Analysis of power circuit of a classic single-chip microcomputer

Analyze today classic single-chip microcomputer power supply circuit. The schematic diagram of circuit is shown in figure below:

▲ Simplified breaker circuit diagram

Before turning on circuit. The "TEST" switch is off and MCU is not powered through VCC. At this time, T1 base is grounded through R9 (100kΩ) and is in off state. The test connected to base resistor R7 of T3 and T1 is in cutoff state, so T3 is also in cutoff state.

The +9V of power supply is isolated to T3 and is not loaded on voltage regulator IC IC2, and the VCC output of IC2 remains low.

▲ Closed circuit status

Press "TEST" button to start circuit, base of T3 is grounded through R7, Test, b-e T2, so that T3 turns on. At this time, +9V is added to voltage regulator IC IC2 via T3. An IC2 VCC output is added to microcontroller.

After MCU is running, it outputs a high voltage through IO2 and turns on T1 through R8. Even if a test is run at this time, base of T3 can be grounded through R7, LED1 and T1 to open electric lock.

▲ Press TEST to run circuit

▲ After starting circuit, MCU provides base voltage T1 to maintain T3 conduction

The microcontroller software can then pull IO2 low again to disable T1 and then T3.

You can read state of T2 switch according to IO1 port and then determine if user has pressed a function key. After determining that user presses Test, after user releases Test, IO2 can be set to low.

According to software function, it can also automatically delay power off, thus reducing power consumption.