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The PCB board path detection circuit based on this idea should achieve three main functions: 1) Automatic selection and measurement of the “pin pairs” to be tested; 2) Automatic determination of the path relationship between “pin pairs”; 3) Automatic recording of the measurement results.
2. Automatic Selection and Measurement of Tested Pin Pairs
2.1 Automatic Switching of Tested Pin Pair
To enable the detection circuit to select different pins for measurement according to the combination principle from the multiple measuring heads connected to component pins, a corresponding switch array can be established. The program controls the on/off state of different switches to direct the component pins into the measurement channel. Since the measured values are analog voltages, an analog multiplexer is used to form the switch array. Figure 1 illustrates the use of an analog switch array to switch the measured pins.
2.2 Measurement of On/Off Relationship
The analog multiplex switch’s open/closed status is controlled by the program through a decoding circuit, allowing only one switch to be closed between two analog switches I and II simultaneously. For instance, to detect the channel relationship between measuring head 1 and measuring head 2, switches I-1 and II-2 are closed, creating a measurement channel between point A and ground via measuring heads 1 and 2. If a channel exists, the voltage at point A is VA=0; if it’s an open circuit, VA>0. VA serves as the basis for determining the path relationship between measuring heads 1 and 2. This instantaneous measurement of the on/off relationship between all pins connected to the measuring head is based on the combination principle. When measuring the pins of components clamped by the test fixture, this method is referred to as “in-clamp measurement.” For non-clampable component pins, test leads are used, connecting one test lead to an analog channel and the other to ground. Closing switch I-1 enables measurement, termed “pen-pen measurement.” Measurement between all clampable pins of the connected measuring head and non-clampable pins touched by the grounding test lead is termed “clip measurement.” In clip measurement, switches of channel I are sequentially closed while switches of channel II remain open.
3. Judgment of Pathway Relationship
3.1 Proposition of Threshold Voltage
VA, the measured voltage, theoretically reads VA=0 for an open circuit and VA>0 for a path, varying with the resistance value between the two measurement channels. Due to the analog multiplexer’s on-resistance RON, however, VA isn’t zero even for a path but equals the voltage drop across RON. As the measurement aims to detect the on/off relationship rather than VA’s specific value, a voltage comparator compares VA against the voltage drop on RON. The comparator’s threshold voltage equals the RON voltage drop, providing a digital output directly readable by the microcontroller.
3.2 Determination of Threshold Voltage Value
Experimentally, RON varies due to individual differences and ambient temperature. The threshold voltage needs individual setting using a D/A converter programmed to sequentially close switch pairs I-1, II-1; I-2, II-2; …; I-N, II-N. The voltage comparator output is monitored, and when it shifts from 1 to 0, the corresponding VA is noted. This VA approximates half the voltage drop on the respective RON of the closed switch pair under current temperature conditions, serving as the threshold data for analog switches.
3.3 Dynamic Setting of Threshold Voltage
A threshold data table is built from measured values. During clip measurement, data from this table based on closed switch pairs is summed and sent to the D/A converter to set the threshold voltage. Pen-clip and pen measurement require loading only one switch’s threshold data since their measurement channels use only one analog switch. Considering circuit errors and contact resistance during actual measurements, an additional correction amount is added to the threshold voltage to avoid misjudging a path as an open circuit. The correction amount must be chosen based on practical conditions, balancing against submerging small resistances.
4. Several Special Cases of Measurement Results
4.1 Influence of Capacitance
When a capacitor bridges tested pins, it appears as a path due to charging during switch closure. To prevent this false path reading, increase the measurement current to hasten charging or include software to verify path authenticity.
4.2 Influence of Inductance
Inductance typically reads as an open circuit due to its large static resistance impedance. However, rapid acquisition by the detection circuit can correctly detect inductance-induced electromotive force, contrasting capacitance measurement.
4.3 Effects of Analog Switch Jitter
Analog switches exhibit voltage fluctuation during transition from open to closed, requiring confirmation of initial measurement consistency.
4.4 Confirmation and Recording of Measurement Results
To address these issues, the software block diagram confirms and records measurement results, extending measurement time to counteract capacitive effects and incorporating induced electromotive force for inductance detection. Two counters, passage count (N) and open circuit count (n), are set to eliminate false readings due to capacitor charging and analog switch jitter, respectively. The appropriate values for counters and delays are determined by experimental evaluation to balance conflicting measurement scenarios. Software checks ensure accurate path determination despite potential errors.
This paper introduces a novel approach to measuring path relationships between component pins on large-scale PCB boards, analyzing the function and implementation principles of the PCB board path detection circuit. Experimental results demonstrate its efficiency, accuracy, and comprehensive measurement and recording capabilities, supported by measurement and navigation software.
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This revision focuses on clarity, consistency, and technical accuracy, maintaining the original content’s meaning while enhancing readability and coherence.
