Supply voltages: Verifying power sequencing for FPGAs, CPUs and DSPs

Components such as FPGAs, CPUs and DSPs must usually meet very specific requirements regarding the order in which the different supply voltages are powered up. If the voltages are powered up in the wrong order, this may result in damage to the components. The voltages must not exceed the tolerances specified by the manufacturer, and in many cases they must also comply with defined slew rates and specific delays upon reaching a target value. The numerous supply voltages for FPGAs, CPUs and DSPs therefore need to be applied in a specific order to ensure reliable operation. It is crucial to verify the correct power-up order, also referred to as power sequencing, during circuit design and product development.

Observing the correct order

During power-up of complex electronic components such as FPGAs, CPUs and DSPs, several supply voltages must be applied in a specific order and with defined delays and ramp-up times. It is necessary to minimize current draw and ensure that the I/Os are at high impedance during power-up.

The recommended power-down sequence is usually (but not always) the reverse of the power-up sequence. If required sequences are not observed, or if currents exceed specified limits, this can cause malfunctioning or even damage to the components. During circuit design, it is important to capture and analyze the characteristics of multiple voltages during power-up and power-down, as well as during voltage interruptions.

Using a suitable probe

To verify correct power sequencing, circuit designers need the right oscilloscope along with a suitable probe such as the R&S RT-ZVC from Rohde & Schwarz. This multichannel oscilloscope probe offers up to four voltage and four current channels with a very high dynamic range. Each channel has an ADC with 18-bit resolution at 5 Msample/s and 1 MHz bandwidth. The probe shows its strengths in combination with an R&S RTE1000 or R&S RTO2000 oscilloscope. Using two R&S RT-ZVC probes on one of the four-channel oscilloscope models, up to 20 voltages can be analyzed in parallel. For this configuration, the current channels must be operated as highly sensitive voltmeters in external shunt mode.

The right oscilloscopes

The R&S RT-ZVC probe is recommended for use with a series R&S RTE1000 or R&S RTO2000 oscilloscope. The R&S RTE1000 is a cost-effective, Windows based oscilloscope designed for touchscreen operation and providing analysis functions for challenging development tasks. With bandwidths from 200 MHz to 2 GHz and comprehensive time, frequency, protocol and logic analysis functions, it is a full-featured multi-domain test solution for fast debugging of complex electronic circuits. Its very high acquisition rate of over 1 million waveforms per second combined with deep memory of up to 200 Msample helps developers rapidly isolate even rare or random errors. Applications include embedded design development and analysis of power electronics components.

The R&S RTO2000 laboratory oscilloscopes with bandwidths up to 6 GHz are ideal for demanding tasks such as power integrity measurements. Depending on the model, they also support testing of radio interfaces on 802.11ac WLAN components for IoT modules in the 5 GHz band and of high-speed communications interfaces such as USB 3.1 with data rates of 5 Gbit/s. Thanks to the oscilloscopes' multi-domain capability, developers can investigate complex and challenging components and modules in a time-saving manner. The synchronized results from time, frequency, and protocol and logic analysis enable system-oriented, highly focused debugging. The acquisition rate is around one million waveforms per second.

Advanced analysis functions

To verify power-up and power-down sequences of FPGAs, CPUs and DSPs, the startup and shutdown behavior of their supply voltages needs to be characterized. There exist various voltage properties that must fulfill certain requirements:

• Power-up/power-down delay: The different supply voltages need to be applied with defined delays that can range from a few nanoseconds to several milliseconds, depending on the specific component.
• Voltage ramp-up times: The levels of the different supply voltages of FPGAs, CPUs and DSPs are usually between 1 V and 5 V. Each voltage has recommended minimum and maximum ramp-up times that range from a few microseconds to several milliseconds. Consequently, the recommended slew rates range from a few V/μs to several V/ms.
• Difference between supply voltages: During ramp-up (see above), the difference between the various voltages should not exceed defined values.

The specific supply voltage characteristics can be analyzed using the measurement and math functions integrated in the R&S RTE1000 and R&S RTO2000 oscilloscopes:

• Cursors allow manual analysis of multiple parameters, such as the delay between different channels.
• Automated measurement functions allow the straightforward verification of properties such as the delay between channels and the rise time of individual voltages. The sampling rate of 5 Msample/s of the R&S RT-ZVC makes it possible to measure typical slew rates of several volts per millisecond.
• Using math functions for individual oscilloscope channels, the required voltage difference between the channels can be verified.

High accuracy for tight supply voltage tolerances

In addition to power sequencing, stable and clean power rail voltages are a key requirement for adequate performance of almost any electronic design. Power rail voltages and their tolerance windows are generally decreasing to minimize power consumption and increase battery life.

Analyzing the small supply voltages and narrow tolerance windows of FPGAs, CPUs and DSPs calls for an instrument with adequate sensitivity combined with high accuracy. The R&S RT-ZVC offers excellent accuracy of 0.1 % for voltage measurements and 0.2 % for current measurements, which is more than 10 times higher than in the case of standard oscilloscope channels.

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