# A Recipe for Embedded Systems

One thing that is always important for engineers, is the need for us to deliver our projects on quality, schedule and budget. When it comes to developing embedded systems there are a number of lessons, learnt by embedded system developers over the years which can be used to ensure your embedded system achieves these. Let us explore some of the most important lessons learned in developing these.

# Hardware Considerations – Power Architectures Part One

One of the more interesting aspects to start looking at is the power architecture of a design, and how we go about powering FPGA’s (and other devices) on the board. Normally the system will have an intermediate voltage which comes from an AC/DC convertor or other DC supply which powers the system. The first stage of the design is to correctly specify this interface in terms of voltage and current required by the design. Determining this intermediate voltage is the easier task of the two, as the current required will have to take into account the downstream convertor efficiencies.

The first stage in defining power architecture is the determination of all the voltage rails and currents drawn by each of these rails. For example when considering a FPGA based imaging system as shown below you may have a number of voltage rails

For this example all of the power supplies have a requirement to be within +/-5%.

As can be seen from the table above the highest voltage required is 3.465V which is the 3.3V at its maximum acceptable tolerance. Knowing this value allows us to determine the voltage supplied by the AC/DC or other DC supply within the system, the sensible thing to do here is to select a convertor which has an output compatible with the 3.3V required and save a conversion stage (increasing the overall efficiency).

The next stage is to determine the power required by each of the rails. The requires that you use power estimation tools such as Xilinx XPE and read the datasheets for other devices to ensure you can determine the power required, I tend to collate all of this in a spread sheet as this comes in useful later on once we are determining the conversion architectures.

As you can see above when I have calculated the power required by the board I have performed two calculations the nominal and the maximum power, this is because at this point in time I have not calculated the maximum rail voltages provided in the worse case by the convertors therefore I have assumed they will be at maximum voltage. This is important as it is needed to determine the power required in the worse case by the AC/DC convertor (You should always design to address worse case requirements) while the difference above 146.5 mW is not large it could be in a larger system.

However having determined the load power we need to determine the overall power required including loses in the power convertors before we can specify the power required from the AC/DC convertor or DC Supply.

Having determined the power required by each device the next step is to determine the power required by each rail, this can then be used to determine the conversion architecture, although of course other requirements also come into play to determine this.

Regarding power architecture there are two main types of convertors

Switching regulators, generate the regulated output voltage by switching storage inductors into and out of the circuit to maintain a regulated output voltage when this switching is controlled via either an analogue or digital control loop. With a switching regulator theoretically 100% efficiency is achievable, however sadly the real world intervenes as components are not ideal however efficiencies greater than 90% can be achieved and GAN FET’s promise even better performance.

Linear Regulators generate the regulated voltage by dissipating the excess power across the pass transistor. This is dissipation is controlled via a control loop to adjust for fluctuations, as there is no switching involved the LR is often used where quieter power supplies are required however that does not mean all ripple on the voltage rail is rejected. As can be seen in the image below as the frequency increases the ripple rejection decreases.

# Defining and Selecting Module Connectors

When designing any electronic system the modules connectors will have a significant impact upon the system reliability.

The system could be designed for a traditional high reliability application like railway, aerospace, medical, military or maybe an emerging one like high frequency trading.

Perhaps the first and simplest approach is to group the connectors in the functional types Power, Control, Data and Clocks etc. as each of these will be addressed in a different manner.

For instance it is possible to have prime and redundant power connectors, but if your system has a large number of data interfaces then it is not possible to have prime and redundant connectors for each input. This may lead to the need for system level redundancy in the worse case.

Regardless of the connector function we need to consider the following aspects

• Pin Derating, maximum reliability of a component is achieved by reducing the electrical stress placed upon it. There are many different standards for this (ESA, NASA, US Military etc) however; the basic idea is to reduce the voltage and current applied to the pins.

• Connector pin out, can a power pin short to a ground pin which will effect the overall power distribution system. It is therefore a good idea to ensure separation of power and ground separating them correctly. If necessary you can use unused pins to add isolation.

• Use of different connector types, styles and keying to prevent incorrect mating of connectors when the system is assembled. An incorrect assembly and power application could result in many hours of design analysis to prove no parts have been subject to electrical overstress.

• Number of mating cycles, the number of times the modules are mated / de-mated from the system has to be recorded. For this reason many designers use connector savers which can be connected to the system and reduce the number of mating cycles.

• Suitability for the job at hand for example if your system uses high speed serial links to communicate then your connector requirements will be very different from a power interface or low speed interface.

• Environmental and Dynamic considerations, many high reliability systems see extremes of temperature, vibration and shock. Can the connector system survive the demands and still stay connected.

Once you have determined your connector philosophy the next stage is ensuring you have a reliable system is in ensuring you cannot propagate a fault outside of your unit should a failure within occur.

# Six Aspects to consider designing your PCB

Designing a PCB for current devices is a very complex and often over looked area instead focus falls upon the more interesting FPGA or Processors. However, the fact remains that without getting the board correct in the first place you may find you have issues either sooner or later so what are the main aspects of a modern PCB should we be concerned about.

• PCB Stack up – the keystone of the entire PCB this defines the number of layers within the PCB (More layers can increase the cost) along with allowing the engineering team to establish the characteristic impedance on the required layers. This like many things in engineering becomes a trade-off between fabrication processes and layer count to achieve the reliability, yield and cost targets.
• Via Types – Via’s enable interconnection between the layers and components however, there are many different types Through, Buried, Blind, and Micro (are these single layer, multi-layer or stacked). The best designs minimise the different types of via, close discussion with your selected PCB supplier is also important to ensure you’re via types are within their capabilities. You will also need to ensure the current carrying capacity of the different via types to ensure for high current paths you can parallel up.
• Design Rules – These will address both rules for the design i.e. component placement, crosstalk budgets, layer allocation, length matching / time of flight analysis and so on. It will also include design for manufacture rules which ensure the finished design can actually be manufactured for instance are the via aspect rations correct.
• Breakout strategy – before you can begin to verify your signal and power integrity you must first ensure you can break out and route all of the signals on high pin count devices. This will also affect the stack up of the PCB board for instance should you use micro via break out (most probably yes), how deep should these be is stacked micro via required. Once you have a defined stack for the PCB you can think of your routing strategy will it be the traditional North South East West, a layer based breakout or a hybrid style.
• Signal Integrity – the most commonly considered aspects of designing a good PCB typically an engineer will consider aspect such as signal rise and fall times, track length and characteristic impedance, drive strength and slew rate of the driver and termination. To ensure the best performance SI simulations will be performed pre layout and post layout of the PCB, you will also need to consider the Cross talk budget.
• Power Integrity – high performance devices especially modern FPGA and ASICs can require large currents at low voltages. Ensuring both the DC and AC performance of the power distribution network is of vital importance

Of course the list above is by no means complete however, it provides a good starting point

# FPGA Forum 2015 Key Note

The last 20 years have seen the explosion of FPGA technology used in many different end applications, including those within harsh environments. It therefore follows that system developers wish these devices to operate correctly and safely regardless of environment. When engineers design for a space flight mission, there are a number of environmental factors that may impact mission performance: radiation; temperature; and the dynamic environment. How much weighting each of these environmental factors has depends upon the end space application which are typically grouped into one of three categories Launcher, Science / Exploration or Telecommunication.  Regardless of the end application the engineer must consider FPGA technology, Mitigation strategies at both the FPGA and System level along with lessons learned from previous missions. However, these techniques and mitigation strategies are not just limited to space applications but can also be applied to terrestrial applications

Slides

# Design Reliability: MTBF Is Just the Beginning Issue 88

When most engineers think about design reliability, their minds turn to a single, central metric: mean time between failures. MTBF is, in fact, an important parameter in assessing how dependable your design will be. But another factor, probability of success, is just as crucial, and you would do well to take note of other considerations as well to ensure an accurate reliability analysis and, ultimately, a reliable solution.

# Calculating Mathematically Complex Functions Issue 87

Thanks to their flexibility and performance,
FPGAs have found
their way into a number of industrial,
science, military and other
applications that require the calculation
of complex mathematical problems
or transfer functions. It is not uncommon to
see tight accuracy and calculation latency
times in the more critical applications.
When using an FPGA to implement mathematical
functions, engineers normally
choose fixed-point mathematics (see Xcell
Journal issue 80, “The Basics of FPGA
Mathematics,” http://issuu.com/xcelljournal/
docs/xcell80/44?e=2232228/2002872).
Also, there are many algorithms, such as
CORDIC, that you can use to calculate transcendental
functions (see Xcell Journal issue
79, “How to Use the CORDIC Algorithm
publications/archives/xcel l/Xcell79.pdf).
However, when confronting functions that
are very mathematically complex, there are
more efficient ways of dealing with them than
by implementing the exact demanding function
within the FPGA. To understand these
alternative approaches—especially one of
them, polynomial approximation—let us first
define the problem.

# How to use Interrupts on the Zynq SoC Issue 87

In embedded processing, an interrupt is
a signal that temporarily halts the processor’s
current activities. The processor
saves its current state and executes
an interrupt service routine to address
the reason for the interrupt. An interrupt can
come from one of the three following places:
• Hardware – An electronic signal connected
directly to the processor
• Software – A software instruction loaded by
the processor
• Exception – An exception generated by the
processor when an error or exceptional
event occurs
Regardless of the source, interrupts can also
You can safely ignore a maskable interrupt
by setting the appropriate bit in an interrupt
mask register. But you cannot ignore a
non-maskable interrupt, because these are the
types typically used for timers and watchdogs.
Interrupts can be either edge triggered or
level triggered. The Xilinx® Zynq®-7000 All Programmable
SoC supports configuration of the
interrupt either way, as we will see later.

# Nuts and Bolts of Designing an FPGA into Your Hardware Issue 82

To many engineers and project
managers, implementing
the functionality within an
FPGA and achieving timing
closure are the main areas of focus.
However, actually designing the FPGA
onto the printed-circuit board at the
hardware level can provide a number
of interesting challenges that you must
surmount for a successful design.