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.
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6.? If the above points are OK, it is reasonable to say that the hardware should run normally (in case, you can also write a shorter parallel port lighting program to test the smallest system)... If the test program runs properly. It is basically determined that the problem is the control program, in keil repeatedly trace the debugger, pay attention to the working register group, accumulator, DPTR, etc., after calling the subroutine is expected.
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How to judge the quality of ic chip
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First, no road detection
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This method is carried out when the ic is not welded into the circuit, under normal circumstances, a multimeter can be used to measure the positive and reverse resistance values between each pin corresponding to the ground pin, and compare with the intact ic.
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Second, on the road detection
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This is a multimeter to detect the DC resistance of each ic pin in the circuit (ic in the circuit), AC and DC voltage to ground and the total working current detection method. This method overcomes the limitation of substitutable ic and the trouble of disassembling ic by substitution test method, and is the most common and practical method for detecting ic.
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2. Dc operating voltage measurement
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This is a way to measure the DC supply voltage and the working voltage of peripheral components by using a multimeter DC voltage block in the case of power-on. Detect the DC voltage value of each ic pin to ground, and compare with the normal value, and then compress the fault range to find the damaged component. Note the following eight when measuring:
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(1) The multimeter should have a large enough internal resistance, at least more than 10 times the resistance of the measured circuit, so as not to cause large measurement errors.
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(2) The potentiometer is usually rotated to the middle position, if it is a TV, the signal source should use a standard color bar signal generator.
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3) The stylus or probe should take anti-slip measures. Because any instant short circuit is easy to damage the ic. The following methods can be taken to prevent the sliding of the pen: take a bicycle valve core set on the nib of the watch, and grow the nib of the watch about 0.5mm, which can make the nib of the watch well in contact with the test point, and can effectively prevent slipping, even if it hits the adjacent point, it will not short circuit.
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(4) When a certain pin voltage is not consistent with the normal value, it should be analyzed according to whether the pin voltage has an important impact on the normal operation of the ic and the corresponding changes in other pin voltages, and the ic can be judged.
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(5) ic pin voltage will be affected by peripheral components. When the peripheral components have leakage, short circuit, open circuit or variable value, or the peripheral circuit is connected to a potentiometer with variable resistance value, the position of the potentiometer sliding arm is different, and the pin voltage will change.
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(6) If the ic each pin voltage is normal, the ic is generally considered normal; If the ic part of the pin voltage is abnormal, you should start from the maximum deviation from the normal value, check the peripheral components have no fault, if there is no fault, the ic is likely to be damaged.
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(7) For dynamic receiving devices, such as television sets, the ic pin voltage is different when there is no signal. If it is found that the pin voltage should not change but change greatly, and the change with the signal size and different positions of the adjustable element does not change, the ic damage can be determined.
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(8) For devices with multiple working modes, such as video recorders, the ic pin voltage is also different under different working modes.
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3. Ac operating voltage measurement method
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In order to master the AC signal changes of the ic, you can use a multimeter with a db jack to approximate the AC operating voltage of the ic. When testing, set the multimeter in AC voltage gear and insert the front pen into the db jack. For a multimeter without a db jack, connect a 0.1 to 0.5 μf straight isolating capacitor to the front pen in series. This method is suitable for ic with low operating frequency, such as video amplifier stage of TV, field scanning circuit, etc. Since the natural frequencies of these circuits are different and the waveforms are different, the measured data is approximate and can only be used for reference.
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4. Total current measurement method
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This method is a method to judge the quality of ic by detecting the total current of the ic power supply line. Since the vast majority of the ic is directly coupled, when the ic is damaged (such as a certain pn junction breakdown or open circuit), it will cause post-stage saturation and cutoff, so that the total current changes. Therefore, the ic can be judged by measuring the total current. It is also possible to measure the voltage drop of the resistance in the power supply path and calculate the total current value using Ohm's law.