How to choose a logic analyzer

Since 1973, when the first instrument for testing the logical and time relationships between multiple signals in a digital system, the Logic Analyzer, came to the fore among data domain test instruments, users have begun to accept this type of data domain test instrument as the ultimate solution to digital circuit testing.

In recent years, the basic trend of logic analyzers has been to seek development direction in the continuous integration of computers and instruments. Relying on the continuous advancement of computer technology, virtual logic analyzers have absorbed and integrated the functions of many digital measuring instruments such as logic pens and protocol analyzers, using the Windows system platform and equipped with a simple and easy-to-use user interface, further simplifying the triggering problem and building a unique digital analysis and measurement platform.

What is a Logic Analyzer?

A logic analyzer is a waveform testing device similar to an oscilloscope. It is used to monitor the logic level of the hardware circuit when it is working, store it, and express it intuitively in a graphical way, so that users can detect and analyze errors in circuit design (hardware design and software design). A logic analyzer is an indispensable device in design, especially digital design. Through it, errors can be quickly located and problems can be solved, achieving twice the result with half the effort.

Compared with oscilloscopes, logic analyzers often give people the following feelings:

1. The operation is complicated and has high requirements on users;

2. The function is similar to that of an oscilloscope;

3. Expensive.

In fact, this is not the case. In recent years, logic analyzers have developed rapidly with the widespread use of digital circuits and have become one of the must-have equipment for many electronic development companies.

Principle of Logic Analyzer

The structure of a logic analyzer is shown in Figure 2.1. It mainly consists of two parts: data capture and data display. Logic analyzers generally collect and store data first, then analyze and display the data.

The data capture part includes signal input, comparison sampling, trigger control, data storage and clock circuit. The external measured signal is sent to the signal input circuit through the probe, and compared with the set threshold voltage in the comparator. The signal greater than the threshold voltage value is a high level, and vice versa. The sampling circuit samples the signal under the control of the sampling clock (external clock or internal clock), and sends the data stream to the trigger module to generate a trigger signal. The data storage circuit performs corresponding data storage control under the action of the trigger signal. After the data capture is completed, the analysis and display circuit processes the stored data and displays it in an appropriate manner.

 

Figure 2.1 Principle structure of logic analyzer

Most logic analyzers are actually composed of a timing analyzer and a state analyzer.

Timing analysis: also known as asynchronous timing analysis. Driven by the internal high-speed sampling clock of the logic analyzer, the input signal is sampled asynchronously, and the sampled data is displayed in the form of a square wave. Driven by the internal high-speed clock, the logic analyzer performs asynchronous sampling on the signal input, and its measurement results are used to distinguish the timing relationship between related signals, such as setup time, hold time, protocol response, etc. According to the sampling theorem, the internal sampling clock must be more than 3 times higher than the highest frequency of the measured signal to obtain the correct sampling data. The higher the internal sampling clock frequency, the higher the timing resolution, the higher the accuracy, and the more accurate the timing relationship. The LAB6000 series logic analyzers produced by Guangzhou Zhiyuan Electronics Co., Ltd. have a maximum sampling frequency of 5GHz and a timing resolution of up to 200ps. The timing analysis mode is generally used for testing hardware systems. [page]

State analysis: also known as synchronous timing analysis. Driven by an external synchronous clock, the logic analyzer performs synchronous data sampling on the input signal. When displaying, it uses binary code or mapping diagrams or disassembly into mnemonics with software. Since the collected state data is completely consistent with the state of the measured signal data flow, it can be used to directly observe the source code of the program. The state analysis mode is generally used to test system software.

