“Today’s embedded designs based on microcontrollers (MCUs), field programmable gate arrays (FPGAs), and digital signal processors (DSPs) typically contain both analog and digital components. Design engineers have traditionally used oscilloscopes and logic analyzers to test and debug these mixed-signal embedded designs. Now, a new class of measurement tools—mixed-signal oscilloscopes (MSOs)—enable you to better debug these MCU, FPGA, and DSP-based designs.
Today’s embedded designs based on microcontrollers (MCUs), field programmable gate arrays (FPGAs), and digital signal processors (DSPs) typically contain both analog and digital components. Design engineers have traditionally used oscilloscopes and logic analyzers to test and debug these mixed-signal embedded designs. Now, a new class of measurement tools—mixed-signal oscilloscopes (MSOs)—enable you to better debug these MCU, FPGA, and DSP-based designs.
So what is a mixed signal oscilloscope, what are its usage requirements and how can it help you test?
When it comes to mixed-signal oscilloscope MSOs, there are several key specifications to be aware of: channel count, bandwidth, and sample rate. Meeting these requirements is key to effectively monitoring the various analog and digital I/O signals in a typical MCU/FPGA/DSP-based design. You’ll also learn about the various mixed-signal trigger types that you should focus on in your mixed-signal oscilloscope MSO to effectively test and debug your embedded designs.
What is a Mixed Signal Oscilloscope (MSO)?
The Mixed Signal Oscilloscope MSO is a hybrid test instrument that combines the full measurement capabilities of a digital storage oscilloscope (DSO), including auto-scaling, trigger holdoff, infinite persistence for analog and digital channels, and probe/channel skew correction. Some of the measurement functions of a logic analyzer are combined in one instrument. Using the MSO, you will be able to see multiple time-aligned analog and digital waveforms on the same Display screen, as shown by the individual oscilloscopes in Figure 1. While MSOs may lack many of the advanced digital measurement capabilities and numerous digital acquisition channels found in full logic analyzers, for today’s embedded design debug applications, MSOs have some unique advantages that traditional oscilloscopes and logic analyzers do not have. .
One of the main advantages of a mixed signal oscilloscope MSO is the way it is used, which in many respects is the same as an oscilloscope. Design and test engineers are often reluctant to use logic analyzers — even when they need to efficiently debug complex designs — because of the time it takes to learn or review how to use a logic analyzer. Even if the engineer is familiar with how to use a logic analyzer, the setup steps to make a specific measurement are far more cumbersome than setting up an oscilloscope. In addition, the advanced measurement capabilities of logic analyzers are often too complex for many of today’s MCU, FPGA, and DSP-based designs.
Oscilloscopes are the most commonly used test instruments in R&D environments. All embedded hardware designers should have a basic understanding of how to use an oscilloscope to measure the signal quality and timing characteristics of mixed-signal embedded designs. But to monitor important timing interactions between multiple analog and digital signals, 2- or 4-channel oscilloscope measurements are generally insufficient. And that’s the MSO’s strength.
Because the mixed-signal oscilloscope MSO provides “just enough” logic analyzer measurement capability without a significant increase in operational complexity, it is an ideal tool for debugging in embedded designs. As mentioned earlier, MSOs are used in the same way as DSOs. In fact, you can simply think of an MSO as a multi-channel oscilloscope where the analog channels provide high vertical resolution (usually 8 bits) and the added logic/digital channels provide low resolution (1 bit) Measurement. The highly integrated MSO mixed-signal measurement solution is simpler to use than a DSO, has a faster waveform update rate, and operates more like an oscilloscope than a logic analyzer.
The waveform update rate is an important metric for any oscilloscope. Slow speed and slow response will affect the normal use of the oscilloscope, whether it is DSO or MSO is no exception. Therefore, when the oscilloscope manufacturer adds the logic acquisition channel to the DSO to form the MSO, the waveform update rate must not be sacrificed. Otherwise, the way traditional oscilloscopes are used will suffer. Mixed-signal measurement solutions can be unresponsive and difficult to use if they use a dual-box configuration, or use an external communication bus such as USB to connect the logic interface. The MSO adopts a highly integrated hardware architecture, which is not only more responsive, but also much easier to use.
