Embedded system hardware architecture ii

EMBEDDED
SYSTEM
HARDWARE
ARCHITECTURE II
MODULE IV
GRACE ABRAHAM

2
ELECTROSTATIC DISCHARGE EFFECTS
• ESD is an important design consideration in embedded
applications because of the potential for failure &
erroneous operation in the presence of external electric
fields
• ESD voltages are commonly impressed on embedded
interfaces-on the order of 10’s of 1000’s of volt
• This issue is in the keyboard or user i/p device
• This effect can damage or may cause latent failures
• Shielding & grounding techniques to minimize this effect
• Several useful resources like publications, seminars, test
labs and consultants who specialize in solving EDC problems

3
• FAULT TOLERANCE
 Fault tolerance has become a requirement in embedded system
 Many hardware & software solutions have been developed
 Each fault is categorized as “hard” or “soft” fault
 Hard faults cause an error that does not go away
 Soft faults are due to transient events or program errors
 Self test and diagnostic programs may be able to identify &
diagnose the failure if it is not severe
 Depending on type of fault that occurs and which devices are
effected, it is possible to design a system to detect the fault
o Eg. Pushing reset or powering down does not result in recovery from
fault

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 In the event of a soft failure, it is possible for designers to make
the system recover from fault automatically
 A BIST program can be used to detect faults in following devices
 The data contents of ROM devices can be tested for errors using
following techniques
o Processor
o Memory
o ROM
o RAM
o E/EEPROM
o Peripheral devices
o Parity
o Checksum
o Cyclic redundancy check (CRC)

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o Hardware error detection and correction
o Data/address pattern tests
o Data structure integrity by checking stack limits and address range
validity
 RAM memories & the integrity of information stored in RAM by
processor can be tested for proper operation using following
techniques
 The integrity of the program and proper execution sequence by
the CPU can be checked using following techniques
o Hardware parity error detection
o Duplicate, redundant hardware and cross checking or voting
o “Watch dog” timer that operates the CPU chip’s reset line
o Diagnostics that run constantly, when CPU has nothing else to do

6
HARDWARE DEVELOPMENT TOOLS
• Two general classes of hardware development tools are
• Passive analysis tools allow looking at the operation of the
system
• Active tools allow the designer to intrude on the operation
of the system while it’s running
• System under test – “target” system
• Computer that is used to develop, edit, compile, assemble,
and download the code to target system – “Host” system
 Passive tools
 Active tools

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• Passive tools include :
• Active tools include :
 Logic probes to look at static logical level and detect pulses
 Oscilloscopes to look at single waveform
 Logic analyzers, with processor specific probes
 Software to assist hardware development, scope loops
 In-circuit emulators (ICE) for HW/SW integration are plugged
to target system in the place of CPU, allowing designer to see
inside the microcontroller, download & execute the programs
selectively
 ROM emulators(ROM ICE) allow designer to reduce the time to
edit, compile, debug programs by replacing program EPROM
with a RAM that can be loaded quickly from host computer

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• INSTRUMENTATION ISSUES
 One of the most significant problem is the proper selection and
use of test instrumentation
 Improper selection and application of tools is the source of
wasted time & confusion for the designer
 Two common usage problems relate to the use of oscilloscope
logic analyzer probes
 2 common causes for measurement problems
 Problems causes a potential for erroneous measurements
 2 things are done two mitigate these problems :
o Excessive ground lead inductances
o Excessive capacitive loading

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1) Use the shortest possible test leads
2) Use high-impedance probes, especially designed for high-speed
applications

10
SOFTWARE DEVELOPMENT TOOLS
• 3 categories of software development tools
• Most of available tools are available as freeware,
shareware, or low-cost commercial products
• Translators include :
 Language translators
 Debuggers
 Utility programs that generally run on the host computers
 Assembler
 Compiler
 Linker
 Interpreter

