# So We Just Consider the Resistor’s Tolerance Right?

When designing precision electronics or performing a detailed worst-case analysis, one quickly learns to consider parameters that may not be so important in other applications. One of the more interesting things to learn is that the tolerance of a resistor is just the starting point. It does not actually define the maximum or minimum value the resistor could be within your circuit.

The key parameters associated with a resistor are as follows.

Tolerance: This defines how close to the nominal value is allowable for the resistor when it is manufactured. A nominal 1,000Ω resistor with a tolerance of ±5% will have a value ranging between 950 and 1,050Ω. This value will be fixed; the value of the resistor will not vary during its life due to the tolerance. However, the engineer has to consider the tolerance in design calculations and ensure the circuit will function across the entire potential value range.

Temperature coefficient: This describes how the value of the resistor changes as a function of temperature. It is defined as parts per million/Kelvin; common values are 5, 10, 20, and 100 PPM/K. Actually, the best way to think of this is parts per million per ohm/Kelvin. A 1,000Ω resistor with a temperature coefficient of 100 PPM experiencing a ±60K temperature change over the operating temperature range (240-360K, based on an ambient room temperature of 300K) will experience a resistance change of ±6Ω based on its nominal value. Obviously, the lower the temperature coefficient, the more expensive the resistor will be. (This is the same for low-tolerance resistors.)

Resistor self-heating: For really high-precision circuits, it is sometimes necessary to consider the power dissipation within the resistor. The resistor will have a specified thermal resistance from the case to ambient, and this will be specified in °C/W. The engineer will know the power dissipation within the resistor; this can be used to determine the temperature rise and hence the effect on the resistance.
To determine the maximum and minimum resistance applicable to your resistor, you must consider the tolerance, the temperature coefficient, and the self-heating effect. As you perform your analysis, you may notice some of the parameters are negligible and can be discounted, but you have to consider them first to know whether or not you can discount them.

For some precision circuits (gain stages in amplifiers, for example) it may be necessary to match resistors to ensure their values are within a specified tolerance of each other and have the same temperature coefficients.

In certain circuits, it is also important to make sure that critical resistors are positioned correctly to ensure both terminal ends of the resistor are subjected to the same heating or cooling effects. Otherwise, the Seebeck effect may need to be considered. When using forced airflow, for example, it may be necessary to ensure that both resistor terminals are perpendicular to the airflow, so the component is of uniform temperature.

# 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

# Getting the most out of your Picoblaze Microcontroller Issue 89

The PicoBlaze™ is a compact 8-bit soft-core microcontroller that the FPGA engineer instantiates within their selected Xilinx® FPGA. Once implemented, this core is completely contained within the FPGA fabric using only logic slices and Block RAMs; it requires no external volatile or nonvolatile memory.

# 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.

# 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.

# A Pain-Free Way to Bring Up Your Hardware Design Issue 85

One of the most exciting
moments in an engineering
project is when the
hardware arrives in the lab
for the first time, ready for commissioning
before integration testing. This
stage in the development process typically
can mean long hours and a certain
amount of stress for all the engineers
on the project. But tools and
techniques are available to help ease
the way and move the project along.
Let’s take a look at how we can minimize
any issues that may arise in getting
a design to the next level, and how
to get through the commissioning
phase in a timely manner.

# 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.

# Design West 2013

Space: The Final Frontier – FPGAs for Space and Harsh Environments

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 spaceflight mission, there are three main
environmental factors that will impact performance: radiation; temperature; and
vibration and shock

Paper available here :- ESC-322Paper_Taylor

Slides Available here :- ESC-322Slides_Taylor

White Paper – Flying High-Performance FPGAs on Satellites: Two Case Studies

When considering flying an FPGA within a satellite mission, ensuring the device and design will work
within the radiation environment is the first of a number of parameters to take into account. In this
paper I am going to consider the parameters which must be considered when flying a highperformance
FPGA in two very different missions.

• Ukube1, a CubeSat mission scheduled for launch in late 2013
• A generic FPGA processing card for use in a number of GEO missions

Of these two missions, one UKube has been delivered for launch, while the generic FPGA processing
card is currently in development. Both of these missions have their own challenges and unique
requirements which need to be addressed. At the same time, however, both missions also have
common driving requirements.

Paper available here :- STS-401Paper_Taylor

Slides available here :- STS-401Slides_Taylor