To learn more about the importance of waveform update rates, download the Keysight application note “Oscilloscope Waveform Update Rate Determines the Probability of Capturing Infrequent Events” (listed at the end of this article).
In the evaluation process before purchasing a mixed signal oscilloscope MSO, you should first compare the operating characteristics and measurement performance described in each manufacturer’s printed manuals and online literature (technical data). This has a certain reference value for evaluating the applicability and responsiveness of the instrument; but the only really effective method is to get started and test it in person.
Typical Mixed Signal Oscilloscope MSO Measurement Application and Required Performance
While mixed-signal oscilloscopes MSOs are ideal for capturing analog and digital signals on mixed-signal devices such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), their measurement applications also include verification and debugging of MCU/FPGA/DSP-based And includes a mixed-signal design of embedded address and data buses. Figure 2 is a block diagram of a typical mixed-signal embedded design with a microcontroller core.
Figure 2. A typical embedded design based on a mixed-signal oscilloscope MSO
Although microcontrollers and DSPs are generally thought of as simple digital control and processing devices, the vast majority of today’s MCUs, FPGAs, and DSPs are actually mixed-signal devices that include embedded analog circuitry. Therefore, it is necessary to monitor and verify the analog I/O, digital parallel I/O ports, and signals on digital serial communication buses such as I2C and SPI in the system.
Note that the block diagram in Figure 2 does not show any address or data bus signals. This is because the internal bus structure of most MCUs and DSPs also includes embedded memory (RAM and ROM).
Since today’s mixed-signal oscilloscope MSOs generally have 16 digital acquisition channels, some engineers mistakenly believe that MSOs can only be used for 8-bit processing applications (8-bit data + 8-bit address = 16-bit). But MSOs are primarily used to monitor analog and digital I/O, all signals typically available in MCU- and DSP-based designs. Do not try to correlate the number of digital acquisition channels in an MSO with the number of processing bits in an internal bus-based MCU or DSP, as they are usually not related. To monitor and verify designs based on 8-bit, 16-bit or even 32-bit MCU/DSP, 16 digital acquisition channels and 2 to 4 analog acquisition and trigger channels are generally more than enough.
However, monitoring parallel address and data lines in an external bus-based design (such as a computer with a 32-bit microprocessor) is not the primary measurement application for a mixed-signal oscilloscope MSO.
If you need to capture signals from multiple address and data buses to verify timing and source code flow in external bus-based systems, a logic analyzer with state analysis and disassembly capabilities may be a better measurement tool. But if you also need to time correlate analog and/or analog characteristics of digital signals at the same time, the multi-vendor dual solution (oscilloscope + logic analyzer) involves importing the oscilloscope waveform into a time-correlated display screen. in the logic analyzer. But when you adopt this higher-performance dual-box test solution, you also have to accept the logic analyzer’s more complex operation, including slow or single-shot waveform update rates.
But even in 32-bit systems with external memory devices, a mixed-signal oscilloscope MSO with 16 logical time channels and 2 to 4 analog channels is usually sufficient for measuring critical time parameters. Figure 3 is an example of using MSO to verify the setup time of a high-speed memory device (SDRAM) in a 32-bit system (IBM PowerPC 405GP). Using the MSO’s pattern trigger function, only 4 MSO digital channels can complete the measurement of specific read and write commands (CS, RAS, CAS, WE). The analog channel of the oscilloscope is further qualified to trigger on one edge of the high-speed clock signal and make critical timing measurements on the 100 MHz clock signal (top yellow trace) relative to a specific data signal (middle green trace), The resulting settling time for this external memory device is 8 ns. Such measurements are not possible with a conventional 2- or 4-channel DSO, and would be extremely time consuming with a logic analyzer connected to a high-speed oscilloscope.