According to the differences in hardware device design, the logic analyzers currently on the market can be roughly divided into desktop logic analyzers and PC-based virtual logic analyzers. Desktop logic analyzers integrate all test software, operation management components and display parts into one instrument; virtual logic analyzers need to be used with a PC to display the final results. Compared with desktop logic analyzers that cost tens of thousands of yuan, virtual logic analyzers have the advantages of low price, high cost performance, strong analysis ability, user-friendly interface, simple operation and small size. In addition, the logic analyzers produced by Guangzhou Zhiyuan Electronics Co., Ltd. also integrate the functions of other digital test equipment, such as logic pens, frequency meters, bus analysis, protocol analysis, etc., making the logic analyzer more powerful in digital testing.

Oscilloscope vs. Logic Analyzer

In the field of electronic testing, oscilloscopes are mainly used for the acquisition and reproduction of signal waveforms, and are mainly used for testing analog signals and analog circuits. With the development of digital technology, digital signal testing has become increasingly important. The earliest digital signal testing was often done with the help of oscilloscopes. Later, timing analyzers and state analyzers appeared to analyze and test multiple digital signals from the perspective of timing and state. Since timing analyzers and state analyzers were expensive at the time, the concepts of the two in the market were good, but the impact was not great, and the test range was very narrow. With the development of digital testing technology, logic analyzers that integrate digital timing and state analysis were born.

When users generally choose between an oscilloscope and a logic analyzer, most of them will choose the familiar oscilloscope. However, oscilloscopes are more inclined to measure analog signals at the application level; logic analyzers can provide more powerful functions than oscilloscopes in digital signal analysis, which is more helpful for user development. In an era full of digital signals, most of the types of signals users face when developing products are also digital signals. In the process of user product development, oscilloscopes may be an indispensable instrument, but with the help of logic analyzers, it can greatly improve work efficiency and save users' precious time.

A logic analyzer is an instrument that uses a clock to drive the acquisition and display of digital signals from a test device. Its main function is to analyze digital circuits.
Since a logic analyzer does not have many levels like an oscilloscope, it usually only displays two values ​​(logical "1" and "0"). After the threshold voltage is set as shown in Figure 3.1, the logic analyzer compares the measured signal with the threshold voltage through an ultra-high-speed comparator. If it is higher than the threshold voltage, it is a logical "1", and if it is lower than the threshold voltage, it is a logical "0". For example: a signal to be measured uses a logic analyzer with a sampling rate of 500MHz. When the reference voltage is set to 1.7V, the logic analyzer will sample a point every 2ns (500MHz) on average during measurement. If it exceeds 1.7V, it is "1", and if it is lower than 1.7V, it is "0". All the sampled "1" and "0" are connected into a waveform according to the time relationship, and the user can find abnormalities in this continuous waveform.

 

Figure 3.1 Logic analyzer sampling

In general, oscilloscopes are mainly used to observe the analog characteristics of signals, such as edge time, voltage amplitude, and whether there is parasitic interference. Logic analyzers mainly measure digital circuits. Due to the inherent characteristics of digital circuits, logic analyzers do not measure the specific value of voltage and some analog characteristics of the measured signal, but specifically measure the signal level. At the same time, logic analyzers have the following advantages over oscilloscopes:

Monitor multiple inputs simultaneously

Generally, logic analyzers have 32 or more channels and can detect dozens of input signals at the same time. You can easily view the relationship between the input channels.

Complete trigger function

Unlike oscilloscopes, logic analyzers do not only have rising edge and falling edge triggers. Logic analyzers generally also have triggers for bus range, bus relationship, event count, etc. Logic analyzers such as LAB6052 and LA2532 also have a variety of powerful trigger modes such as visual trigger, protocol trigger, plug-in trigger, and user-defined trigger.

