance (for some examples by a factor of 2.355). In addition, the description styles of fault-tolerant FSMs neutralize design errors when transitions are described in each state but not for all possible values of input variables. Conclusions. In this paper, the problem of designing fault-tolerant FSMs when the values of bits in the state register or in the input vector of the FSM change because of the negative external impact is described. Different ways of solving the problem at the level of FSM description in HDL are considered. Two description styles for fault-tolerant FSMs are proposed: safe0 and safe1. The fault tolerance of FSMs is provided in the following manner. When the input vector is not defined in the FSM specification for a specific state, the FSM will remain in the initial transition state, i.e. the FSM will not transit to another state. If the code of the illegal state is set in the state register, the FSM will transition to the start state. For all these faults, the safe0 style provides a zero output vector at the FSM output, whereas the safe1 style preserves the value of the previous output vector. A promising direction for future research seems to be the development of new styles and methods of FSM description, aimed at improving the FSM parameters (an area, a performance and a power consumption), as well as improving the reliability and fault tolerance of FSMs.