ΔX = 8.00ns
1/ΔX = 125.00 MHZ
DY(1) = 0.0V
Figure 3. Critical Settling Time Measurements Using a Mixed Signal Oscilloscope MSO in a 32-Bit System
For these types of signal integrity measurements in mixed-signal embedded designs, the analog and digital acquisition performance of the MSO is often far more important than the channel count. The most basic specifications of an oscilloscope’s analog acquisition performance are bandwidth and sample rate. For more accurate analog measurements, the oscilloscope bandwidth should be at least five times the maximum system clock rate. For example, if you need to monitor a digital signal with a maximum trigger/clock frequency of 200 MHz with the oscilloscope’s analog channels, the oscilloscope’s analog bandwidth should be 1 GHz in order to capture the 5th harmonic with reasonable accuracy. For real-time/single measurements, the sample rate of the oscilloscope should be 4 times the oscilloscope bandwidth or faster. To learn more about the relationship between bandwidth and sample rate, download the Keysight application note “What You Need to Know About Sampling Rates” and the application note “Evaluating the Oscilloscope Bandwidth Needed for Your Application”.
Unfortunately, some users of oscilloscopes and logic analyzers do not fully appreciate what kind of digital acquisition performance a mixed-signal oscilloscope MSO and logic analyzer needs to have. It is very important that the digital acquisition performance of the MSO should be commensurate with the analog acquisition performance of the oscilloscope. But that doesn’t mean it’s a simple combination of a high-performance oscilloscope and a low-performance logic timing analyzer. Keysight recommends that the MSO’s digital/logic acquisition system sample rate be at least twice the bandwidth of the oscilloscope’s analog acquisition channel. In the example we just discussed above, a 1 GHz oscilloscope needs to capture the analog characteristics of a digital signal with a trigger/clock rate of 200 MHz, while the same signal needs to be captured on the MSO’s digital/logic channels with suitable timing accuracy, the digital / The logical channel must achieve a sampling rate of 2 GSa/s.
When you use logic/digital acquisition channels, the measurement resolution is limited to ±1 sample period. For example, if you intend to capture digital signals at a maximum trigger/clock rate of 200 MHz (period = 5 ns), each high or low pulse might be as narrow as 2.5 ns (assuming a 50% duty cycle). This means that if your MSO digital acquisition system samples at a maximum rate of 2 GSa/s, the timing measurement error on any one pulse edge may reach ±500 ps, which is the worst-case condition for time-difference measurements. 1 ns peak-to-peak error, or 40% error on a 2.5 ns pulse. We believe that more than 40% timing error is unacceptable for both MSOs and logic analyzers, which is why we recommend that the sample rate of the digital acquisition channel must be at least twice the bandwidth of the oscilloscope.
In addition to bandwidth and sample rate, another important factor to consider is probe bandwidth; both analog and digital systems probe bandwidth. If the analog or digital signal you want to capture contains significant frequency content over 500 MHz, you need to use active probes on the analog channels. Likewise, a digital acquisition system probe must be able to provide a higher frequency signal to the digital system’s sampling circuitry to reliably capture each pulse in a higher frequency pulse train.
Mixed Signal Triggering
The higher number of acquisition channels of a mixed-signal oscilloscope MSO (compared to a DSO) means that you can now implement more triggering methods to specifically acquire specific interactions of analog and digital I/O signals. While not as sophisticated as a high-performance logic analyzer, the triggering capabilities of an MSO far exceed the limited triggering capabilities of a standard 2- or 4-channel oscilloscope.
Most mixed-signal oscilloscope MSOs and mixed-signal measurement solutions on the market today are capable of triggering on at least a single-level parallel pattern trigger condition, and some MSOs even offer bi-level pattern sequence triggering including a reset condition. However, even if you are using relatively simple single-level pattern triggering, you will find huge differences in triggering capabilities between the various MSO/mixed-signal measurement solutions. First and foremost, the MSO can trigger on a combination of analog and digital inputs. For somewhat loose mixed-signal measurement solutions, reliable triggering can only be implemented on one side or the other of the acquisition system due to the large signal skew between analog and logic channels. That is, you can only trigger on the oscilloscope on a traditional analog trigger condition, or on only one of the parallel digital conditions — not on both conditions at the same time. The MSO should provide mixed-signal triggering with precise time alignment between the analog and digital channels being triggered. We will exemplify the need to trigger on mixed-signal conditions later in this article so that the oscilloscope can acquire simultaneously on a specific output phase of the DAC controlled by the MCU.
