Generally speaking, the microcontroller is inactive for the most part and just waits for some external signal in order to takes its role in a show. This can cause some problems in case batteries are used for power supply. In extreme cases, the only solution is to set the whole electronics in sleep mode in order to minimize consumption. A typical example is a TV remote controller: it can be out of use for months but when used again it takes less than a second to send a command to TV receiver. The AT89S53 uses approximately 25mA for regular operation, which doesn't make it a pover-saving microcontroller. Anyway, it doesn’t have to be always like that, it can easily switch the operating mode in order to reduce its total consumption to approximately 40uA. Actually, there are two power-saving modes of operation:Idle and Power Down.

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. In 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.

Power Down mode

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 microcontroller 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.

PCON register

The purpose of the Register PCON bits is:

• SMOD Baud rate is twice as much higher by setting this bit.
• GF1 General-purpose bit (available for use).
• GF1 General-purpose bit (available for use).
• GF0 General-purpose bit (available for use).
• PD By setting this bit the microcontroller enters the Power Down mode.
• IDL By setting this bit the microcontroller enters the Idle mode.