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Transcript
Engineering of Embedded Systems
Steven Rogacki
Senior Engineer in Research
SPRL: Space Physics Research Laboratory
Office: 1204 Space Physics Bldg.
(734)936-3104
[email protected]
SPRL Engineering
SPRL provided multiple electronic assemblies for SAM, the primary
instrument suite on the NASA/MSL Rover mission
Also built electronics for:
•
MAVEN (Mars Atmosphere and Volatile EvolutioN)
•
LADEE (Lunar Atmosphere & Dust Environment Explorer)
•
CASSINI/Huygens probe
•
Rosetta (Comet rendezvous)
•
Solar Orbiter
•
Remote sensing radiometry: SMAP, GeoSTAR, HIRad, LRR
FIPS Mass Spectrometer,
Mercury Orbiter
Electronics Box, incl:
+15kV swept,
-15kV adjustable and
+3.6kV adjustable power
supplies; digital and
Analog signal processing
ASPS automatic precipitation sampling
system
PENGUIN Polar magnetometer
SPRL designed and built the TIDI Instrument on theTIMED spacecraft.
Still taking wind speed data after 14 years in orbit
Design of Space systems involves some unique constraints: thermal (vacuum operation); radiation
environment; launch vibration; limited parts choices; hard delivery schedules, no service calls
Embedded Systems -November 2015
2
Embedded Systems




Have limited or specific
functionality
Are not general purpose
computers, but they are
programmed...

ASIC (generally high
volume)

FPGA - Field
Programmable Gate
Array

Microcontroller
Limited user interface if any
“Real-time” response is often
critical
Embedded Systems -November 2015
3
Embedded Systems
Discuss:
Practical design methods and avoiding problems
Power Supplies for embedded systems
Linear vs. Switching
Terminology
Introduction to SPICE Modelling
Online resources
Real-life examples
Note to self: slides with pictures are better
Embedded Systems -November 2015
4
A Real Design Process is often (usually) chaotic
System
Requirements
Analysis
Electrical
Schematic
PC Board Layout
Mechanical
Design
System
Requirements
Oops, we
forgot something
Not enough
memory
Electrical
Schematic
PC Board Layout
Mechanical
Design
Fabricate
Program & Test
Program & Test
Embedded Systems -November 2015
Spec can't
be met
Analysis
Fabricate
This framework can help
to schedule milestones
Poorly defined
requirement
Real parts don't
Work that way
I can't
test this
at all
How can I test
this?
This is more realistic and you aren't doing anything wrong...
Try to anticipate problems, be suspicious of “small
changes”, do trade studies, build limited prototypes to test
uncertainties, begin programming early, revise
documentation as you go, keep a lab notebook.
5
Design Process: System Requirements
Determine what you are trying to do...
Document Functional Requirements: To design a system with ambiguous specifications,
write your own and update as needed. Requirements must be testable or verifiable
through analysis. “Fast as possible”, “Low noise”, “Small size”, etc. are NOT
requirements.
Block diagrams can be helpful to keep a global view. Most practical designs progress
from the top down AND from the bottom up.
Simple Top Level
Functional Block Diagram
(FIPS Mass Spectrometer)
Embedded Systems -November 2015
6
Block Diagram showing interface details
(PENGUIN Polar Magnetometer)
Embedded Systems -November 2015
7
Example: Top Level Requirements
Embedded Systems -November 2015
8
Detailed Requirements: Snow Sensor
Embedded Systems -November 2015
9
Energy Estimate: Snow Sensor
Embedded Systems -November 2015
10
Design Process

Analysis and Simulation
Information collection phase: study the problem, gather information, consider
alternatives. Dig deeper into the real goals. Create or reference top-level and
detailed requirements. Meet and discuss. Find examples of similar designs.

Design Tools

MATLAB, Mathematica, Excel (license free: Octave, SciLab)

SPICE (license free: LtSPICE, more on this later...)

Development Boards: low cost way to test possible designs

Prototype boards: The goal is to test ideas with speed and flexibility

Online tools: use parametric parts searches on device producers
websites. Linear Technology, Analog Devices, National
Semiconductor, Texas Instruments, STMicro, Microchip, Microsemi,
Micrel, Maxim, OK that's enough but of course there are others...

Most of these companies also produce Application Notes and
Reference Designs for their parts. Always use these if you can.
Embedded Systems -November 2015
11
Application Notes
A wealth of information from many companies. For example, Linear
Technology AN47 discusses high speed analog circuits; how to
accurately measure them, layout advice, numerous examples, and
how to successfully build fast prototype circuits. The terse comments
on the pages shown here are a bit of engineering humor.
Embedded Systems -November 2015
12
One Prototyping Method


Printed circuit boards are relatively low cost and can be produced in less
than a week; they can be an efficient prototyping tool if you can afford
multiple iterations.
But, if you are inventing something new, the proper circuit design may
not be known. The technique shown here is good for testing high speed
analog circuitry, allowing very rapid component or topology changes.
Keep a hot soldering iron handy.
Embedded Systems -November 2015
13
Design Process

Electrical Schematic Design

An electrical schematic diagram is not just a means of capturing a
design. It should also clearly communicate the design functionality.

Signals generally progress from left to right, (+) Power feeds in at the
top, (-) Power at the bottom. Named nets should be used to limit
clutter and identify signals across sheets, but don't create a graphical
netlist.

