Verifying mixed-signal circuits using mixed-signal oscilloscopes

As the functions of electronic products become increasingly complex, mixed signals are increasingly appearing in products designed by engineers. Although mixed signals can bring flexibility to the design, the difficulty of debugging and testing products by engineers has also increased due to the different frequency and amplitude characteristics of analog and digital signals. This article details how to use Agilent's mixed signal oscilloscope to complete design debugging and testing.

Nowadays, whether in the computer field, communication field or consumer electronics field, when you pick up a circuit board, you will find that the devices used in it are diverse, often mixed with analog devices and digital devices. The analog part includes real-world physical signals such as light, sound, temperature, pressure, as well as power signals, video signals, AM/FM and other modulation signals, while the digital part includes single-chip microcomputers, microprocessors, programmable logic devices, DSP , etc., while ADC , DAC , and some single-chip microcomputers combine analog and digital signals. Such a mixed structure certainly brings flexibility to our design, but it also brings complexity to debugging and testing. Its complexity is manifested in:

1. The need to test and verify analog signals still exists, but at the same time there are many digital signals that need to be displayed, verified and tested simultaneously, especially the need to verify whether the control signal is controlling the related signals correctly at the right time.
   
2. Isolated signals are becoming less and less, and correlation debugging and verification of multi-channel signals are necessary in many cases. The speed of analog signals is often much lower than that of digital signals, requiring the instrument to support a high sampling rate while capturing a complete cycle of a slow signal. This requires the instrument to have a deep storage depth and multiple channels, while the price must be acceptable.
   
3. High-speed digital signals themselves exhibit analog characteristics (such as overshoot, ringing, etc.), and signal integrity testing is required.
   
4. Serial buses, such as I2C , SPI , CAN , LIN, USB , SATA , PCI-E, etc. ^, are widely used for communication between different devices or chips . The demand for instruments to be synchronized with serial communication protocols to debug and verify circuits is increasing rapidly .

Special packaging forms such as BGA make it impossible to measure many signals, and the use of programmable devices means that many key signals are not brought out at the pins.

Leading test equipment manufacturers have been committed to the research of mixed circuit test technology. The newly developed mixed signal oscilloscope ( MSO ) helps engineers solve the problems of mixed signal debugging and testing. Some users compared this instrument with a digital camera and found many similarities, such as:

1. With wide-angle lens capability, it can capture all-round scenery, and when taking pictures of emergencies, it can also record the surrounding people and environment. The mixed signal oscilloscope can capture up to 18 or 20 analog and mixed signals in all directions, and determine whether the abnormal signal is related to other multi-channel digital signals or analog signals.
   
2. High pixel count and one-shot imaging can not only record the whole picture, but also magnify the local details without distortion. The mixed signal oscilloscope is equipped with fast response deep storage as standard, which can capture and display up to 18 signals or 20 channels on one screen at the same time. Each signal is captured in depth, with a standard storage depth of 1MB to 8MB. There are also options that can be configured to be deeper, which can magnify tens of thousands of times to observe and analyze details.
   
3. The shutter captures the moment in synchronization with the focus of attention. The flexible trigger function allows you to synchronize the mixed signal oscilloscope with the operating status of the object under test, such as synchronizing with serial bus protocols such as I2C and SPI, as well as ^SDRAM control commands, PCI bus commands, LCD driver circuit commands, etc.

Mixed Signal Oscilloscope Measurement Solution

Due to the complexity of mixed signal circuits, even if you only need to observe the quality of one signal, digital oscilloscopes and analog oscilloscopes cannot do it. For example, when you need to observe the signal quality of a data line of DDR SDRAM, eye diagram analysis is a common method. During the analysis, the oscilloscope must first be synchronized with the read and write operations of the DDR SDRAM. According to the commands of the DDR SDRAM (see Table 1), this requires occupying 5 channels connected to the RAS, CAS, CS, WE, and CLK signals respectively, and using another channel to observe the eye diagram of the data signal you are concerned about. The result is shown in Figure 1. The mixed signal oscilloscope can easily obtain 8 consecutive read operations (i.e., 8 eye diagrams) of the DDR SDRAM.

A digital storage oscilloscope (DSO) or analog oscilloscope can tell whether a signal is normal, but it cannot tell you when the signal becomes abnormal. In other words, it cannot help you verify whether the quality of the key signal is up to standard under a specific circuit operating state, but this is a very simple thing for a mixed signal oscilloscope. As shown in Figure 2, engineers can use a mixed signal oscilloscope to find that after the DMA controller returns the bus control to the CPU, the firmware inside the PCI bus data acquisition card occasionally runs away. The root cause is that the clock will have an unexpected amplitude drop at this time, causing the circuit to mistakenly believe that a new clock cycle has arrived, resulting in malfunction. Based on this, the engineer further discovered the cause of the amplitude drop and solved this problem. When using it, just pay attention to connecting the control signal to the logic channel and set the trigger condition according to the PCI bus command.

