Detection Circuit in PCB Reverse Design System
As for unknown PCB, when an electronIC engineer reverse designs or maintains electronic equipment, he first needs to understand the connection relationship between components Therefore, it is necessary to measure and record the connection relationship between the pins of each component PCB board The SIMple method is to set the multimeter to the "short circuit buzzer" position, use two test pens to measure the connection between pins in pairs, and then manually record the closing state between/"pin pairs" In order to obtain a complete set of connection relationships between all "pin pairs", the "pin pairs" to be tested must be organized according to the combination principle When the number of components and pins on PCB is large, the number of "pin pairs" to be measured will be huge Obviously, if this work is performed manually, the measurement workload, recording and proofreading will be huge In addition, the measurement accuracy is low As we all know, when the resistance impedance between the two test leads of the universal multimeter reaches about 20 ohms, the buzzer will still sound, indicating that it is a channel In order to improve the measurement efficiency, it is necessary to realize automatic measurement, recording and proofreading of "pin pair" of components Therefore, the author designed a path detector controlLED by a microcontroller as the front-end detection equipment, and designed a set of powerful measurement and navigation software for back-end processing, to jointly achieve automatic measurement and the path relationship PCB between component pins in measurement Record This paper mainly discusses the design idea and technology of path detection circuit for automatic measurement The prEMIse of automatic measurement is to connect the pin of the tested component to the detection circuit For the detection equipment, multiple measuring heads are set and led out through cables The measuring head can be connected to various test fixtures to establish a connection to the component pin The number of pins determines the number of pins connected to the detection circuit in the same batch Then, under the control of the program, according to the combination principle, the detector will merge the measured "pin pairs" into the measurement path one by one In the measurement path, the closed state between the open/"pin pair" indicates whether there is resistance between the pins. The measurement path converts it into a voltage to judge the relationship between them and record it
PCB board

This PCB path detection circuit is based on this idea should primarily realize three functions:
1) Automatically select the "pin pair" to be tested and measure;  2) Automatically determine the path relationship between "pin pairs";  3) Automatically record the measurement results.
2. Automatic selection and measurement of tested pin pairs
2.1 Automatic switching of the tested pin pair
In order to enable the detection circuit to select different pins for measurement according to the combination principle from the many measuring heads that have been connected to the pins of the components Enter the measurement channel to obtain its open/close relationship Because the measured value is analog voltage, analog multiplexer shall be used to form switch array Figure 1 shows the idea of switching measurement pins using analog switch arrays
2.2 On Measurement/Close Relationship
The closing or not of the analog multiplex switch is controlled by the program through the decoding circuit For example, check whether there is a channel relationship between measuring head 1 and measuring head 2, turn off switches I-1 and II-2, and form a measuring channel between point a and the ground through measuring heads 1 and 2 If it is a channel, the voltage at point A is VA=0; If it is an open circuit, then VA>0. VA value is the basis for judging whether there is a path relationship between measuring heads 1 and 2 With this pipe, the disconnection between all pins connected to the measuring head can be measured instantaneously by opening/according to the combination principle Since this measurement process is carried out between the component pins clamped by the test fixture, I call it clamp measurement If the pin of the component cannot be cut, it must be measured with a test lead Connect one test lead to one analog channel and the other to ground At this time, as long as the control switch I-1 is closed, measurement can be carried out, which is called pen measurement The measurement between all the clamping pins connecting the measuring head and the non clamping pins contacting the grounding test wire can also be completed in a moment At this time, it is necessary to control the closing of channel I switch in turn, and channel II switch is always open, This measurement process can be called clip measurement
3. Judgment of pathway relationship
3.1 Proposition of Threshold Voltage
If VA is used as the measured voltage, theoretically, when VA=0, it should be an open circuit, and when VA>0, it should be an open circuit. The value of VA varies with the resistance between the two measurement channels However, since the analog multiplexer itself has a non negligible on resistance RON, after the measurement path is formed, if it is a path, VA is not equal to 0, but equal to the voltage drop on RON Because the purpose of measurement is only to understand/off relationship, there is no need to measure the specific value of VA For this purpose, it is only necessary to use a voltage comparator to compare whether VA is greater than the voltage drop on RON The threshold voltage of the voltage comparator is set as the voltage drop on the RON. The output of the voltage comparator is the measured value, which is a digital quantity that can be directly read by the microcontroller
3.2 Determination of threshold voltage value
The experience found that there are individual differences in RON, which is also related to the ambient temperature. In conclusion, it is necessary to use closed analog switch channels to set the load threshold voltage one by one, which can be realized by programming D The method is to sequentially close switch pairs I-1, II-1; I-2, II-2；  a ?| a ?|；  I-N, II-N；  The number of transmissions increases from SMAll to large. At this time, measure the output of the voltage comparator When the output of the voltage comparator changes from 1 to 0, the current data corresponds to VA When each channel is opened, measure VA, which is the voltage drop through RON when a pair of switches are closed For high-precision analog multiplexer switches, the individual difference of RON is very small. In summary, half of the VA automatically measured by the system can be approximated to the corresponding data of the voltage drop on the respective RON of the switch, that is, the current temperature threshold data of the analog switch
3.3 Dynamic Setting of Threshold Voltage
Build a table using the threshold data measured above.  During the measurement in the clamp, according to the number of two closed switches, the corresponding data is obtained from the table, and their sum is sent to the D/converter that forms the threshold voltage It is used for pen clamp measurement and pen measurement because the measurement channel only passes through the No analog switch 1. Only one switch's threshold data needs to be loaded In addition, because the circuit itself (D/converter, voltage comparator, etc.) has errors, in the actual measurement process, there is contact resistance between the test fixture and the pin to be tested, and the actual load threshold voltage should be within the threshold value determined by the above method Therefore, a correction amount is added to avoid misjudging the path as an open circuit However, the new threshold voltage will submerge the small resistance, that is, the small resistance between two pins is judged as a channel. In this case, the correction amount of the threshold voltage should be reasonably selected according to the actual situation Through experiments, the detection circuit can accurately determine the resistance between two pins with a resistance value greater than 5 ohms, and its accuracy is significantly higher than that of a multimeter
4. Several special cases of measurement results
4.1 Influence of Capacitance
When a capacitor is connected between the tested pins, it should be in the open circuit relationship, but when the switch is closed, the capacitor is charged by the measuring channel, and the two measuring points are like a channel At this time, the measurement result read from the voltage comparator is the path For this false path phenomenon caused by capacitance, the following two methods can be used to solve this problem: add measuring current appropriately to shorten the charging time, so that the charging process ends before reading the measurement results; Add test software to check the true path and false path segments
4.2 Influence of Inductance
If there is an induction connected between the tested pins, this should be an open circuit relationship. However, because the static resistance impedance of the inductor is very small, the multimeter measurement result is always an open circuit Contrary to capacitance measurement, when the analog switch is closed, due to the induced electromotive force in the inductance, the inductance can be correctly judged by using the fast acquisition speed of the detection circuit But this is in conflict with the requirements of capacitance measurement

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