It is shown that the problem of evaluation of states both object and controller arises at digital control. In multi-circuit embedded systems time intervals of residence of system in any state depends on both time complexity of control algorithm and dispatching discipline. Two simplest disciplines, of most common use are investigated: the cyclic dispatching and foreground (quasi-stochastic) one. With use the semi-Markov process as fundamental theory, models of functioning of control programs under investigated dispatching disciplines are worked out. Mathematical relationships for time of return to any state of semi-Markov process and time between switches are obtained. The parameters obtained are essential for working out of efficient regimes of data processing in embedded systems.