Tools include: ORCAD, Altium, Eagle (free: KiCAD)

The Art of Electronics, 2nd ed. Horowitz, Hill, is another wonderful
resource for practical advice. One example from that book:
An awful schematic
Embedded Systems -November 2015
A good schematic
14
A reasonably good schematic
LT1964, LT1461 LDO's provide +/-1.5V, +1.8V
(Low Dropout Regulators)
Embedded Systems -November 2015
15
Schematic Examples



Showing an embedded power supply providing multiple
voltages with both linear and switching regulators.
Outputs: +15V (unregulated), +5V, +3.3V, +1.2V using
LT3502A (switcher) and LTC1844 (LDO regulator)
Snow Sensor
Embedded Systems -November 2015
16
Example using battery directly
Embedded Systems -November 2015
17
Design Process: Circuit Board Layout



Bypass capacitors for every IC, as near to the chip as possible. Datasheets usually
offer specific recommendations for bypassing and sometimes board layouts.
Minimize inductance of high frequency or sensitive signal traces. Inductance is a
function of the enclosed circuit path area.
High frequency signals may need controlled impedance transmission lines or even
differential paired conductors.

A ground plane layer is recommended for all but the simplest designs.

Consider where to add test points.
PCB Calculator
Embedded Systems -November 2015
18
Design Process: Mechanical Design; Fabrication and Assembly




Don't forget a Bill of Material

Schematic capture tools can create BOM reports

Include Component type; description; Manufacturer and Part number;
Reference designation; Package; voltage, tolerance, or power rating if
needed; if possible, distributor and their part number.
3D Modelling is extremely useful:

Befriend a mechanical engineer, or

Learn Autocad Inventor, Solidworks, or an equivalent
This class will introduce you to bench reflow soldering for circuit
board assembly. There is also hand soldering; not as easy as it
looks.
Some parts will likely require specialized equipment, eg. Ball Grid
Arrays. Services are available which will assemble your boards for
reasonable cost. They may even buy all the electronic parts as
well (Advanced Assembly is one such vendor)

Embedded Systems -November 2015
19
Design Process Specifics: Power Subsystem


Specified Power Sources: If you are designing a subsystem in a larger
assembly, your primary power source may be specified for you:

DC Bus voltage; There may be inrush current limits or even conducted noise
limits in addition to specified voltage range, current range, and power limits.

AC Line voltage; Chips exist which can regulate rectified 120VAC.
Alternative embedded power sources

Solar Cells: Will almost certainly require a battery system as well...


Battery - Primary cell or rechargeableExample: Li-Ion Rechargeable Battery


PPT (Peak Power Tracking) may be needed using a switching power
supply.
Huge variety of types: besides energy capacity and cell voltage,
consider internal resistance, peak or pulse current required,
temperature range in operation, safety, discharge characteristics.
Rechargeable batteries or battery packs can have charge monitors
build-in to protect the battery or balance charge in series cells.
Energy harvesting: Use vibration, thermal, light, sound, but get very low power:
supercap can replace need for battery.
Embedded Systems -November 2015
20




Primary battery
datasheet (nonrechargeable).
Capacity 2000mAH
meets the
1547mAH
calculation.
But, this battery
won't work for the
Snow Sensor
project.
Any guesses why?
alternate battery
Embedded Systems -November 2015
21
Once the primary power source is defined, all subsystem load voltage and current requirements are
specified, and other factors considered such as voltage stability, response to transient loads and
desired efficiency, actual part selection for voltage regulation can begin. Some terminology:

Quiescent Current: the current or power that the regulator uses to function; does not flow to the load.

Dropout Voltage: minimum voltage between input and output

Linear Voltage Regulator: Non-LDO Devices usually capable of higher current

LDO Low Drop-Out Regulator

Voltage Reference: High accuracy voltage control, can sink as well as source current

Switching Regulator: More efficient for higher currents or large difference between input and output
voltages.

Buck: Output voltage lower than input

Boost: Output voltage higher than input

Buck/Boost: Input voltage can range above and below output

Resonant: Many topologies, higher power.

Peak Power Tracker: get maximum power out of a solar array

Application note: Linear Technology AN140 has a good description of linear and
switcher supplies
Embedded Systems -November 2015
22
Design Process Specifics: Power Subsystem
Type
Linear
Voltage
Regulator
Output
Current
Dropout
Voltage
Input Voltage
Range
Voltage
Accuracy
0 to 1A
2V
7 to 35V
+/-4%
0 to 150mA
90mV @
150mA
1.6 to 6.5V
+/-1.75%
4uA
-2mA to
+10mA
200mV
2.7 to 12.6V
0.15%
25uA
0 to 100mA
800mV
1.9 to 3.9V
+/-2.5%
Example Part Quiescent
Current
5mA
Fixed 5V
35uA
Low Dropout
Fixed or adj. V
Linear
range from
Regulator
1.25 to 6V
Voltage
Reference
Switcher:
Buck
2.5 to 5V
1.9 to 2.3V
fixed
To paraphrase AN140: The choice between switcher and linear
supply is not always easy, but linear regulators or LDO's are often
selected for their simplicity, ease of use, low cost, low noise, and
fast transient response. And if Vout is close to Vin, an LDO may be
more efficient than a SMPS (switching mode power supply).
Embedded Systems -November 2015
23
Testing Notes





Write firmware with testing in mind.
Include low-level test commands for
every function.
When making direct circuit
measurements, understand 'scope
probes and their limitations: Another
diagram from Linear Technology
AN47 >>>>>
The higher the frequency, the shorter
the probe ground lead.
Adjust probe response before using.
Oscilloscopes are fast and show
waveforms but are not very accurate
for amplitude measurement. Use
appropriate bench instrumentation.
Embedded Systems -November 2015
24
Introduction to SPICE



Demonstrate capabilities using linear regulator
model.
Show how to test specific (Linear Technology)
voltage regulators

LDO Linear Regulator LTC1844 for transient
response

Switching buck converter LT3502 for noise
and efficiency
Questions?
Embedded Systems -November 2015
25