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What do these embedded processors and their applications have in common? To sum up, there are several obvious features: First, these processors have been fully verified by many products, people are well aware of it, and the development can be widely supported, easy to design; Second, compared with the existing high-end program, the clock rate Relatively slow, the bus speed is also very slow; Third, the application system (from vending machine to avionics) must have high reliability; finally, the cost (including design, manufacturing and maintenance costs) must be as much as possible low.
Another feature worth noting is that there is currently a steady development trend. The rate of these embedded devices and bus clocks are constantly increasing, not to mention catching up with the fastest server, but will be toward the "short clock cycle." "The direction of the device development, the clock rate faster than before 5 to 6 times. Compared with previous processors, the new device has the same pin and function, but can do more work in a given time, it can perform more instruction cycles to complete more complex work without slowing down the entire system. This is especially beneficial to software developers because time-consuming code optimization is no longer important, and new products will be brought to market faster and cheaper.
Ordinary oscilloscopes that perform basic digital checks have more than doubled their bandwidth to 200 MHz, and some very useful "high-end" measurement features such as advanced triggering, Fast Fourier Transform (FFT) analysis, and color display have all been added to the low end. Instrument. Today's designers can also use the digital fault detection solution when facing civilian products embedded processors.
The bandwidth-determined application processor that was produced not too long ago has one more hidden “performance†than the same device that was produced 10 years ago, that is, the signal edge transition speed is faster. From the CMOS process used to produce these products 15 years ago to the fast 5V process developed five years ago, the edge conversion speed has increased about 3 times. Many new designs use this fastest 5V process, and some even further reduce The voltage in the core is only 5V at the periphery, and the latter can also achieve a faster clock rate. This speed increase is a byproduct of the shrinking feature size of the silicon.
Faster edge speeds are usually a good thing, reducing latency, set-up time, and conflicts within the system, but shorter propagation delays (mostly due to faster CMOS edge rates) can also have an adverse effect. When such delays are getting shorter and shorter, the address decoding margin, which usually depends on the delay between the address line logic and the bus control line, will encounter more trouble. Therefore, designers need to know and understand these edge conditions, narrower transient phenomena, and other pulse characteristics that may occur during high-speed switching.
When choosing a DSO for a digital design with a 20-MHz embedded processor, one might think that using a 50MHz or 100MHz bandwidth to handle this task is more than sufficient. Of course, some basic fault detection, such as the presence or absence of a signal or the timing and synchronization are exactly the same, but other details may not be so obvious.
DSO with higher bandwidth can understand signal characteristics more deeply than low-bandwidth instruments because oscilloscope rise time has become one of the factors that determine the quality of the observed signal. The formula is as follows:
Measured rise time = √ (oscilloscope rise time) 2+ (signal rise time) 2
Pulses that appear “correct†when observed at low bandwidth may have an amplitude offset at the leading edge, making it appear like two pulses; or a very narrow transient signal on the bus output may be completely unnoticed, causing subsequent device inputs to fail stable. As the above formula shows, a 200MHz DSO can capture details not seen by the 100MHz instrument.
The benefits of large DSO bandwidth are not limited to just observing the edge of the signal. When using high-bandwidth instruments, ground bounce, noise, crosstalk, and many other aberrations are all easier to observe and less easy to ignore. The higher the bandwidth, the more accurate the signal is reproduced. Figure 1 shows the different conditions seen on the 60MHz and 200MHz bandwidth oscilloscopes for the same signal.
Detecting Timing Problems with Conditional Triggering In a digital storage oscilloscope, triggering condition selection is an important but sometimes less well-known and labor-saving tool that allows DSO triggering to meet specified conditions. Like displaying waveforms, conditional triggering is an essential tool for embedded system detection. Many people use noise suppression (usually increasing the trigger lag) to limit short pulses and use various bandwidth limitations to select the desired signal.
One of the most versatile triggering feature pulse width triggers has recently been ported from high-end lab instruments to ordinary DSOs. This setup triggers an oscilloscope when the input signal pulse width is:
Less than the specified time greater than the specified time equal to the specified time (within the nominal error range)
Not equal to the specified time (within the nominal error range)
"Less than" pulse width triggering is one of the quickest ways to find a suspicious transient pulse at the bus or device output. When a short transient pulse caused by crosstalk or timing gating the output enable or chip select input of the device, it will cause intermittent problems, causing the device to send data to the bus at the wrong time, resulting in unpredictable results. The "less than" trigger detects pulses that are shorter than the user-specified width so that the oscilloscope can capture all the signals present at the probe input. This method yields not only the transient phenomenon itself, but also the output enable and data bus results.
