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Hello everyone, today we will learn the standby circuit and power-on circuit on the mainboard

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After the ATX power supply is connected to the mainboard, plug in the AC 220V AC power supply

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At this time, the ATX power supply will output a 5VSB standby voltage and a high-level power-on signal

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After the purple 5VSB is supplied to the mainboard,

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it is stepped down to 3.3V by the voltage regulator on the mainboard.

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Here is the 3.3V output of the regulator

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It converts the purple 5VSB into a 3.3V main standby power supply

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If we get a mainboard without a circuit diagram,

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how to find the 3.3V standby power supply?

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In fact, we can measure on the B10 pin of the PCIE slot

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The PCIE slot refers to this slot

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The side near the CPU is side A, and the other side is side B

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The B10 pin refers to the 10th pin on the B side, counting from the 1st pin here

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B10 pin can measure the standby voltage of 3.26V

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In the absence of a circuit diagram, the 3.3V standby power supply can be measured at this fixed pin

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This 3.3V voltage is for the bridge to provide standby

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In addition to 3.3V standby, there is a 1V standby voltage on the back of the mainboard

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There is a 1V standby power supply here

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Then, there is also a 1V standby power supply

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These are the two 1V standby voltages of this board

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For this mainboard, we tested its standby conditions according to the standard timing,

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and found that in addition to 3.3V main standby power supply and 1.0V main standby power supply,

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it also has RTC circuit conditions

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The RTC circuit is usually next to the button battery and this bridge chip

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Ok, let's take a look at the power supply of the RTC first

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Generally, there is a double diode next to the battery to measure the 3.1V RTC power supply.

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A diode has two positive poles

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Connect the positive side of the battery to the battery voltage of 3.0V,

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and the other side is from the 3.3V standby voltage

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This 3.3V standby voltage has a voltage drop after passing through the diode and becomes 3.1V

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This is the power supply of the RTC

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Clock for RTC circuit

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Next to this bridge is a crystal oscillator

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The crystal oscillator has two pins, one pin 0.2V,

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one pin 0.3V, and the two pins have a voltage difference

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Measured with an oscilloscope, there is a clock frequency of 32.768KHz

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Ok, this is the clock for the RTC circuit

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The reset signal of the RTC circuit has a jumper called "CLR CMOS" on the mainboard

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This jumper cap has three pins, the top pin is ground

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The middle pin is the RTC reset signal,

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which is powered by the RTC to provide a pull-up to 3V high level, which goes to the bridge

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The other pin is an idle pin

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In the normal state, the jumper should jump on the reset signal and the empty pin,

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and the reset signal cannot be connected to the ground.

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If connected to ground, this reset signal will be pulled low, thus clearing the CMOS settings

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Ok, this is the reset signal measurement for the RTC circuit

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Next, there is an IO, which can also be called EC

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The standby condition of IO is power supply, clock, reset and its program

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The IO power supply has an inductance next to the IO that can be measured,

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also called FB, which is a safety inductance

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When there is no circuit diagram and no such obvious power supply insurance,

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we can check whether there is a filter capacitor next to the IO

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Measured on the capacitor pin, you can also measure its standby voltage

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Its clock is next to the IO, there is a 32.768KHz crystal oscillator

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Measure its pin voltage, one side is 0.9V, the other side is 1.3V

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As long as there is a voltage difference between the two pins, there is generally a waveform

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In the absence of an oscilloscope,

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we can use the substitution method to judge whether the crystal oscillator is good or bad

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If there is voltage and no waveform, the crystal oscillator is usually damaged.

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No voltage and no waveform generally means that there is a problem with the IO,

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or the working conditions of the IO are insufficient.

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The IO standby reset signal cannot be found without a circuit diagram

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OK, this skips the reset signal measurement

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The IO program generally has a Flash ROM next to the IO, which stores the IO program

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Usually we measure its first chip select signal with an oscilloscope to see if it has a waveform

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We need to use an oscilloscope to judge

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Ok, this is the standby condition for the IO

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Next, the switch pin should have a voltage

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If we press the switch, the switch signal will be sent to the IO,

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and the IO will send the switch signal to the bridge

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The bridge chip will send ACPI_S4n, ACPI_S3n, ACPI_S5n and other signals to the IO,

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pull down the green line through the IO, and turn on the power supply of each channel

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At this time we can test

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When in standby mode, the yellow line on the CPU power supply side has no voltage,

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the orange line here also has no voltage, and the red line has no voltage.

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These are powered only after pressing the switch

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The gray one is the PG signal, and there is no voltage.

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At this time, we press the switch and the mainboard will be turned on.

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We can measure the orange 3.3V, normal

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The red one is 5V, normal

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The yellow 12V here is also normal

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And the power supply outputs a PG signal of 5 volts

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This power supply has started normally