Power Saving Mode in 8051 | 8051 Power Down and Idle Mode

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Power Saving mode in 8051 microcontroller is used to save energy during idle time of processor and during power down time of processor. So, is important to make microcontroller as a power saving device.
The 8051 uses current of approximately 25 mA during the normal operation.
In power saving mode, the 8051 microcontroller reduces its total consumption to approximately 40μ
The 8051 supports two power-saving modes of operation: Idle and Power Down as shown in Figure.

Idle Mode

After setting the bit IDL in the PCON register, the microcontroller turns into idle power saving mode.
Where serial port, timers and interrupt system continue operating normally consuming 6.5 mA.
In Idle mode, the content of all registers and I/O ports is remains unchanged i.e. the CPU status is preserved entirely.
SP, PC, PSW, Accumulator and other registers remains as it is their data during IDLE mode.
In order to operate the microcontroller normally, interrupt or reset signal can be used.
Then the IDL bit is automatically cleared and the continues execution from program instruction following that instruction which has set the IDL bit.

After setting the bit PD in the PCON register, the microcontroller turns into idle power saving mode.
In this mode, 8051 turns off its internal oscillator which reduces power consumption.
In power-down mode, the 8051 microcontroller operates using only 2V power supply where the total power consumption is less than 40 μA.
The microcontroller can be brought to normal mode by using hardware reset signal only.
In Power Down mode, the contents of all SFR registers and I/O ports remains as it is.
But after the microcontroller brought into the normal mode, the content of the SFR register is lost, but the content of internal RAM is saved.
Reset signal must of approximately 10 ms so that 8051 microcontroller can stabilize quartz Oscillator operating.
In the Power Down mode, the oscillator is completely stopped, and the instruction that invokes Power Down is the last instruction executed by 8051 microcontroller.
The internal RAM and Special Function Registers save their values till the Power Down mode is removed.

Sr. No.	Power Down Mode	Idle Mode
1.	Power down mode is set by setting the PD bit of the PCON register.	Idle mode is set by setting the IDL bit of the PCON register.
2.	In the power down mode, the on-chip oscillator is stopped.	In the Idle mode, the internal clock signal is gated off to the CPU, but not to the Interrupt, Timer, and serial Port functions.
3.	With the clock frozen, all functions stopped but the state of all SFR registers, contents on-chip RAM and I/O ports remains unchanged.	The CPU status is preserved, the Stack Pointer, Program Counter, Program Status Word, Accumulator and all other registers maintain their data during Idle.
4.	Hardware reset required to terminate power down mode.	To terminate Idle mode, execute any interrupt or reset which will clear IDL bit automatically.
5.	Operate using only 2V power supply while a total power consumption is less than 40μA.	Microcontroller turns off CPU unit while peripheral units such as serial port, timers and interrupt system continue operating normally consuming 6.5mA.

The purpose of the Register PCON bits is: