How can I know whether the microcontroller is still usable?

First, you should confirm whether the power supply voltage is normal. Use a voltmeter to measure the voltage between the ground pin and the power pin to see if it is the power supply voltage, such as the commonly used 5V. Next, check whether the reset pin voltage is normal. Measure the voltage values ​​when the reset button is pressed and released to see if they are correct. Then check whether the crystal oscillator is oscillating. Generally, an oscilloscope is used to view the waveform of the crystal oscillator pin. Note that the "X10" position of the oscilloscope probe should be used.

Another way is to measure the IO port level in the reset state. Press and hold the reset button, and then measure the voltage of the IO port (except the P0 port that is not connected to the external pull-up) to see if it is a high level. If it is not a high level, it is mostly because the crystal oscillator has not started. Another thing to note is that if you use the on-chip ROM (this is the case in most cases, and now there are very few people who use external expansion ROM), you must pull the EA pin high, otherwise the program will run around. Sometimes it works with the emulator, but it doesn't work when burning into the chip. It is often because the EA pin is not pulled high (of course, the crystal oscillator does not start oscillating is also one of the reasons). After checking the above points, the fault can generally be eliminated. If the system is unstable, it is sometimes caused by poor power supply filtering. Connecting a 0.1uF capacitor between the power pin and the ground pin of the microcontroller will improve it. If the power supply does not have a filter capacitor, you need to connect a larger filter capacitor, such as 220uF. When the system is unstable, you can try to connect a capacitor (the closer to the chip, the better).

In addition, you must be patient when debugging the system, calm down and adjust it bit by bit, and don't be anxious.