The above function is essentially that the mixed signal oscilloscope can synchronize with the control command of the parallel bus. The third problem that the mixed signal oscilloscope can solve is synchronization with the serial bus. For example, the I ² C bus consists of only two lines (clock line SCL and data line SDA). How to judge and verify whether the circuit can correctly read a certain data (such as 0x07) to a certain address (such as 0x50)? The mixed signal oscilloscope can judge whether two devices have completed communication through the I ^{2 C bus based on the I 2} C protocol . The same method is used for other buses such as SPI and CAN. In other words, the mixed signal oscilloscope can first decode the protocol of the serial bus and then synchronize with it.

The fourth problem solved by the mixed signal oscilloscope is to directly display the captured deep storage data in high definition (Figure 3). Abnormal signals may occasionally appear in the pulse width modulation (PWM) signal. The mixed signal oscilloscope can directly distinguish the abnormal signal from other signals in the deep memory in the form of bright spots or other eye-catching forms. Even for a single acquisition, there is no problem. In addition, these abnormal bright spots can be magnified, observed, measured and analyzed. As can be seen from Figure 4, after magnifying one of the bright spots, it is found that the abnormality is a short amplitude drop at the end of the positive pulse. The engineer can then specifically measure the time and amplitude information of the abnormal signal.

For special packaging forms such as BGA and circuits using FPGA, the circuit itself does not have many testable pins. 18 or 20 channels are often good. In addition, the development tools provided by FPGA suppliers often have limited pins. If Xilinx chips are used, the Agilent FPGA debugger E5904B can be used with a mixed-signal oscilloscope to simultaneously observe the interaction between the internal nodes of the FPGA and the peripheral signals.

Currently, most of the digital oscilloscopes in use are 2-channel or 4-channel. When a large number of digital signals need to be debugged, engineers with good conditions will use logic analyzers. However, the debugging efficiency of mixed signal circuits using logic analyzers or digital oscilloscopes in isolation is often very low. For example, in many cases, the key handshake activities in the circuit or the verification of the execution of specific tasks often involve analog signals and multiple digital signals that must appear in a certain time period and in a certain sequence. Therefore, it is necessary to synchronize the oscilloscope and logic analyzer and use them together. The current solutions are:

1. Allows the use of oscilloscope modules in logic analysis systems;
   
2. Use a time-correlated fixture to synchronize two instruments so that when the cursor of one instrument moves, the cursor of the other instrument also moves (i.e., cursor linkage function).

Compared with the mixed signal oscilloscope solution, the above two solutions are suitable for circuits that can lead out dozens or even hundreds of signal test points. The advantage is that the logic analysis function is very complete and powerful, and it can do disassembly and even advanced source code analysis. The disadvantage is that it can only lead out circuits with more than a dozen test points, which is obviously a bit overkill, and the price is relatively expensive, and it is more complicated to use than the mixed signal oscilloscope. Especially the second solution using the time-related fixture, if you want to transfer the oscilloscope data to the screen of the logic analyzer and display it together with the digital channel, the screen refresh rate will be very slow. If the oscilloscope has a 4M sampling point storage depth per channel, it may take 1 minute to transfer the data of the four channels of the oscilloscope to the logic analyzer for display once. For the example of the PCI bus data acquisition card mentioned above, the oscilloscope must be set to infinite persistence mode to discover the occasional clock signal amplitude drop. If the screen refresh rate is very slow, it is difficult to solve the problem, and the same is true for observing the DDR SDRAM signal eye diagram. Of course, you can let the two instruments display their own waveforms, which will not affect the waveform refresh rate of the oscilloscope, but observing multi-channel mixed signals is not very intuitive, and some manufacturers' time-correlated fixtures do not support the cursor linkage function, which makes it even more inconvenient to use.

Mixed signal oscilloscopes are products developed based on the characteristics and test requirements of analog and mixed signal circuits, and their price positioning is the same as that of digital storage oscilloscopes ( DSOs ). In today's circuits, many test points cannot be touched or brought out, logic analyzers are not fully utilized, or there is only money to buy oscilloscopes but not logic analyzers. In this case, mixed signal oscilloscopes are a good choice.