| 摘要 |
<p>PURPOSE:To prevent a drop of the processing efficiency by providing a first detecting circuit for detecting an ON state of a body power source, and a second detecting circuit for detecting a voltage value of a stand-by power source, and executing a selection of both these power sources. CONSTITUTION:A microcomputer 1 is provided with 30 terminals in addition to a feeding terminal VDD and an input/output port, and uses two ports P, Q among them. When this port P is in an H level, in case a body power source is connected to the feeding terminal VDD, the microcomputer 1 executes all operations as per a command, and in the case of an L level, each port of the microcomputer 1 becomes a holding mode, in a state that the command has been received. Also, as for the port Q, if the port P is H, whichever input/output of H or L are provided, no influence is exerted on the port P, and when the port P is L, when the port Q becomes L, the holding mode of the microcomputer 1 is released. Subsequently, by a voltage comparator Comp, an ON state of the body power source, and an operating voltage of a stand-by power source are detected, and an appropriate power source selection can be executed.</p> |
