Single-chip microcumputer with clocksignal switching function

Claim 1

A single-chip microcomputer (19) provided with a clock-signal generating circuit (14) having: a main system-clock oscillation circuit (5) and a sub system-clock oscillation circuit (4); a selector (7) For outputting a clock-signal (103) by making a selection between a main clock-signal (102) which is an output of said main system-clock oscillation circuit (5) and a sub clock-signal (101) which is an output of said sub system-clock oscillation circuit (4); a selector flag setter (3) for setting a selector flag signal (116) to designate the selection to be made by said selector (7); an oscillation control flag setter (2) for setting an oscillation control flag signal (105) to control the oscillating operation of said main system-clock oscillation circuit (5); and a synchronization control circuit (6) for making the synchronization between said main clock-signal (102) and said sub clock-signal (101) and outputting a synchronization signal (121); characterized in that said clock-signal generating circuit (14) further comprises: a means for taking a logical AND operation of said oscillation control flag signal (105) from said oscillation control flag setter (2), said selector flag signal (116) from said selector flag setter (3) and said synchronization signal (121) from said synchronization control circuit (6) and outputting an oscillation control signal (120) for stopping the oscillating operation of said main system-clock oscillation circuit (5).