Oscilloscope Serial Data Measurements and DAC: Trigger, Decode, Measure/Graph and Eye Diagram Software

Figure 1. Serial bus measurements made available
with "TDME" and "TDMP "decoder options.

All Teledyne LeCroy oscilloscopes support a rich set of standard waveform measurement parameters, but the installation of any "TDME" or "TDMP" serial decoder software option adds special parameters designed for measuring serial data buses. Besides automating the measurement of serial bus timing, these parameters allow you to access encoded serial data and extract it to analog values for what is essentially a Digital-to-Analog Converter (DAC)!

What’s in a Name?

Teledyne LeCroy has adopted the convention of using a key in the name of our serial trigger and decode products that tells you what capabilities they offer.  The “ME” or “MP” in the name of a Teledyne LeCroy serial decoder option (e.g., CAN FDbus TDME or USB4-SB TDMP) refers to "Measure/Graph and Eye Diagram" or "Measure/Graph and Physical Layer Tests." All these options include the following 10 serial bus measurements. Physical Layer Test options will also include measurements designed specifically to meet the requirements of the standard.

Oscilloscope Basics: When to Use Trend to Graph Oscilloscope Measurements

Figure 1: Applying the Trend operator to the
same input waveform illustrates how the
Trend is asynchronous to the input waveform.  

In a previous post, we described the characteristics of the Track math function and two key applications of using Tracks to graph oscilloscope measurement data: anomaly detection and waveform demodulation. In this post, we'll discuss the characteristics and uses of the Trend function.

To illustrate an important distinction between Tracks and Trends, the Trend math operator in Figure 1 is now applied to the same signal as was the Track in our previous post without first reacquiring the input waveform. 

Note that unlike a Track, the Trend is not time-synchronized to the input waveform. Only the order of events, and not the timing of events, is retained. The underlying shape of the Track may be displayed in the Trend because the same measurement values from a single acquisition are displayed in the same sequence—however, the timing information of when each of the values has occurred is not retained in the Trend. Therefore, unlike the Track, the Trend does not point to the location of an anomaly. Without time scaling, the Trend does not have the frequency information needed to demodulate an input waveform.

Oscilloscope Basics: When to Use Track to Graph Oscilloscope Measurements

Figure 1: Pulse Width Modulated waveform (yellow)
and Track math operator (blue),
where the X-axis scaling is identical for both.

Modern oscilloscopes contain many tools that can be used for analyzing data, including Track and Trend math functions. Both Tracks and Trends graphically display measurement results and locate anomalies. The main similarity between Tracks and Trends is that the Y-axis of both operators is the measurement parameter itself (for example, Pulse Width, Duty Cycle, Rise Time, Slew Rate, etc.). The main difference between the two math operators is their X-axis, in which the Track uses the identical X-axis and synchronous horizontal scaling as the input waveform, whereas the Trend uses units of chronology. A Track, in essence, is a waveform of the measurement values. A Trend is a data logger showing the history of change in measured parameter values, but points are not necessarily synchronous with the measured waveform.

Use Tracks for Anomaly Detection

The Track provides valuable debugging information by directly pointing to an area of interest. 

Notice the negative-going spike in the Track waveform in Figure 1. Figure 1 occurs at the point in time where the input waveform reaches its most narrow pulse width, and the Track instantly finds it, indicating when one measurement deviates from the others in the graph. The Track identifies the exact location in time where the narrowest or widest pulse width has occurred, and fully describes the measurement changes occurring throughout the entire waveform. Since oscilloscopes can acquire thousands or even millions of waveform edges within a single acquisition, the Track allows an engineer to quickly "find the needle in a haystack".

Correlating Sensor and Serial Data in Complex Embedded Systems

Figure 1: Voltage output of a temperature sensor.
As the temperature rises, the output voltage falls. 

The microcontrollers/microprocessors in deeply embedded systems often are set up to monitor and control operational parameters.  Take, for example, a deeply embedded system where a microcontroller is used to control temperature that has been sensed by a temperature sensor.  The sensor is read through one of the microcontrollers analog interfaces.  As temperature changes are sensed, the microcontroller adjusts the speed of a cooling fan, which is driven by a pulse width modulated signal. The microcontroller uses a program algorithm to convert the DC level of the sensor into a PWM signal with an appropriate duty cycle to set the fan speed to correct any changes in temperature. 

Where it is possible to probe the temperature sensor, the output is a DC signal that changes very slowly over time.  Figure 1 shows a direct measurement of the temperature sensor using a heavily filtered oscilloscope channel to minimize noise pickup.