PCB Blog - Detection circuit in PCB board reverse design system

On the unknown printed circuit boards,when an electronic engineer performs the inverse design or maintenance of electronic equipment, he first needs to understand the connection relationship between the components. Therefore, it is necessary to measure and record the connection relationship between the pins of each component on the PCB board. The simple method is to set the multimeter to the "short-circuit buzzer" gear, measure the connection between the pins pair by pair with two test pens, and then manually record the on/off status between the "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 the PCB is large, the number of "pin pairs" that need to be measured will be huge. Obviously, if this work is carried out manually, the workload of measuring, recording and proofreading will be very large. Moreover, the measurement accuracy is low. As we all know, when the resistive impedance value between the two test leads of the general multimeter is as high as 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 achieve automatic measurement, recording and proofreading of component "pin pairs". For this reason, the author designed a path detector controlled by a microcontroller as a front-end detection device, and designed a set of powerful measurement and navigation software for back-end processing, to jointly realize the automatic measurement and measurement of the path relationship between the component pins on the PCB board. Record. This paper mainly discusses the design idea and technology of the path detection circuit to realize automatic measurement. The premise of realizing automatic measurement is to connect the pins of the component under test to the detection circuit. For this detection device, set up several measuring heads, which are led out through cables. The measuring heads can be connected to various test fixtures to establish connections with the component pins. The number of pins determines the number of pins connected to the detection circuit in the same batch. Then, under program control, the detector incorporates the "pin pairs" under test into the measurement path one by one according to the combination principle. In the measurement path, the on/off state between the "pin pair" is presented as whether there is resistance between the pins, and the measurement path converts it into a voltage quantity, thereby judging the on/off relationship between them and recording it .

The PCB board path detection circuit based on this idea should mainly 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, the corresponding switch array can be set up, and different switches can be turned on/off by the program to switch the pins of the components. into the measurement channel to obtain its on/off relationship. Since the measured value is an analog voltage, an analog multiplexer should be used to form a switch array. Figure 1 shows the idea of using the analog switch array to switch the measured pins.

2.2 Measurement of on/off relationship
The closing or not of the analog multiplex switch is controlled by the program through the decoding circuit, and only one switch can be closed at the same time among the two analog switches I and II at the same time. For example, to detect whether there is a channel relationship between measuring head 1 and measuring head 2, close the switches I-1 and II-2, and form a measuring channel between point A and ground through measuring heads 1 and 2. If it is a channel, Then the voltage at point A is VA=0; if it is an open circuit, then VA>0. The value of VA is the basis for judging whether there is a path relationship between the measuring heads 1 and 2. In this way, the on/off relationship between all pins connected to the measuring head can be measured in an instant according to the combination principle. Since this measurement process is performed between the pins of the components clamped by the test fixture, I call it in-clamp measurement. If the pins of the component are not clippable, they must be measured with test leads. 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, the measurement can be carried out, which is called pen-pen measurement. It is also possible to complete the measurement between all the clampable pins of the connected measuring head and the non-clampable pins touched by the grounding test lead in an instant. At this time, it is necessary to control the closing of the switches of channel I in turn, and the switches of channel II are 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, and the value of VA varies with the resistance value 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. Since the purpose of the measurement is only to know the on/off relationship, it is not necessary 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 equal to the voltage drop across RON, and the output of the voltage comparator is the measurement, which is a digital quantity that can be read directly by the microcontroller.

3.2 Determination of threshold voltage value
The experiment found that there are individual differences in RON, and it is also related to the ambient temperature, so the loaded threshold voltage needs to be set one by one with the closed analog switch channels, which can be achieved by programming the D/A converter. The method is to sequentially close the switch pairs I-1, II-1; I-2, II-2; ……; I-N, II-N; The number sent increases from small to large, and the output of the voltage comparator is measured at this time. When the output of the voltage comparator changes from 1 to 0, the data at this time corresponds to VA. This measures VA when each channel is on, which is the voltage drop across RON when a pair of switches is closed. For high-precision analog multiplex switches, the individual differences in RON are small, so half of the VA automatically measured by the system can be approximated as the corresponding data of the voltage drop on the respective RON of the pair of switches, that is, at the current temperature Threshold data for analog switches.

