Power Saving mode operation of microcontroller

In this mode PCON (power control register).in this register two bit play important role for power control of 8051.
Below show the format of PCON

8051 has two power saving modes.they are

1. idle mode
2. power down mode.

• The two power saving modes are entered by setting two bits IDL and PD in the special function register (PCON) respectively.

POWER DOWN MODE:

the schematic diagram of power down mode shown below.

• By setting the PD bit of the PCON register from within the program, the microcontroller is set to Power down mode.
• thus turning off its internal oscillator and reduces power consumption enormously.
• The microcontroller can operate using only 2V power supply in power- down mode, while a total power consumption is less than 40uA.
• The only way to get the micro controller back to normal mode is by reset.
• While the microcontroller is in Power Down mode, the state of all SFR registers and I/O ports remains unchanged.
• By setting it back into the normal mode, the contents of the SFR register is lost, but the content of internal RAM is saved.
• Reset signal must be long enough, approximately 10mS, to enable stable operation of the quartz oscillator.

IDLE MODE:

• Upon the IDL bit of the PCON register is set, the microcontroller turns off the greatest power consumer- CPU unit while peripheral units such as serial port, timers and interrupt system continue operating normally consuming 6.5mA. 
• In Idle mode, the state of all registers and I/O ports remains unchanged.
• n order to exit the Idle mode and make the microcontroller operate normally, it is necessary to enable and execute any interrupt or reset.
• It will cause the IDL bit to be automatically cleared and the program resumes operation from instruction having set the IDL bit. 
• It is recommended that first three instructions to execute now are NOP instructions.
• They don't perform any operation but provide some time for the microcontroller to stabilize and prevents undesired changes on the I/O ports.

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