Image forming apparatus having an improved power-mode switching function

A method of controlling an image forming apparatus having a communication unit, the image forming apparatus including a power supply unit, a system control unit controlling the entire image forming apparatus, the system control unit having a CPU that is set in one of a normal mode, a power-saving mode and a sleep mode by controlling the power supply unit, a real-time clock keeping track of hours, minutes and seconds of a current time and outputting a signal indicating the current time, a register, and a comparator, the method comprising the steps of:

• setting a return time in the register, the return time indicating a time the CPU is to be switched from one of the power-saving mode and the sleep mode to the normal mode;
• causing the comparator to compare the current time output from the clock with the return time stored in the register, and to output, when a match occurs, a control signal to the CPU;
• determining whether an error in the image forming apparatus is detected when an image formation is performed;
• switching the CPU from the normal mode to the power-saving mode when it is determined that no error is detected;
• determining whether reduction of power supply to the system control unit by the power supply unit during the power-saving mode is possible;
• switching the CPU from the power-saving mode to the sleep mode when it is determined that the reduction is possible;
• and
• switching the CPU to the normal mode when the comparator outputs the control signal to the CPU.