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Source | TsinghuaJoking
Today we analyze a classic single-chip microcomputer power supply circuit. The schematic diagram of the circuit is shown in the figure below:
▲ Simplified circuit of the switch circuit
before powering up the circuit. The switch "TEST" is off, and the MCU is not powered through VCC. At this time, the base of T1 is grounded through R9 (100k), and is in a cut-off state. The Test connected to the base resistor R7 of T3 and T1 are both in the cut-off state, so T3 is also in the cut-off state.
The +9V of the power supply is isolated by T3, and the voltage regulator chip IC2 is not loaded, and the output VCC of IC2 remains low.
▲ Circuit closed state
Press the button "TEST" to start the circuit, the base of T3 is grounded through R7, Test, and the be of T2, so that T3 is turned on. At this time, +9V is added to IC2 voltage regulator chip through T3. IC2 output VCC is added to the microcontroller.
After the MCU works, it outputs a high voltage through IO2, and turns on T1 through R8. Even if Test is released at this time, the base of T3 can be grounded through R7, LED1, and T1 to realize the self-locking opening of the power supply.
▲ Press TEST to start the circuit
▲ After the circuit is started, the MCU provides the base voltage of T1 to maintain the conduction of T3
Afterwards, the single-chip software can make the IO2 port become low level again, so that T1 is cut off, and then T3 is cut off.
According to the IO1 port, the switch state of T2 can be read, and then it can be judged whether the user presses the function key. After judging that the user presses Test, after the user releases Test, the IO2 can be set to a low level.
It is also possible to automatically delay power-off according to the software function, thereby reducing the consumption of power supply.
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