The role of logic analyzer_How to use logic analyzer

A logic analyzer is a waveform testing device similar to an oscilloscope. It can monitor the logic level (high or low) of a hardware circuit when it is working, store it, and express it intuitively in a graphical way, making it easy for users to detect and analyze errors in circuit design (hardware design and software design). A logic analyzer is an indispensable device in design. With it, errors can be quickly located, problems can be solved, and the result can be twice the result with half the effort.

The role of a logic analyzer

A logic analyzer is an instrument that uses a clock to collect and display digital signals from test equipment. Its main function is to determine the timing. Since a logic analyzer does not have many voltage levels like an oscilloscope, it usually only displays two voltages (logic 1 and 0). Therefore, after setting the reference voltage, the logic analyzer uses a comparator to determine the measured signal. Those higher than the reference voltage are High, and those lower than the reference voltage are Low, forming a digital waveform between High and Low. For example: a signal to be measured uses a logic analyzer with a sampling rate of 200MHz. When the reference voltage is set to 1.5V, the logic analyzer will take a point every 5ns on average during measurement. Those above 1.5V are High (logic 1), and those below 1.5V are Low (logic 0). The subsequent logic 1 and 0 can be connected into a simple waveform, and engineers can find abnormal errors (bugs) in this continuous waveform.

Generally speaking, when a logic analyzer measures a signal, it does not display the voltage value, but only the difference between High and Low. If you want to measure voltage, you must use an oscilloscope. In addition to the different display of voltage values, another difference between a logic analyzer and an oscilloscope is the number of channels. A general oscilloscope has only 2 or 4 channels, while a logic analyzer can have 16 channels, 32 channels, 64 channels, or even hundreds of channels. Therefore, a logic analyzer has the advantage of performing multi-channel testing at the same time.

Based on the differences in hardware design, the logic analyzers currently on the market can be roughly divided into stand-alone (or single-machine) logic analyzers and PC-based card-based virtual logic analyzers that need to be combined with a computer. Stand-alone logic analyzers integrate all test software, computing management components, and other components into one instrument; card-based virtual logic analyzers need to be used with a computer, and the display screen is also separate from the host.

In terms of overall specifications, stand-alone logic analyzers have developed into products with quite high standards, such as sampling rates up to 8GHz, channels that can be expanded to more than 300 channels, and relatively high storage depth. Stand-alone logic analyzers used to be expensive, ranging from tens of thousands to hundreds of thousands of RMB, and ordinary users could rarely afford them. Recently, Taiwan OItek Technology Co., Ltd. launched the OLA2032BTM stand-alone desktop EasyDebugTM logic analyzer, which is less than 20,000 RMB and affordable for every engineer. Especially in the teaching of digital circuits, it has changed the previous situation where teachers used virtual logic analyzers to reduce costs, which led to problems such as unintuitiveness and trouble. At the same price, we can easily pick up a desktop stand-alone logic analyzer.

The card-type virtual logic analyzer based on computer interface provides corresponding performance at a lower cost, but the card-type virtual logic analyzer also has a big disadvantage. It needs to be equipped with a computer to use. Especially in digital testing, engineers are often trapped in a pile of PCB boards. Instruments with rotary buttons are more convenient than moving the mouse on the screen. The development of technology has gradually integrated the functions of oscilloscopes and logic analyzers into a hybrid instrument (MSO), also known as a mixed signal test instrument. The comparison between logic analyzer and oscilloscope is as follows, so that we will know when to use a logic analyzer or an oscilloscope.

How to use a logic analyzer_How to use a logic analyzer (tutorial)

1. Hardware channel connection: First, we need to connect the GND of the logic analyzer and the GND of the board to be tested to ensure the integrity of the signal. Then connect the channel of the logic analyzer to the pin to be tested. The pin to be tested can be brought out in a variety of ways.

2. Channel number setting. Generally speaking, most logic analyzers have 8 channels, 16 channels, 32 channels, etc. However, when we collect signals, we often don’t need so many channels. In order to observe the waveform more clearly, we can hide the unused channels.

3. Sampling rate and sampling depth settings, first of all, we need to have a rough estimate of the highest frequency of the signal to be measured, set the sampling rate to more than 10 times of it, and also roughly judge the length of time of the signal we want to collect. When setting the sampling depth, try to set a certain margin. The sampling depth divided by the sampling rate is the time we can save the signal.

4. Trigger setting. Since the logic analyzer has a depth limit, it is impossible to save data indefinitely. When we use the logic analyzer, if no trigger setting is used, the time will be calculated from the beginning of the capture until the storage depth we set is full, and the capture will stop. In the actual operation process, the first section of the captured signal may be a useless signal, and the useful signal may be one of the sections, but the useless signal still occupies our storage space. In this case, we can improve the utilization rate of the storage depth by setting the trigger. For example, if we want to capture the UART serial port signal, and the serial port signal is a high level when there is no data, we can set a falling edge trigger. From the click to start capturing, the logic analyzer will not save the captured signal to our memory, but will wait for a falling edge to be generated. Once the falling edge is generated, the real signal acquisition will begin, and the collected signal will be stored in the memory. In other words, the useless signal from the click to start capturing to the falling edge is shielded by the trigger we set, which is a very practical function.

5. Capture waveform. Logic analyzer is different from oscilloscope. Oscilloscope displays in real time, while logic analyzer needs to click "Start" to start capturing waveform until the storage is full of the storage depth we set. Then we can slowly analyze the captured signal, so it is necessary to click "Start Capturing".

6. Set protocol analysis (standard protocol). If the waveform you captured is a standard protocol, such as UART, I2C, SPI, the logic analyzer is generally equipped with a special decoder. By setting the decoder, you can not only display the waveform like an oscilloscope, but also directly parse the data and display it in various forms such as hexadecimal, binary, and ASCII code.

7. Data analysis: Similar to an oscilloscope, a logic analyzer also has various measurement markings that can measure pulse width, waveform frequency, duty cycle and other information. Through data analysis, we can find out whether our waveform meets our requirements, thus helping us solve the problem.