Another important consideration for a mixed-signal oscilloscope MSO is whether its pattern triggering includes any kind of time qualification. In addition to entering and/or exiting trigger qualifications, pattern trigger conditions should include minimum time constraints. An easy way to illustrate this is to trigger on an unstable transition state first; then demonstrate what tools an oscilloscope can use to avoid this instability. Figure 4 is an example of triggering on pattern CE (1100 1110) using the Keysight 6000 X-Series MSO. We can see this status from the top half of the display (which gives a better overall picture of the signal).
CE and EE are unstable transition states between DE and E4 on the bus. It is most likely not the trigger that the user intended. At this point, the user can use the time limit menu (Qualifier) of the oscilloscope to set the time threshold for the trigger, that is, the trigger state must remain longer or shorter than the specified time; or remain within or outside the specified time range.
The minimum time limit is important to avoid triggering under transitional/unstable conditions. When parallel digital signals change states, the switching process may be nearly simultaneous — but not exactly simultaneous. In addition to the limited rising and falling edge speeds of the signal when it is not high or low, even in the best designed systems there will be slight delays between signals. This means that your system will always have jittery/unstable signal conditions when switching signals. You certainly want your DSO/MSO or logic analyzer to avoid triggering under these unstable conditions if possible.
Oscilloscopes (including mixed-signal oscilloscopes MSOs) are capable of triggering precisely at the analog trigger level/threshold crossing point, while logic analyzers typically use sample-based triggering. Sample-based triggering will produce a peak-to-peak trigger jitter/uncertainty of ±1 sample period (worst-case peak-to-peak uncertainty = 2 sample periods). With “sample-based triggering”, we first let the instrument randomly sample the input signal, and then establish a trigger reference point based on the sampled data. This type of triggering produces significant trigger jitter, which may be acceptable for some typical logic analyzers, but not acceptable for regular oscilloscope or MSO measurements used to observe repetitive signals.
Figure 4. The oscilloscope triggers on an unstable transition state without a minimum time limit.
Figure 5 is an oscilloscope with a mixed-signal option that generates trigger events from sampled data. Figure 6 shows an example of a Keysight mixed-signal oscilloscope MSO that utilizes analog hardware comparators to trigger on all analog and digital input signals.
Figure 5. Sample-based pattern triggering produced 4 ns trigger jitter (using LeCroy WaveRunner with MSO option for mixed signal oscilloscope).
In this mixed-signal measurement example, each oscilloscope is set up to trigger on a specific 8-bit pattern on the MCU’s digital output port. This condition is synchronized with the rising edge of digital input channel D4 (A4). To measure the signal integrity of the D4 (A4) signal, we set up one of the oscilloscope’s analog channels to “double-probe” the same digital signal. As you can see in Figure 5, an oscilloscope with digital triggering on sampled data produces a peak-to-peak trigger jitter of almost 4 ns because its maximum digital/logic channel sample rate is only 500 MSa/s (±1 uncertainty sampling period). Note: There is a 4 ns peak-to-peak “smear” in the repetitive analog trace (green trace in the middle) when using the scope’s infinite persistence display mode.
Figure 6 shows the results of the same repeated trigger measurement performed using the Keysight Mixed Signal Oscilloscope MSO, which generates trigger events based on real-time analog comparator hardware technology rather than sample-based triggering. With the scope set to 5 ns/div, we were able to observe very stable analog traces with the scope’s infinite persistence display mode, even though triggering was only based on the scope’s digital/logic channel inputs. We can now make more accurate signal integrity measurements on repetitive input signals using one of the oscilloscope’s analog input channels.
Figure 6. Real-time comparator hardware pattern triggering in the Keysight MSO produces very low trigger jitter.
One last thing to consider when evaluating various MSO/mixed-signal measurement solutions for your mixed-signal embedded application is whether the oscilloscope can trigger on. Serial I/O is very widely used in today’s embedded designs. In the next section of this article, we’ll show an example where serial triggering is required to cause the oscilloscope to synchronously acquire a specific analog output “chirp” signal based on a serial input command in a mixed-signal embedded design.
Activate and debug true mixed-signal embedded designs
Let’s now look at the activation and debug process of a mixed-signal embedded product designed by Solutions Cubed, Inc. in Chico, California. Figure 7 is the structure diagram of the product.
