Panasonic MN101C77C, F77G user manual II 20 Standby Functions

Chapter 2 CPU Basics

„HALT Modes (HALT0, HALT1)

−The CPU stops operating. But both of the oscillators remain operational in HALT0 and only the high- frequency oscillator stops operating in HALT1.

−An interrupt returns the CPU to the previous CPU operating mode that is, to NORMAL from HALT0 or to SLOW from HALT1.

„STOP Modes (STOP0, STOP1)

−The CPU and both of the oscillators stop operating.

−An interrupt restarts the oscillators and, after allowing time for them to stabilize, returns the CPU to the previous CPU operating mode - that is, to NORMAL from STOP0 or to SLOW from STOP1.

„SLOW Mode

−This mode executes the software using the low-frequency clock. Since the high-frequency oscillator is turned off, the device consumes less power while executing the software.

„IDLE Mode

−This mode allows time for the high-frequency oscillator to stabilize when the software is changing from SLOW to NORMAL mode.

To reduce power dissipation in STOP and HALT modes, it is necessary to check the stability of both the output current from pins and port level of input pins. For output pins, the output level should match the external level or direction control should be changed to input mode. For input pins, the external level should be fixed.

This LSI has two system clock oscillation circuits. OSC is for high-frequency operation (NORMAL mode) and XI is for low-frequency operation (SLOW mode). Transition between NORMAL and SLOW modes or to standby mode is controlled by the CPU mode control register (CPUM). Reset and interrupts are the return factors from standby mode. A wait period is inserted for oscillation stabilization at reset and when returning from STOP mode, but not when returning from HALT mode. High/low-frequency oscillation mode is automatically returned to the same state as existed before entering standby mode.

To stabilize the synchronization at the moment of switching clock speed between high speed oscillation (fosc) and low speed oscillation (fx), fosc should be set to 2.5 times or higher frequency than fx.

II- 20 Standby Functions