Composition of online tester module for double-sided PCB
I. Circuit online test technology
- Principle of online test: The basic principle of online test is that the tester provides input stimulus for the chip under test on the printed circuit board, and automatically collects and records the output response and state value of the chip under test under computer control, and compares all the state values recorded by the computer with the standard state truth table to judge the fault condition of the object under test.
- Post-drive test technology: Post-drive test technology is mainly used for online testing of digital circuits. Its essence is to inject or pull out transient large current at the input stage of the device under test (the output stage of the previous driver chip), forcing its potential to become higher or lower as required, so as to achieve the purpose of applying test stimulus to the device under test online.
In order to ensure the functional test of the device on the circuit board, the logic level of the drive device must be forced, and each pin driver must be able to absorb or output sufficient current. According to the post-drive safety standard recommended by the international protection standard document (00-53/1), the maximum drive current of the tester is designed to be 240mA, and the test time is within 200ms. Through the experiment, the device under test can be basically isolated well, and the safety of the device under test is also ensured.
II. Composition of the tester
- Hardware module: The tester consists of a portable computer, a single-chip test platform and test analysis and processing software. The single-chip test platform completes the data collection of the object under test under computer control. Some functions and descriptions are as follows:
The single-chip circuit mainly completes data acquisition, control, command processing, and data exchange with the computer. The MCS-51 series 8031 single-chip microcomputer is used in the design of the tester, 2764 is selected as the extended ROM, and 6264 is selected as the extended RAM. The decoding chip circuit is 74LS138. For serial communication with the computer, MCl488 and MC1489 are used for mutual conversion between RS-232C level and TTL level. The single-chip system clock frequency uses a 6MHz crystal oscillator, the communication baud rate uses 2400, and the single-chip uses working mode 3 for serial communication. Timer T1 is set to mode 2. Set SMOD=1, time constant F3H.
The bus driver expands the microcontroller bus to improve its driving capability. 74LS244 and 74LS245 line drivers are selected.
The drive control circuit mainly completes the control of TTL and CMOS test thresholds during the test process. A 4-layer SPST (single-pole single-throw) DG211 analog switch is selected. The switch control is completed by the decoding circuit and the 74LS373 latch. To ensure that DG211 is in the normally open (OFF) state when it is turned on, a pull-up resistor (10kΩ) is added to the control line. The test drive circuit applies a test input signal to the chip under test, and a micro relay is used to control the input signal. The test signal is generated by the data buffer 74ACT244. To ensure that the input current meets the design requirements, a 4-way parallel connection is used. To prevent damage to the device, an LC network is added for large current buffering, and a diode protection circuit is designed.
The data acquisition circuit reads the output response of the chip under test, and a dual voltage comparator LM393 is used for output signal control. It has low power consumption, high comparison accuracy, and is compatible with TTL logic. The output of LM393 is connected to the data latch of 74LS373, and the comparison data is read by the single chip microcomputer.
The voltage-driven D/A circuit completes the output of the step voltage during the VI test. The 8-bit parallel D/A converter MC1408 is used. The chip power supply voltage is +5V and -12V. The reference voltage is provided by the constant current voltage regulator TL431. The output selects bipolar output, which is completed by the two-stage amplifier LM348.
The current conversion acquisition A/D circuit implements the collection of test point current data. The circuit uses load resistors and differential amplifier circuit LM343 to follow the voltage of the test point and convert the current value of the test point into a voltage that can be processed by the A/D conversion circuit. The AD7574 eight-bit successive comparison high-speed A/D conversion circuit is selected. The conversion time is 15μS, and the single +5V power supply is used. The reference voltage is selected as VREF=-8V. The input voltage range is 0~+|VREF|. The A/D conversion can be started by generating a negative pulse at the RD end of the program control chip.
Software module: The tester is controlled by a portable host computer through a serial port. The single-chip test platform completes the work of excitation control, data acquisition, etc. All data analysis and processing and command control are completed by the portable host computer. The whole set of test software consists of several main modules, including host control software, data communication software, offline test software, online functional test software, online status test software, VI characteristic test software, node voltage test software, electronic manual, test development software, system self-test software, etc.
2.Main functions of the tester
The tester adopts circuit online test technology, which can be used to test and analyze common faults of various small and medium-scale integrated circuit chips online or offline, and test the V/I characteristics of analog and digital devices
The basic principle of the functional test of digital chips is to detect and record the input/output status of the chip, compare the recorded status with the standard status truth table, and judge whether the function of the tested chip is correct. Each digital device on the digital chip status test circuit board has three state characteristics after power-on: the logical state of each pin (power, ground, high resistance, signal, etc.), the connection relationship between the pins, and the logical relationship between input and output. When a device fails, its state characteristics generally change.
The tester can extract the state characteristics of each IC device on a good circuit board, store them in the computer database, and then compare them with similar faulty circuit boards to accurately find the fault location
.
VI characteristic test analysis This test function is based on analog characteristic analysis technology and can be used to test analog, digital, special-purpose devices, programmable devices, and large-scale and ultra-large-scale devices. The tester automatically extracts the characteristic curve of the measured point through a test probe or test clip, displays it on the microcomputer screen, and finally stores it in the computer. When diagnosing special faults, the measured VI curve is compared with the pre-stored standard curve to find the fault. Node voltage test Since the test object of the tester includes not only digital circuit devices, but also a large number of analog circuit devices, in order to further improve the scope of application of the tester, the node voltage test technology is used in the tester. By applying a working voltage to the object under test, the voltage response value of the test node is read by the computer, and a standard test information library is established for the operator to analyze and determine the fault location.
In addition to the above main functions, other functional testers also have auxiliary testing functions such as electronic manuals, test development, and system self-test.
