PCB Technical - The detection circuit in the PCB system of PCB design

1 Introduction

In the reverse design or maintenance of electronic equipment, electronic engineers first need to understand the connection between unknown PCB components, so it is necessary to measure and record the connection relationship between the pins. Every component on the PCB. The easiest way is to hit the "short circuit buzzer" file with a multimeter, measure the connection between the pins with two pens, and then manually record the pass/disconnection status between the "pin pairs". In order to obtain a complete set of connections between all "pin pairs", it must be organized according to the principle of "pin pairs". When the number of components and the number of pins on the PCB is more, the "number of pins" "need to be measured" It will be very large. Obviously, if the manual method is used to complete this work, the amount of measurement, recording and proofreading will be very large.

And the measurement accuracy is low. As we all know, the resistance value of the general multimeter between two pens is as high as 20 ohms, and the buzzer still generates noise, which is expressed as a path. In order to improve measurement efficiency, we must try to realize automatic measurement, recording and calibration of component "pin pairs". To this end, the author designed a path detector controlled by a microcontroller as a front-end detection device, and designed a set of powerful back-end processing measurement and navigation software to achieve automatic measurement and recording of connections between components. Components on the PCB.

This article mainly discusses the design idea and technology of the path detection circuit, which realizes the automatic measurement. The prerequisite for automatic measurement is to connect the pins of the component under test to the detection circuit. For this, the testing equipment is equipped with multiple measuring heads. Through cables, the measuring heads can be connected to various test fixtures and component pins. The number of headers determines the number of pins of the detection circuit connected in the same batch. Then, according to the program control of the detector, the "pins" are measured one by one into the measurement path according to the principle of combination.

In the measurement path, the pass/break condition between the "pin pairs" is expressed as a resistor between the pins, and the measurement path converts it into a voltage amount, thereby judging the pass/break relationship between them and Record it.

The PCB path detection circuit based on this idea should mainly realize 3 functions:

• Automatically select test "pin pairs" and perform measurement;

• Automatically determine the path relationship between "pin pairs";

• Automatically record the measurement results.

2 Automatically select and measure a pair of measurement pins

2.1 Automatic switching of measurement pin pairs In order to make the detection circuit select different pins for measurement from multiple measuring heads of additional component pins according to the combination principle, the corresponding switch array can be set by the program/different switches can be turned off, and the component pins Insert the measurement path to obtain its pass/break relationship.

Since it is measured as an analog voltage, an analog multiplexer should be used to form a switch array

2.2 Measurement of channel/breaker relationship The design principle of the detection circuit can measure the pass/break relationship between all the pins connected to the measuring head in an instant. Combination principle.

Since this measurement process is carried out between the pins of the component clamped by the test fixture, the author refers to it as in-fixture measurement. If the pin of the component is not clamped, it must be measured with a pen. As shown in Figure 2, the pen is connected to one analog channel and the other is grounded, so as long as i-1 is turned off, the control switch can be measured, which is called stylus measurement.

3.1 If VA is used as the measurement voltage, theoretically, when VA = 0, it should be a path, VA> 0 should be an interruption, and the value of VA varies with the resistance between the two measurement channels. However, since the analog multiplexing itself has a non-negligible conductive resistance RON, after the measurement path is formed, if VA is the path, VA is not equal to 0, but equal to the voltage drop on RON. Since the purpose of measurement is only to know the pass/break relationship, there is no need to measure the specific value of VA, so only a voltage comparator is needed to compare whether VA is greater than the voltage drop on RON.

Set the threshold voltage of the voltage comparator to be equal to the voltage drop on RON. The output of the voltage comparator is the measurement result, that is, the digital quantity that the microcontroller can read directly.

3.2 Determination of threshold voltage

It is found that RON has individual differences and is related to ambient temperature, so it is necessary to set the load threshold voltage separately by closing the analog switch channel, which can be achieved by programming the D/a converter. By turning off the switch to 1.-1, 2-1; ⅰ-2, ⅱ-2; The circuit shown in Figure 2 can easily determine the threshold data. ; 1.-N, 2-N; form a path loop, after each pair of switches are closed, the number is sent to the D/A converter, and the number is sent from small to large increments. At this time, the output of the voltage comparator is measured, when the voltage When the output of the comparator comes from 1 to 0, the data corresponds to VA at this time. This allows the VA of each channel path to be measured, that is, the voltage drop across RON when a pair of switches are closed.

3.3 Dynamic setting of threshold voltage Use the threshold data measured above to construct a table. When making in-clamp measurements, the corresponding data is removed from the table according to the numbers of the two closed switches, and they form a threshold voltage with the sending D/a converter.

For pen clamp measurement and pen measurement, since the measurement path only passes through the analog switch of No. I, only the threshold data of the switch can be loaded. In addition, since the circuit itself (d/a converter, voltage comparator, etc.) has errors, and the actual measurement value of the test fixture and the tested pin has contact resistance, the actual load threshold voltage should be added based on the threshold determined by the above method Correction amount, so as not to misjudge the path as open circuit. However, the increased threshold voltage will overwhelm the small resistance resistance, that is, the small resistance between the two pins is judged as a path, so the threshold voltage correction amount should be selected reasonably according to the actual situation.

Through multiple PCB testing experiments, the testing circuit can accurately determine the resistance value of the two pins greater than 5 ohms, and its accuracy is significantly higher than that of a multimeter.