Most embedded systems use more than one power cable, many using 4 or more. A single IC, such as an FPGA, DSP, or microcontroller, may have specific timing requirements. For example, a chip manufacturer may recommend applying an I/O supply voltage after the core voltage supply is stable. Another manufacturer may require that power be supplied for a relatively specified period of time to avoid prolonged voltage differences across the supply pins. The power-up sequence between the processor and external memory can also be critical.
The chip manufacturer may specify that a particular power supply must be started in a single sequence to avoid multiple power-on resets. This can be challenging because inrush current can place high transient demands on the point-of-load regulator. In this case, the power line startup shape is just as important as the timing sequence.
Once you've combined the various chip power requirements, overall power, reference power, and other IC point-of-load regulators, you'll quickly encounter seven or eight power lines.
Using a 4-channel oscilloscope to verify power line timing in an embedded system can be time consuming, but this is what most engineers must do. As we communicate with oscilloscope users, evaluating the power-on sequence and shutdown sequence is one of the most common reasons engineers want more than four channels. In this article, we will briefly describe the use of a 4-channel oscilloscope to evaluate the power-on sequence and shutdown sequence, and demonstrate some examples of using an 8-channel oscilloscope.
Traditional 4-channel oscilloscope method
One of the methods is to analyze the power system in a sub-module manner, that is, use multiple acquisitions to check the timing on a module-by-module basis. To compare different modules, you can use one of the power-on trajectories or the Power Good/Fail signal as a trigger to perform multiple captures to determine the start-up time and shutdown time relative to the reference signal. Since the acquisition is performed in multiple power cycles, it is difficult to characterize the relative timing deviation of the power supply. However, by using the infinite persistence function on the oscilloscope, you can determine the range of variation for each power supply in different cycles over multiple power cycles.
Another common method is to "cascade" multiple oscilloscopes, usually by triggering the oscilloscope on one of the power supplies or on a common Power Good/Fail signal.
Both methods take a long time and require special attention to synchronization:
· Be cautious when dealing with synchronization and time uncertainty
· Can collect data and develop system timing diagrams, but it takes a long time
· Complexity increases with the number of observed power trajectories
· Settings must be perfectly unified
· A measurement channel must be used to provide synchronization
Use MSO to expand the number of channels
Mixed-signal oscilloscopes provide more channels for power sequencing. To this end, the MSO must have an appropriate voltage range on the digital inputs and can independently adjust the threshold. For example, the Tektronix MDO4000C with the MSO option provides 16 digital inputs with independent thresholds for each channel up to 200 MHz supporting ± 30 Vp-p dynamic range, suitable for most voltage levels in a typical design. Note that if your goal is to strictly measure timing relationships, then this method is particularly suitable, but you cannot measure the rise/fall time or shape (monotonicity) of the power on/off.
8-channel oscilloscope speeds up processing
Compared to all previous methods, using an oscilloscope with 8 analog channels can significantly reduce time and reduce clutter. In an 8-channel oscilloscope, you can use an analog probe to characterize up to eight power cables. To measure the timing of power-on and off-time with more than 8 power cables, you can also use a mixed-signal oscilloscope with digital signal input and independently adjustable thresholds.
Now let's look at some typical power sequencing applications.
Boot delay with remote on/off
The switching power supply tested in the screenshot below generates a high current, regulated 12 VDC output. This power supply is remotely controlled via a switch on the front panel of the instrument. Shortly after the switch is pressed, the +5 V standby power is turned on and the switching converter is turned on. After the +12 V output is stable, the Power Good (PW OK) signal goes high, indicating to the load that the power supply is reliable.
The +5 V standby voltage signal provides a simple rising edge trigger for correlated signal acquisition. The auto-measurement function verifies that the output voltage start-up delay is <100 ms, and the delay from the output voltage start-up to PW OK is within the specification range of 100 – 500 ms.
This screenshot shows how the AC/DC switching power supply starts when the front panel switch is pressed.
Shutdown delay with remote on/off
After the main power switch is turned off, the switching converter is turned off and the output voltage is lowered. According to the specification, the power supply must be kept at least 20 ms after the switch is pressed. Most importantly, according to the specification, the PW OK signal is degraded by 5 – 7 ms before the +12 V output voltage falls outside the regulation range, allowing load time response and clean shutdown.
As shown in the figure below, the PW OK signal provides a falling edge trigger for the acquisition related signal. The waveform cursor measurement verifies that the PW OK warning signal works in a manner that meets the specifications.
Waveform cursor measurements can be used to verify that the PW OK warning signal works in a manner that meets specifications.
Verify timing in multiple power cycles
In order to verify that the power-on timing is always within the specification range in multiple power cycles, an infinite persistence can be used to display the signal timing change, and the automatic timing measurement statistical screen quantifies the deviation. In the settings shown below, the 50% point of the +5V standby voltage is used as the timing reference. The power-on sequence is repeated 10 times, and the timing deviation in 10 power-on cycles is slightly higher than 1%.
Repeated on-time measurement can be achieved using infinite persistence and measurement statistics.
Load point regulated power supply timing
The screenshot below shows the turn-on time for a system board to power up seven load points during startup. The input power to the board is the +5V standby signal and the +12 VDC overall voltage in the above example.
The automatic turn-on delay measurement in this test is performed between the 50% points automatically calculated for each waveform, so each measurement has a different configuration with a different set of measurement thresholds. The first measurement shows the delay from the +5 V standby signal to the overall +12 V supply, and the second measurement is the +5V supply delay. The remaining measurements are the key delay sequence for the mains +5 V supply.
This measurement shows the turn-on timing of the seven regulated power supplies.
Shutdown timing of regulated power supply
The automatic shutdown delay measurement in this test was performed between each waveform point below 5% of the nominal value. Unlike previous percentage-based measurement thresholds, each measurement has an absolute voltage threshold. The Power Good signal drops when the power is turned off. As shown in the screenshot below, some of the power supply load is heavier and the shutdown is faster.
As can be seen from the figure, part of the power supply load is heavier and the shutdown is faster.
Boot timing of more than 8 tracks
Automatic delay measurements are based on when the signal crosses the respective threshold voltage. Since each auto-measurement configuration includes a unique threshold (typically 50% of the signal amplitude), each digital channel may have a unique threshold (generally set to 50% of the supply voltage), so a mixed-signal oscilloscope can do the following The power supply delay shown is measured until the number of available digital inputs. According to the MSO model, the number of channels can be between 8 and 64.
This shows the use of digital channels to verify the turn-on timing of more than 8 regulated supplies.
Power rise time measurement
In addition to power sequencing, the rise time of the power supply must be controlled to meet the specifications of some of the key components in the system. Auto rise time and fall time measurements are also based on voltage reference points. By default, the voltage reference point is automatically calculated to be 10% and 90% of the signal amplitude of each channel. In the simple example shown below, the result box on the right side of the display shows the rise time of the positive supply and the fall time of the negative supply.
The screenshot shows that the rise time and fall time measurements are shown in the results box on the right side of the screen.
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