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This lesson is about the temperature detection circuit of Xiaomi mobile phone,

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which is related to the circuit that prompts abnormal temperature.

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In this circuit, we first draw its line channel

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There are two lines responsible for temperature detection,

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one leads to the third pin of the MOS tube Q5408, and one leads to the 6th pin of it

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After the D pole and S pole of the MOS tube are turned on,

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it becomes the I2C master to be responsible for temperature detection.

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The I2C bus leads here

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Both lines go to the same place

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And the I2C bus also leads to the CPU and the charging chip

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This is its link

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It passes through a composite MOS tube from the battery holder,

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and the composite MOS tube contains two MOS tubes

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After the MOS tube is turned on, it needs to go to the pull-up circuit,

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and the I2C bus must have a pull-up circuit

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After the pull-up circuit, it also leads to the charging chip

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Generally, the charging chip is responsible for detecting the temperature of the battery

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At the same time, this I2C bus also leads to the CPU

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During maintenance, if the ground value measured on the pin 3 or 4 of the battery holder is infinite,

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it is generally considered that the MOS tube Q5408

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The fastest solution is to remove the MOS tube and short-circuit its first set of d and s poles

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and the second set of d and s poles to repair the fault

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If the voltage of 1.8 volts cannot be measured at the three and four pins of the battery holder,

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check the one and four pins of the MOS tube

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If the voltage of 1.8 volts can be measured at pin 1 and pin 4 of the MOS tube,

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you need to remove the Q5408 at this time,

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and short-circuit its pin 6 and pin 1, pin 3 and pin 4 to repair it

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If the voltage of 1.8 volts cannot be measured on the battery holder,

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the voltage of 1.8 volts cannot be measured on the first and fourth pins of the MOS tube

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At this time, we have to doubt whether the pull-up circuit is normal

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If the pull-up voltage is normal, the voltage cannot be measured on this pin,

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which is usually caused by the damage of the pull-up resistors R3202 and R3203,

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just need to replace the resistors

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If the voltage of 1.8 volts cannot be measured at the power supply terminal,

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it is mostly caused by a broken wire

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Because this 1.8 volt power supply is also to power the CPU

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If the phone can be turned on, it means that the power supply is generally not damaged

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If the voltage of 1.8 volts cannot be measured,

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find the 1.8 line on other lines and use the flying lead to come over

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This is Xiaomi's temperature detection circuit

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That's all for this lesson