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Hello everyone, today we will learn about the RTC circuit standby conditions of the domestic computer mainboard.

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The content of this lesson includes: the power supply of the RTC circuit, the reset of the RTC circuit,

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and the clock signal of the RTC circuit

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First of all, after plugging in the ATX power supply,

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the power supply will provide a 5VSB standby power supply to the mainboard in the standby state

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The 5VSB standby power supply will use this regulator to reduce the 5VSB to 3.3 VSB

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Then, after the mainboard converts 5VSB into 3.3VSB through UP8801S,

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it will be sent to a double diode next to the battery holder

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3.3VSB will replace the button battery for power supply, generate RTC power supply,

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and provide power for the bridge RTC circuit module

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When the ATX power supply is not plugged in, and there is only a button battery on the mainboard,

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only the button battery provides power for the RTC circuit

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Let's zoom in on this piece of the circuit to have a look.

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D37 is a double diode

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What it comes over here is 3VSB, which comes from the UP8801S above

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The diode end comes from the button battery holder

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When plugged into the ATX power supply,

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it will be powered by the 3.3VSB main standby power supply instead of the button battery

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When the ATX power supply is not plugged in, the above 3.3VSB is 0V

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At this time, the button battery continues to provide power to the RTC circuit,

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so the RTC circuit is always powered

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The measured voltage at the cathode of this diode is 3.12V

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The 3rd pin of UP8801S is 5VSB power supply

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Its pin 6 output is 3.3VSB, and the actual measured voltage is 3.269V

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This is where its RTC is powered and where it is measured

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In the absence of a circuit diagram, we almost always find a double diode next to the battery holder

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to measure the power supply of the RTC

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There are three RTC circuits.

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In addition to the RTC power supply, there are also RTC reset signals and clock signals.

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The reset signal is called RTC_RTCRSTn

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The clock of the RTC circuit is a 32.768KHz crystal oscillator

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The measurement point for this RTC reset signal usually has a jumper cap in the mainboard

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The location of this jumper is called CLR_CMOS2

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This is pin 1, pin 1 is NC

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The 2nd pin is the RTC reset signal, and the 3rd pin is the ground

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When we connect its pin 2 and pin 3 together through a jumper cap,

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it will pull down the reset signal and clear the CMOS settings

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In the normal state, we need to jump this jumper on the two pins of the NC pin and the reset signal.

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The measured voltage of this RTC reset signal is 3.26 V

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This is the reset signal measurement point, usually measured on the jumper cap

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Let's take a look at its clock

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Regarding the RTC clock, there is usually a crystal oscillator next to the bridge

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After zooming in, we can see this iron shell crystal oscillator

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To measure its waveform, it is generally measured on the resonant capacitor next to it.

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The two resonant capacitors are very light in color and small in size

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Its capacity is picofarad level

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The actual measured waveform is 32.768KHz

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The waveform is this shape, the peak value shown here is 576 millivolts

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There is no way to measure its frequency without an oscilloscope

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Ok, these are the standby conditions for the RTC circuit