At the heart of this mixed-signal embedded product is the Microchip PIC18F452/PT microcontroller, which operates using an internal 16-bit instruction set. Because this particular MCU has an internal bus structure and includes an embedded analog-to-digital converter (ADC), the mixed-signal device and corresponding peripheral circuits are extremely useful for activating and debugging embedded mixed-signal designs using a mixed-signal oscilloscope MSO. Good example.
The ultimate goal of this design is to generate analog “chirp” output signals of varying lengths, shapes, and amplitudes based on a variety of analog, digital, and serial I/O input conditions. (A “chirp” is an RF pulsed analog output signal that includes a specific number of cycles, often encountered in aerospace and defense and automotive applications).
Figure 7. A mixed-signal embedded design that produces an analog “chirp” output from analog, digital, and serial I/O
The MCU simultaneously monitors the following three inputs to characterize the output chirp signal:
1. Use a parallel digital I/O port on the MCU to monitor the status of the system control panel to determine the shape of the chirp signal (sine, triangle, square) produced by the output.
2. Monitor the output level of the acceleration analog input sensor through an ADC input on the MCU to determine the amplitude of the chirp signal generated by the output.
3. Use the dedicated I2C serial I/O port on the MCU to monitor the status of the serial I2C communication link to determine the number of pulses generated in the output chirp. This I2C communication input signal is generated by another smart subsystem component in the embedded design.
Based on the three states of analog, digital, and serial input, the MCU is programmed to continuously output a string of parallel signals to an external 8-bit DAC to generate analog chirps of various amplitudes, shapes, and lengths. The unfiltered staircase output of the DAC is fed to an analog low-pass filter to smooth the signal and reduce noise. This analog filter also introduces a predetermined amount of phase shift to the output signal. Finally, the MCU generates parallel digital outputs through another available digital I/O port to drive the LCD display to provide the user with system status information.
In this design, the first step in designing/programming the MCU is to configure the MCU’s I/O with the appropriate number of analog and digital I/O ports. Embedded system designers have to consider the trade-off between the number of analog I/Os and the number of digital I/O ports in a MicroChip, a particular microcontroller.
Before attempting to program the MCU to monitor various inputs and generate the specified final output signals, the design team decided that it would be best to develop test code that activates a certain part/function of the embedded design, before adding interactive complexity. Verify that it works correctly and signal integrity. The first part of the circuit/first function to activate and debug is the external DAC output and input and the analog filter. To verify that the circuit and internal firmware were working correctly, we initially coded the MCU to generate a continuous/repetitive sine wave of fixed amplitude regardless of input signal conditions.
Figure 8 is a screenshot of the Keysight InfiniiVision Series Mixed Signal Oscilloscope MSO, which simultaneously captures the continuous digital input of the external DAC (the output of the MCU’s digital I/O port), as well as the DAC’s staircase output and analog filtered output. Since this particular signal is a relatively low-level output signal, using only 16 levels of an 8-bit DAC (up to 256 levels), we can easily observe the converter’s staircase waveform output characteristics on the oscilloscope display screen (green trace).
Set this particular acquisition to trigger when the DAC output reaches its highest output level (center of the screen). Triggering at this particular point is not possible with a regular oscilloscope, as oscilloscope triggering requires edge transitions. To trigger at this point/phase of the output signal, we first establish a simple single-level pattern trigger condition based on a digital input signal that corresponds to the highest output analog level of the external DAC. To trigger at exactly this point on the waveform, the designer entered the parallel binary pattern “1110 0110”. Because this MSO uses “time-qualified” pattern triggering, the oscilloscope always triggers at the beginning of the specified pattern and never triggers on unstable/jump conditions.
Figure 8. The Keysight InfiniiVision Series MSO captures the parallel digital input and analog output of a DAC controlled by an MCU.