The "greater than" trigger helps to find some "stuck" data or other signals that have not been returned to the default state after processing, causing the oscilloscope to trigger when the falling edge of the pulse does not occur at the specified time. For example, a data bus output signal transitions to “1†in response to the output enable action and does not transition to a new state afterwards. This may be due to the inaccuracy of the output enable signal itself, the excessively long three-state transition time of the driven device, or data The next value of the bus does not appear for a variety of reasons, such as the "greater than" trigger, the error can be found, the signal that affects all oscilloscope channels will be reproduced, and after some checks, it will be possible to find out what causes it. problem. The time range here is the same as other pulse width trigger settings, ranging from tens of nanoseconds to a few seconds, to provide sufficient time to ensure that the measured is actually a "stuck" signal without delaying the signal.
"Equal" trigger provides a method to replace the voltage threshold trigger when the trigger signal (such as the output enable) is triggered by a transient signal or noise to cause oscilloscope pseudo triggering. With a basic embedded microprocessor can illustrate this situation. Most of these devices include an external bus that allows the processor to extend the built-in memory or peripheral interface. This bus typically allows peripheral circuitry to control the timing of data transfer between the processor and the processor. The processor first gives an address and then sends an address strobe. The selected peripheral circuit finally sends a "receive" signal to confirm that the processor's instruction was received. The clock delay required for this process is known (usually user-specified), and the specific circuits vary.
Knowing this delay time is the key to distinguishing the peripheral circuits and checking the response activity of the test points. The method is simple, use the address strobe as a trigger and set the pulse width trigger time equal to the number of peripheral clock delays. The address strobe rises and the countdown starts. The trigger circuit waits for a preset time, and the oscilloscope triggers and probes. The signal condition at the test point. By definition, this is the time that the peripheral is active on the bus, so "equal to" the pulse width trigger makes the oscilloscope to some extent can assume the work of the logic analyzer.
Frequency measurement with built-in counters Automatic frequency measurement is almost one of the functions of the DSO from the very beginning. In general, the first cycle of the acquired waveform can be checked. This is a useful tool for measuring a one-time event, but it cannot produce a continuous, high-precision waveform averaging frequency.
Another method of frequency measurement is the use of common frequency counters. Such frequency meters are generally available and often cheaper. This method can also be implemented in DSO using the trigger signal as the source signal for the average frequency reading, which is a new feature of the current low-end DSO. Frequency counters are measured in many different ways. The most common and simplest method is a fixed frequency counter that counts the number of input cycles (display counts) over a fixed period of time; or a fixed cycle counter, used to calculate one cycle. The number of hours (showing the reciprocal of the count). Both methods have good accuracy when the count is large, and have poor accuracy when the count is low. A similar method is to divide the measurement interval into two halves. The first half calculates the time and number of pulses. Once half the number of points is reached, the measurement is terminated when the input transitions (the same polarity as the transition at the beginning of the measurement). This method cannot achieve the highest accuracy in extreme cases, but the general accuracy is at 1/2 of the highest accuracy, providing a stable and easy-to-read frequency display (accurate to 6 digits) for effective trigger events. Since any event (within a reasonable range of magnitude) can essentially be used as a trigger event, the "reading" here is actually a universal frequency counter.
When troubleshooting an embedded system, it is often necessary to check the frequency of various local clock signals, including the main crystal. At this point, the oscilloscope triggers the counter to provide a fast internal solution. The measurement is more accurate than the automatic frequency measurement based on the waveform, and there is no need to set up a separate instrument for frequency counting.
The counter also helps to find sources of crosstalk and noise. For example, if the counter finds that the frequency of a noise signal on the bus is 100 kHz, there may be problems with the crosstalk or grounding of the switching power supply. Similarly, if the noise signal frequency is 1/2 of the main clock, the problem may be due to crosstalk from the side bus. Since the signal source can actually be any trigger signal, the counter can measure the frequency of any conditional trigger event, not just the voltage trigger that occurs once per cycle, such as combining the counter with the pulse width trigger, it can also be determined to occur in continuous The frequency of a particular pulse width within the pulse.
Color Waveform Display Color LCDs have previously been available only in high-end experimental instruments, but they are now also found in some common DSOs. Colors add an extra layer of information to the display, making it easier than ever to perform such tests.
The waveform is just a line on the screen. What kind of benefits can be expressed in color? The color can be seen more clearly when observing multiple waveform lines, and each line uses a different color. This color coding method is also used on the front panel of the oscilloscope. For example, the yellow knob controls the yellow wave and is connected through the yellow probe. It can also be extended all the way to the probe, even to the circuit under test, and the test points are marked with different colors. . In addition, color is also useful when superimposing two waveforms for comparison, and some colors will be clearer under dimming lighting conditions.
Color oscilloscopes can increase productivity. Simply put, simple applications can reduce many small human errors. Such small errors often take several hours to resolve.
Using Digital Storage Oscilloscopes to Eliminate Embedded System Digital Faults
Abstract: Although current servers and PCs are increasingly favoring advanced high-speed processors, the low-end 20 or 30 MHz processors are still playing a role in real life. Although these clumsy microprocessors are old and have been around for many years, they still have a place in machinery, consumer electronics and automotive electronics.