3.3 Dynamic Setting of Threshold Voltage
Build a table using the threshold data measured above. During the measurement in the clip, the corresponding data is taken from the table according to the number of the two switches closed, and their sum is sent to the D/A converter to form a threshold voltage. For pen clip measurement and pen measurement, since the measurement channel only passes through the analog switch of No. 1, only the threshold data of one switch needs to be loaded. In addition, because the circuit itself (D/A converter, voltage comparator, etc.) has errors, and there is a contact resistance between the test fixture and the pin under test during actual measurement, the actual loaded threshold voltage should be within the threshold determined by the above method. On the basis, a correction amount is added to avoid misjudging the path as an open circuit. However, the increased threshold voltage will submerge the small resistance resistance, that is, the small resistance between the two pins is judged as a channel, so 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 the two pins with a resistance value greater than 5 ohms, and its accuracy is significantly higher than that of the 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 an open-circuit relationship, but the capacitor is charged by the measurement channel when the switch is closed, and the two measurement points are like a channel. At this time, the measurement result read from the voltage comparator is path. For this kind of false path phenomenon caused by capacitance, the following two methods can be used to solve it: appropriately increase the measurement current to shorten the charging time, so that the charging process ends before reading the measurement results; add the test software to check the true and false paths program segment.

4.2 Influence of Inductance
If there is an inductance connected between the tested pins, it should be an open circuit relationship, but because the static resistance impedance of the inductance is very small, the result measured by a multimeter is always an open circuit. Contrary to the case of capacitance measurement, at the moment 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 contradicts the measurement requirement of capacitance.

4.3 Effects of analog switch jitter
In the actual measurement, it is found that the analog switch has a stable process from the open state to the closed state, which is manifested as the fluctuation of the voltage VA, which makes the initial measurement results inconsistent. Confirm again.

4.4 Confirmation and recording of measurement results
Considering the above situations, in order to adapt to different measured objects, the software block diagram confirms and records the measurement results. In order to eliminate the influence of capacitive elements and analog switch jitter, the measurement time needs to be extended, and to eliminate the influence of inductive elements, it is necessary to use the induced electromotive force to determine in a very short time. Two counters are set in this program: the number of passages counter and the Circuit breaker counter. The purpose of setting the number of passages N is to eliminate the false passage effect caused by the charging of the capacitor at the moment when the analog switch is closed. That is, when the N times of passage results are accumulated, the charging of the capacitor is generally completed, and then it is confirmed that the measured points are passages. The purpose of setting the number of open circuit n is to eliminate the interference caused by the jitter of the analog switch. Generally, when the results of n times of open circuit are continuously measured, it indicates that the jitter of the analog switch has ended, and it can be confirmed as open circuit. However, because the induced electromotive force of the inductance drops rapidly at the moment when the analog switch is closed, if both the second and second measurement results are open circuit, it is confirmed as open circuit. Since several situations contradict each other, the values of the counter and the delay are determined according to the actual situation on the basis of weighing the three situations. Of course, when using the above procedure to judge, if there is a small resistance, small inductance or large capacitance between the tested pins, it may be misjudged as a path. This kind of problem can be easily checked by the software in the measurement results. If the two ends of a two-pin component are in the same network, the above judgment error may exist, which should be eliminated after confirmation. This paper provides a new idea for the measurement of the path relationship between component pins on a large-scale PCB board by analyzing the function and implementation principle of the PCB board path detection circuit. Experiments show that this detection circuit can efficiently, accurately and completely measure and record the path relationship between the pins of various components on the PCB board with the support of the measurement and navigation software.