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• Debuggers include :
• Utilities include :
 Software /firmware monitors
 Processors In circuit Emulator (ICE)
 ROM ICE
 PROM Programming
 Performance measurement
 Execution frequency histogram

12
OTHER SPECIALIZED DESIGN
CONSIDERATIONS
• Thermal characteristics of a system
• Concept of thermal resistance
• Power dissipation
• Effects on device temperature & reliability
• Battery power storage

13
• THERMAL ANALYSIS AND DESIGN
 Reliability of devices are closely related to temperature
 Device’s reliability drops exponentially with increasing temperature
 To calculate operating temperature, consider an electrical analogy
 Temperature rise(°C)=power(watts) * thermal resistance(°C/watt)
 For eg : Given a 5V linear voltage regulator with a 9V i/p providing
1A of load current, the regulator will dissipate :
P = V * I
= (9-5 volts) * 1 amp
= 4 watts
 Total thermal resistance between the semiconductor junction and
ambient air
o Temperature is analogous to voltage
o Power dissipated is equivalent to current
o Thermal resistance is equivalent to electrical resistance

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 ϴja = ϴjc + ϴca
= 1 + 10
= 11°C/watt
 Temperature rise of the junction above that of air surrounding
the regulator is given by:
T = P * ϴja
= 4 watts * 11°C/watt
= 44°C above ambient
 The designer must reduce the input voltage to reduce the thermal
resistance by forced air flow

15
• BATTERY – POWERED SYSTEM DESIGN
CONSIDERATIONS
 Rapid increase in use of portable, battery-operated electronic
devices
 Batteries are divided into 2 common groups :
 Primary cells include :
 Secondary cells include :
o Primary (one- time discharge & discard)
o Secondary (rechargeable)
o Lead-acid cell (older)
o Nickel-cadmium (NiCd) cell (older)
o Nickel metal hydride (NiMH) (newer)
o Rechargeable alkaline and lithium ion cell (newer)
o Non-rechargeable alkaline and lithium cells

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 A wide range of special – purpose batteries that are optimized for
some specific characteristics
 Primary batteries : alkalines and lithium coin cells
 Secondary cells :
o Zinc-air primary cell
o Uses atmospheric air as an “electrode” to provide very high energy density
at low operating current
o Simple to use
o Limited to 1 to 3 years of operation
o Shelf-time limit imposed by internal leakage current that discharges the
battery slowly over time, especially at high temperatures
o Rechargeable battery types each have slightly differing charge/discharge
requirements
o Special algorithms to optimize the performance & service life of batteries

17
 Embedded devices must be designed to operate long periods of
time
 There are CMOS processors & memories which have been designed
with very low power consumption operating modes frequently
referred to as “sleep”, ‘’power down”, or “idle” modes

18
PROCESSOR PERFORMANCE METRICS
• To compare different processor performance we have
performance metrics such as
 IPS (instructions per second)
 OPS (operations per second)
 FLOPS(floating – point OPS)
 Benchmarks

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• IPS (Instructions per second)
 Rate at which the fastest instruction executes on a machine
 MIPS (millions of IPS)
 BIPS (billions of IPS)
• OPS (Operations per second)
 OPS (MOPS & BOPS) are instruction execution times based on a
mix of different instructions
 FLOPS (mega FLOPS, giga FLOPS) are similar expect that they
weight floating – point instructions heavily to represent heavy
computational applications such as computational simulations

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 Problem with OPS metric is that the resulting number is heavily
dependent on the instruction mix that is used to compute it,
which may not accurately represent intended application
instruction execution frequency
• BENCHMARKS
 Benchmarks are short, self contained programs that perform a critical
part of an application
 Used to compare functionally equivalent code on different machines
 Programs are run for some number of iterations, and the time is
measured and compared with that of other CPU
 Problem : Benchmark is not only the measure of the processor but
also of programmer & tools used to implement the program

Embedded system hardware architecture ii

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Embedded system hardware architecture ii