Figure 9. Keysight MSO triggers at 50% crossover using a combination of analog and digital pattern triggering
Figure 9 shows another trigger condition for the MSO in addition to the analog trigger condition. When using pattern triggering on parallel digital input signals, it sets the MSO to trigger precisely at the 50% output level of the DAC. As mentioned earlier, not all MSO/mixed-signal measurement solutions allow mixed-signal triggering on a combination of analog and digital conditions. But since there are two analog output conditions at the same level (50% rising level and 50% falling level), to be consistent with triggering on a rising or falling point requires more than just an 8-bit input pattern trigger on the pattern. By additionally qualifying to trigger on the “0” level of analog channel 1 (top yellow trace), the oscilloscope can use a combination of analog and digital pattern triggering to trigger on the desired phase. Note that the analog signal is considered a “1” above the analog trigger level and a “0” below the trigger level.
Figure 9 also shows automatic parametric measurements of the filtered output signal, including amplitude, frequency, and phase shift relative to the unfiltered DAC output.
After activating and verifying that the external DAC and analog filter circuit are working properly, the next step in the design/activation process is to program to generate a specific number of non-repetitive sine wave pulses (chirps) based on the serial I2C input. Figure 10 shows the overlap (infinite persistence) of chirps of different lengths obtained using standard scope edge triggering. In contrast, edge triggering of traditional oscilloscopes cannot be limited to triggering on a chirp of a specified length.
Figure 10. Conventional oscilloscope edge triggering cannot synchronize triggering on a chirp of a specific length.
Using I2C triggering, Keysight MSO oscilloscopes can start acquisitions synchronously on specific serial input conditions. These conditions instruct the MCU to generate an output chirp of a specified length (number of pulses).
Figure 11 depicts the oscilloscope’s ability to trigger on 3-cycle chirp and I2C trigger on serial address and data content. Data channel D14 and D15 are defined as I2C clock and data input trigger signal respectively. We can actually define any of the 16 digital channels or 2 to 4 analog oscilloscope channels to trigger serially on these two input signals. While monitoring the serial input and analog output signals, D0-D7 are set to monitor the external DAC input (MCU output) signal in the “Bus” overlay display.
Figure 11. Triggering on a 3-cycle chirp using the I2C trigger and decode function in the Keysight MSO.
The bottom of Figure 12 shows the time-correlated I2C serial decode traces. You can also view serial decodes in a more familiar tabular format, shown in the upper half of the display.
Figure 12. I2C signals can be displayed in time-correlated or table-decoded form (see the top half of this screen).
Although not shown, another analog channel of the oscilloscope can be set up to simultaneously detect and acquire the analog input signal from the accelerometer to determine the magnitude of the output signal. Alternatively, you can use free MSO digital channels to monitor and/or further qualify triggering on digital control panel inputs and/or LCD output driver signals.
Mixed-signal oscilloscopes (MSOs) are ideal for debugging and verifying that today’s various MCU, FPGA, and DSP-based mixed-signal designs are working correctly. The MSO’s ability to display time-correlated analog and digital waveforms simultaneously on a single instrument with powerful mixed-signal triggering on all analog and digital channels allows designers to use their familiar, oscilloscope-like user interface and Debug mixed-signal embedded designs faster with the tools of the Way.
Download a free trial version of the Embedded Software Suite to experience MSO triggering and decoding.
There are many MSOs and comprehensive mixed-signal measurement tools on the market today. Before deciding to buy, you must carefully evaluate the measurement capabilities and practicality of these instruments.
You should pay particular attention to the following seven features:
1. The MSO should be used like an oscilloscope you are familiar with — not like a logic analyzer.
2. The MSO should have all the measurement capabilities of an oscilloscope without sacrificing other features such as autoscale, trigger holdoff, infinite persistence (analog and digital channels), and probe/channel skew correction.
3. The MSO should provide a fast waveform update rate like an oscilloscope, but not a slow update rate like a logic analyzer.
4. The MSO’s digital/logic channel acquisition system performance (sampling rate and detection bandwidth) should match the oscilloscope’s analog acquisition system performance.
5. The MSO should be able to trigger across analog and digital channels (mixed-signal triggering) with precise time alignment.
6. The MSO should be able to trigger on the pattern according to the minimum time limit to avoid triggering under unstable/jumping digital switching conditions.
7. The MSO needs to be able to provide both analog and digital triggering based on real-time analog comparator technology — not sample-based triggering (the latter can produce significant trigger jitter on repetitive analog waveforms).