How to measure the chip quality
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1.? Check the power supply: Directly use a multimeter to measure the levels of VCC and GND and check whether they meet the requirements. If the VCC deviates too much from 5V or 3.3V, check the output of the 7805 or other voltage regulator or filter circuit.
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2.? Check the crystal oscillator ? This I also do not know how to check the crystal vibration is good or bad, my method is more soil: generally change a few crystal vibration on the power to try, anyway, quartz crystal vibration is not worth a lot of money:)
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3.? Check the RESET pin level logic, pay attention to whether the model used is a high level reset or a low level reset, if the MCU has been repeatedly reset state, ha ha, the result is self-evident.
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4.? Also check the EA pin if the program is designed to run from an extended external ROM.
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5.? Check whether the MCU is damaged or the flash cannot be downloaded, it is best to try a new chip.