Power On Self Test – Demystified

By | March 10, 2012



As a person who is using a computing equipment , You are aware about the term called POST and you also know it is expanded as Power on Self Test. Yes the same process which begins after you press the power button of your system .  This post is an effort to move a little deeper in to the process of POST and the electronic details which may be useful to a chip level service person for his day to day life with Desktop and Laptop Motherboards .

Let us start our discussion with basic explanation of a typical motherboard . For the ease of explanation , we are using a desktop motherboard , but is also applicable to a laptop motherboard too.

The motherboard diagram given here only shows components  that take part in the power on self test to avoid  complexities


The ATX Power supply of the system comes to life at the moment you press the power button on the system  . the ATX power supply is turned on by the pluse produced by the power on switch . Before the system turns on by the power switch , the system is in Standby with ACPI.

3.3 volt  from the power supply will reach 6th pin of the IO   it turns the NPN  transistor attached to it to  active low

Npn transistor  will open way for 3 vdc to reach BIOS

BIOS will in turn  activate the I/O .

I/O and VRM ( voltage regulator Module ) will  collectively  produce the Power good signal ( PG ).

BIOS will produce data pulse which will reach the crystal oscillator circuit

14.3185  MHz generator by the crystal oscillator  will  produce 2 v ac  and clock generator IC will produce the output

12th pin of clock generator  will produce the pulse on the  27 th pin of the CPU and CPU will initiate the POST routine from the BIOS chip

The POST routine is stored in address location FFFF:O and is the basic program to test the functionality of all the components on your motherboard and add on cards

After the success full completion of thew POST , BIOS will load load the Bootstrap loader program which will initiate the operating system loading process typically called  Booting


( hope we can have a post on the process of Booting )


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