Powerful analytical capabilities

The means of waveform display of oscilloscope is just to depict the waveform, while logic analyzer is much more powerful. Logic analyzer can combine multiple measurement channels into a bus for display, and can display bus data in binary or analog form. At the same time, LAB6000 and LA2000 series logic analyzers also have plug-in analysis function, which can directly perform protocol analysis on the measured signal according to the protocol selected by the user, making the data more intuitive and clear, as shown in Figure 3.1. It saves users from the trouble of only seeing the data waveform but not knowing the meaning of the data, which is particularly useful in data communication analysis. [page]

 

Figure 3.2 LAB6052 logic analyzer I2C analysis results

Correct choice of logical analysis

The important indicators of logic analyzers include the number of input channels, timing sampling rate, state sampling rate, storage capacity, trigger mode, analysis function, other functions, etc. Table 1 shows the main selection parameters of logic analyzers produced by Guangzhou Zhiyuan Electronics Co., Ltd., which are also important selection indicators of logic analyzers.

 

Table 4.1 Logic Analyzer Selection Table

 

Number of sampling channels

Where a logic analyzer is needed, to fully analyze a system, the number of channels of the logic analyzer should at least be sufficient to introduce all the signals that need to be observed into the logic analyzer.

Timing sampling rate

In timing sampling analysis, to have sufficient timing resolution, there should be a sufficiently high timing analysis sampling rate, but not only high-speed systems require high sampling rates. If the sampling frequency is high, more detailed results can be seen. For example, the LAB6503 logic analyzer has a timing sampling rate of 5GHz and a time resolution of up to 200ps between signals, which fully meets the ns-level measurement of modern devices.

State sampling rate

When performing state analysis, the logic analyzer sampling clock uses the external input clock, and data is stored under the drive of the external clock. The highest frequency of the external input clock determines the maximum state sampling rate of the logic analyzer. In addition to the standard state sampling mode, the LAB6000 series logic analyzer also has a state sampling mode with timestamp. The state sampling mode with timestamp allows users to not only observe different input states, but also measure the relationship between each input state and the external synchronous clock.

storage

Storage capacity refers to the number of sampling points that the logic analyzer can continuously store. To put it more directly, it refers to how long the logic analyzer can measure the waveform. The larger the storage capacity, the longer the observation time. However, since the price of high-speed memory is relatively high, the larger the storage capacity, the higher the price of the corresponding logic analyzer. The unique Timing-State mode of the LAB6000 and LA2000 series logic analyzers can automatically balance the relationship between storage capacity, observation time and measurement accuracy. The use of Timing-State technology can enable the logic analyzer to have high measurement accuracy and longer observation time at the same storage capacity.

Trigger method

Compared with oscilloscopes, logic analyzers provide a rich set of trigger modes. In addition to edge triggering, logic analyzers generally have triggering modes for bus range, bus relationship, event count, etc. LAB6000 and LA2000 series logic analyzers can also compare data by range, =, !=, >, <, etc. At the same time, LAB6000 and LA2000 series logic analyzers provide multiple triggering modes such as visual triggering, protocol triggering, plug-in triggering, user-defined triggering, and high-speed SPI triggering.

Analysis capabilities

The analysis function is the biggest highlight of the logic analyzer and is also an important indicator that distinguishes the logic analyzer from other instruments. The logic analyzer analyzes the timing and status of the input signal. The logic analyzer produced by Guangzhou Zhiyuan Electronics Co., Ltd. can further provide bus analysis for I2C, SPI, UART, ModBus, 1-Wire, etc., analysis of high-level protocols such as SD card and CF card, and disassembly analysis for the processor.

Other functions

The logic analyzer produced by Guangzhou Zhiyuan Electronics Co., Ltd. proposed the concept of a digital circuit measurement platform. At the beginning of the design, it integrated functions such as frequency meter, logic pen, bus analyzer, and protocol analyzer. Users can observe multiple measurements simultaneously on one software interface, making one machine multi-purpose and saving users from repeated investment.

The logic analyzer combines the functions of state analysis and timing analysis, and is an indispensable and powerful debugging tool for users in development. With these basic knowledge, users can confidently use the logic analyzer for digital-assisted development and troubleshooting, thereby greatly shortening the R&D cycle and developing high-quality products faster.